* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download teachers` resource book on fundamental particles and
Super-Kamiokande wikipedia , lookup
Theory of everything wikipedia , lookup
Quantum electrodynamics wikipedia , lookup
Eigenstate thermalization hypothesis wikipedia , lookup
Canonical quantization wikipedia , lookup
Renormalization group wikipedia , lookup
Minimal Supersymmetric Standard Model wikipedia , lookup
Technicolor (physics) wikipedia , lookup
History of quantum field theory wikipedia , lookup
Peter Kalmus wikipedia , lookup
Large Hadron Collider wikipedia , lookup
Introduction to quantum mechanics wikipedia , lookup
Double-slit experiment wikipedia , lookup
Weakly-interacting massive particles wikipedia , lookup
Renormalization wikipedia , lookup
Nuclear structure wikipedia , lookup
Atomic nucleus wikipedia , lookup
Identical particles wikipedia , lookup
Relativistic quantum mechanics wikipedia , lookup
Mathematical formulation of the Standard Model wikipedia , lookup
Quantum chromodynamics wikipedia , lookup
ALICE experiment wikipedia , lookup
Strangeness production wikipedia , lookup
Future Circular Collider wikipedia , lookup
Grand Unified Theory wikipedia , lookup
Theoretical and experimental justification for the Schrödinger equation wikipedia , lookup
ATLAS experiment wikipedia , lookup
Electron scattering wikipedia , lookup
Compact Muon Solenoid wikipedia , lookup
SLAC-PUB-4879 LBL-26669 January 1989 CM) TEACHERS’ RESOURCE BOOK ON FUNDAMENTAL PARTICLES AND INTERACTIONS* HELEN R. QUINN AND JONATHAN DORFAN Stanford Linear Accelerator Center Stanford University, Stanford, California 94309 R. MICHAEL BARNETT,ROBERT N.CAHNAND GERSON GOLDHABER~ Physics Division, Lawrence Berkeley Laboratory 1 Cyclotron Road, Berkeley, California 94720 and tDepartment of Physics, University of California GORDON J. AUBRECHT, Department II of Physics, Ohio State University Marion, Ohio 43302 AND MEMBERS OF THE FUNDAMENTAL PARTICLES AND INTERACTIONS CHART COMMITTEE To accompany the FPICC Wall Chart @Copyright 1989 FPICC The U.S. Government is licensed to publish or reproduce this work. t * This work was supported by the Director, Office of High Energy and Nuclear Physics, Division of High Energy Physics, and Office of University and Industry Programs of the U.S. Department of Energy under Contract Nos. DE-AC03-76SF00098 and DE-AC03-76SF00515, and by the National Science Foundation under agreement no. PHY83-18358. . DISCLAIMER This document was prepared as an account of work sponsored : by the United States Government. Neither the United States Government nor any agency thereof, nor The Regents of the University of California, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or : responsibihty for the accuracy, completeness, or usefulness of ‘any information, apparatus, product, or process disclosed, or ’representsthat its use would not infringe privately owned rights. ~Reference herein to any specific commercial products process,or service by its trade name, trademark, manufacturer, or otherwise, does not necessanly constitute or imply its endorsement, : recommendation, or favoring by the United States Government or any agency thereof, or The Regents of the University of Califomia. The views and opinions of authors expressedherein do not necessarily state or reflect those of the United States ~Government or any agency thereof or The Regents of the University of California and shall not be used for adverttsmg or ’product endorsement purposes. Printed in the United States of America Available from National Technical Information Service U.S. Department of Commerce 5285 Port Royal Road Springfield, VA 22161 Price Code: A04 Lawrence Berkeley Laboratory is an equal opportunity employer. . 1 The Fundamental Particles and Interactions Chart Acknowledgments Committee We would like to give credit to the Lawrence William T. Achor (Western Maryland II (Ohio’ State University), Laboratory), Erzberger Robert College), R. Michael Beck Clark Accelerator Center), (Lawrence Berkeley (Texas A & M University), Andria Frederick Area High School, Pennsylvania), Accelerator Center), and Thad Drasko S. Priebe, Helen R. Quinn James Ruebush P. Zaleskiewicz J. Aubrecht, Barnett .(Palo Alto High School, California), National Gordon (St. Charles (University Jovanovic Chair Department Liliana Coordination This project of Roland at Greensburg), project (PA), Funding mittee (PA) organization and internal Area School Physics through can Association and ancillary Office of High District National communications and from materials Energy Science Foundation of University under Contract Nos. and Industry Grant Programs Division of Lawrence Berkeley from the Palmyra TEI-8550610 of Modern to the Ameri- and publication of the chart Office of Energy Research, Physics and of the U.S. Department of Energy and DE-AC03-76SF00515 under Agreement Laboratory, for com- of High Energy ceived via the Center for Science and Engineering Group Support on the Teaching by the Director, Physics, DE-AC03-76SF00098 Science Foundation was received The development was supported and Nuclear of sources. the Conference of Physics Teachers. Office U.S. National from a variety No. PHY83-18358 Education input We are grateful was done by was the creation of George came from Ralph for the excellent Dennis, work of this Director Linear from of the ‘Lawrence is indebted thus far. Among to many Berkeley High School, the valuable (CA), individuals and St. and support Center for Science (SLAC), High School, CA), H. Nelson (Berkeley input 3. Quinn (Lick-Wilmerding High to the Area High School School, (IL). We wish of the chart and book which and C. G. Wohl (LBL), f rom A. Gould and C. Sneider (funding High School, CA), P. Moore (Piedmont (Miramonte was also given by B. Armstrong (Notre High School, CA). Valuable and G. S. Wagman (LBL) and from High School, CA). reData Accelerator Center). This is a Preliminary Version of the Resource January 2 1.11 . 3 we from (Lawrence High School, CA) Sister M. E. Parrot and D. Shortenhaus technical contributions at Palmyra J. D. Jackson, Hall of Science), and from R. Apra (Pioneer KY), for their Charles advice on the content and M. E. Peskin Dame Academy, advice, Laboratory them are physics students received from R. N. Cahn, I. Hinchliffe, M. Karliner the encouragement, Education. FPICC Palo Alto greatly and by the and the Particle and via the Stanford Department. Illustrations design and artwork in the four figures of this work plus artistic benefited Otto, to acknowledge funding The primary Laboratory group. The has received artwork Berkeley Linear and Engineering The FPICC the chart. The airbrush head of the Illustrations High School, Illinois), of Pittsburgh that produced Jach. IIuggard. (Fermi (Palmyra (Stanford I 1989 Book for use in field testing. . 1 I II TABLE Chapter Chapter Chapter 1. Introduction 1.1 Preliminaries 1.2 Using the Chart 2. ..................... and Philology Forces and Interactions 2.3 Particle Types 3.1 Layout 3.2 The Diagram 3.3 Fermion Boson Tables 3.7 Figures 4. Accelerators 4 5.11 Fifth 8 5.12 W and 2 Bosons 12 5.13 Future 5.14 References 6. Further 6.1 The Structure of the Atom ............. 20 20 .................... within an Atom 20 ........ 21 ....................... 29 ....................... Tables 32 ...................... 36 ................ of the Interactions .40 ........................ Introduction to Experimental 49 ............... and Detectors High Energy Physics ...... 49 54 4.2 Accelerators ....................... 4.3 Detectors 5. Experimental 5.1 The Nucleus ..... 5.2 Cosmic Rays ....................... 5.3 Strange Particles 5.4 Parity 5.5 More and More Hadrons .................. 99 5.6 Structure of the Nucleon ................. ,102 5.7 Neutrino Experiments 5.8 Neutral 66 ........................ Basis Violation Currents Chapter 16 of the Structure Tables Properties ‘T Lepton 14 .................. of the Chart of the Chart 3.6 5.10 14 ................... ...................... The Components Hadron J/t+4 Particle 14 ........................ 2.2 4.1 Chapter Overview 3.5 5.9 ....................... Physics 3. CONTENTS ...................... 2.1 3.4 Chapter dF of Particle , Physics ................. ......... 91 91 92 ..................... 95 ..................... 96 ................... .................... ,102 ,103 Chapter .‘iT ........ .............. Quark , ..................... .108 .109 .................... .lll .......................... ............... r- Explanations The Structure of the Nucleus The Structure of Nucleons 6.4 Confinement ...................... 6.5 The Quark Color and Color Neutrality 6.7 Words 6.8 Residual 6.9 Decays of Unstable 6.10 The Higgs Boson 6.11 Particle Theory 6.12 Beyond the Standard 7. Bibliography Strong Baryons .116 ........ .122 .............. .123 ................ from Equations Interactions Particles .125 ............ ............... .134 ............... .135 .138 .................... in Astrophysics Model .119 .121 of Hadrons 6.6 .112 ............... and other .lll .I12 ................ 6.2 and Pictures ....... ................. 6.3 Content .104 .108 ........................ and Cosmology ............... ....... .139 .140 . . . . . . . . . . . . 1’‘. . . . . . . . .141 iE . I . 1 1. Introductioh tive transitions a nucleus some type of radiation. 1.1. PRELIMINARIES People different have long asked “What together ?” Particle Model.” Down to the smallest are answered by a theory The purpose this theory made of?” physics seeks to answer these questions. a clear consensus has evolved. these questions is the world of this chart in a form suitable and “What physicists and the accompanying for presentation it Over the past ten years scales studied that particle holds to beginning a helium process. strong a neutron Model is a well-established progress in our understanding to physics as the periodic particles and their and another that table is to chemistry. by postulating six called leptons it should from which that questions most It is as basic since the slow rate of the process indicates It explains hundreds six basic constituents all matter in an introductory (“What is made. mature called weaker than and important positively charged electrical interaction question: (the with theory dard Model” question, holds the nucleus protons know the two at least down the atom has a x U(1) independently is students this question should become compelling; gravity must be invoked. and neutrons, binding There is one more nuclear phenomenon is too weak. between this interaction. them into nuclei. which needs an explanation. 8 learn that To answer It is called the strong the energetic a electron this process requires yet another compared to the emission of com- We call this a weak interaction, fourth theory Sheldon Glashow, the accuracy of this book. other contributions the development In radioac- Both Glashow, all observed particle contributions that is contributions was first suggested in the context and Abdus and Luciano led to these theoretical interactions The critical Maiani. on the theoretical and the experimental of our understanding. The Nobel importance of a was recognized by and then of the subsequently is given in Chapter but which 5 side there are many were important Prize in 1979 was awarded for their role in the development 9 we ob- was developed Some of the history developments made by the theory Salam and Weinberg of the world x U(1) theory we have not mentioned This Model led to the understanding of the predictions which processes. must be mentioned here. The .. by Murray Gell-Mann. The Salam. of the SU(2) called “the Stan- of the Standard description of the weak and electromagnetic John Ilipoulos which theory as a correct primary theory by Steven Weinberg type of quark experiments tested SU(3) through of the development and theoretical very important SU(2) the nucleus physics clear that by comes from it must be due to an interaction describes to give a history serve. Certain know repulsion satisfactorily and to its acceptance are held in place by their than the electrical which either one. of the theory interaction that electron emitting interactions. are now understood Many experimental strong When force stronger It soon becomes interaction that probably together?” charges repel one another, another It acts between They They which book does not attempt For the most part they have not yet asked the other there must be some attractive the protons. by electrons the nucleus. can answer the electromagnetic These three interactions of course. holds it together?“) level of atoms). nucleus surrounded and neutrons. “What like electrical lo-“m and “What class they -r rays in electrical be explained inter- mediated quarks It is the product general physics reach a physics is it made of?” to a scale of about made of protons students slowly of matter. theory. By the time relatively structure We believe it is a sufficiently now be included It occurs parable of subatomic three a strong in a, transition cannot To explain tremendous energy identified nuclear fission, emitted decays into a proton, represents of the fundamental properties many years of research. theory photon It is an energetic an antineutrino. type of interaction. The Standard in a spontaneous interactions. in that on radioactivity The /3 ray, h owever, interaction. and, as later realized, physics students. experiments in the process emitting called o, /3 and 7 rays. We now know that the o ray is nucleus emitted or electrical transition is to summarize The earliest The y ray is an energetic by an electrical experimentally, call “the Standard book action goes from one state to another, types of radiation, actually I to to of the electroweak . theory. Grll-hlatln undwstantling hat1 \von tile’ Nobc.1 Prize ~II 1969 for his c.olltriljlltiorls of strangencw, the 1984 Nobel side, for the discovery the Standard quarks, and Prize was awarded Model. In 1988, Leon Lederman, berger won the Nobel Prize for their The Standard for all processes, neutrons, which in this theory stituents called quarks. confirmation Melvin 1962 experiment the structure down ii1 van dcr hleer of a prediction Schwartz of and Jack Stein- demonstrating to a scale even finer Standard the existence and the interactions than composite Model how to write Fortunately of matter are themselves The do not yet know most particle 011 thtz c~spwinlc~rlt and Sillion for us, gravity “electric charge is conserved”- and is consistent this with Model has not answered simply correspond being predicted quantum approximation and lepton Newton’s we mean that allows because that also and so in to ignore the effects by it. There are many For example, However, features in the Standard a particle we do have soTe indirect evidence the Standard seeks theories of gravity. data it will surely Because that Model be a part of whatever further than has yet to be the theory cannot including Particle Model” relativity goes heyond Model correctly understanding cal1f.d group theory.* this material and which understand theory. be stressed rather The beauty predictions about that In presenting Model mathematical structure but also to predict the rates of a variety the outcome are well beyond this material, of of experiments. beginning physics are.but a small concepts of lists of particle lies in the fact that III to stress that the names and descriptions than memorization of the Standard processes can be explained to calculate ‘I‘liis parts of the nleaning particles cannot, of such calculations X ri( I)., it is important to name and describe not just can exist of any physical should language more than the most obvious in teaching processes, and to make quantitative part .r .~!I(?) and processes names and masses. hundreds of particles and on the basis of a few types of quarks and leptons and interactions. are familiar with basis of particle physics the need for a microscope should be emphasized. to see small objects. The accelerators that are the basic tools of particle physics experiments act as gigantic There between reached is an inverse accelerator relationship and the sizes of objects that reason why progress in this field requires celerators. At this time physicists the energy in the This the construction of ever higher-energy are proposing to construct (SSC) to seek answers to some of the fundamental tions. details on accelerators microscopes. by particles they can probe. Super Collider Further Students and detectors is the fundamental ac- the Superconducting unanswered will be discussed ques- in Chapter 4. the possible the Standard of general the Standard rather does not address. go “beyond theory Model the values of any of the othcar masses. There are many deeper questions, today such as of the data that Model, for it; particles The experimental the Standard called the top quark have, nor can it explain that is made- this is true in the Standard all questions. physicists they should gravity is S(i(3) in this chart and book is based on an extensive students, theory of fact at present. the same way as Einstein’s theory a statement data of all interactions, research whenever all experimental what mass it should somewhat many book to a choice of parameters although physics that their Throughout unification everything Ilowever, Model in a mathematical we will not deal pith made up of con- is a very weak interaction physics processes it is an excellent Model. quark this book, of this language. and of protons does not include a consistent the parts of the theory the mathematics The wall chart and this book describe the world in the language of the Standard predict dcnotcs nanle of the Standard Although that objects re- of gravity. observed, ‘1‘11~tcchltical which Model explains gravity. iolks. Rubbia type. sponsible includes interact of the W and Z bosons, a critical of more than one neutrino physicists their to Carlo t11(, to I describes we reach. in so * One reads this expression as “S U 3 cross S U 2 cross U 1”. The symbols SCI(n) stand for the Simple Unitary group of n dimensions. For those readers who really want to know: The simplest representations of this group the set of all n-component complex vectors and the transformations among them given by e~~‘l”*p where the sum is over Q = 1,2,3,. ., (n2 - l), f” are (n* - 1) arbitrary real numbers, and A, are n x n traceless, hermitian matrices. There are n* - 1 such matrices in .57(n). V( 1) is the one dimensional unitary group, represented most simply by the transformations of e’@ acting on the set of all complex numbers. 10 11 I . I d Chapter 5 presents a brief summary It is important scribing matter. The interactions us to explain particles of matter why are constantly while and decay patterns. combina- Further, is static; at every level we find they explain The Standard others Model allows decay very rapidly. into the Standard of the interactions, In Chapter which in de- particles. in motion. are stable the dynamics Model Nothing laws of physics are built laws, along with as well as constituents the rules which explain are observable objects. some objects of the conservation Standard provide of the composite the constituents experiments. to stress the role of interactions tions of the fundamental the structure of some crucial that Model. explain All It is these particle lifetimes 6 of this book we discuss the application of the to such questions. concept then isotopes and again in particle the lightest reappears baryon. cussion of nuclear chart. Finally, just chart Typically for use in an introductory such a course covers mechanics, and waves and modern provided should primarily physics. Clearly heat, electricity the introduction by the wall chart belongs to the modern come after However, the introduction the fundamental concepts early as possible in the course. or another of the four fundamental are first discussed portion interactions does not immediately between should It is important of quantum should of the course-the course; for example, tromagnetic the introduction mechanics. be introduced with electron sion principle the concept to provide levels in atomic and its crucial physics one should role in explaining more than a static The notion that as notion that friction in the surfaces of the two students that in particle into stable objects them of gravity particles the Standard in the our understanding picture of objects introduce the Pauli of levels. made from can first be introduced also should Model be discussed. is built It should between in mechanics in the solar system and then carried through the atomic in the various be made clear to on the basis of hundreds this brief outline applies to an introductory the college level the chart can also be profitably level course on modern of experiments course, it is clear that at incorporated, physics or even an introduction Another physics proposed teachers presented which the wall chart about use of the chart modern in this chart is important in their teaching is to educate particle will physics. provide into an intermediate- to particle program. This In Exclusame 12 13 I both physics. high school and college Workshops general even for teachers of the elec- wave in the wave section. the “filling” simplifies of physics. Although material as the quantum are our understanding there must be some interaction of the course on electromagnetism of an electromagnetic simplifies and neutrons level. Refer when forces view as early as possible of the photon field can occur first in the section and reappear discussing to the quantum of protons picture in the dis- only a few of which and nuclear levels to the quark level. The role of the four interactions occur to most students. to begin referring mesons and baryons, to see how the quark pieces together. this subject It is useful of known we come to the quark decays of unstable be introduced the atoms scale of structure is decay process shown on the optics, the idea that every force is due to one interactions in the mechanics is due to the electrical materials For example, physics from the neutron and magnetism, physics which is of stable why the proton course. physics segment of the course and of particle for example, physics to particle of the basic concepts starting as the concept the pieces to bind was designed to explain, the patterns of the many elements. with the example This physics at the smallest shown on the chart, particles level in explaining The process of nuclear P-decay should be included physics, to the very large number putting 1.2. USING T.HE CHART at the nuclear for teachers background on the information on who do not plan to incorporate . I ? 2. “the force of gravity” Overview strong The terminology student of particle and others that usage. This chapter physics contains have technical introduces many words meanings that differ some of the terminology face type is used to denote technical that are new to the from their of particle terms that are explained everyday physics. more fully weak. Bold- elsewhere in this book. 2.1. everyday to evolve a physics meaning one is not new. Consider is related to the everyday Standard Model the tendency “flavor” have technical meanings. a variety is carried that of properties, a step further. have nothing but those properties sounds silly. However, physics vocabulary name a new particle that there choose the name for something the naming system are also many or a new concept. tries The words to do with the idea that finishing entirely It is usually their quarks up with new words the privilege new. Once patterns to incorporate In the “color” and everyday come with in the usual something the pattern, how to of the discoverer to There a charge Model, associated cles are given on the chart. lepton flavors, sometimes just experiences with interaction. that later. Strong and are carried SU(3) Model contains experimental which an interaction force” evidence Electric if and only if it charges for all parti- are associated with charges are called color by quarks overall and by gluons. three parts, the composite denoted quark and charges (or Particles for which that their con- may be neutral. in the technical name as a x SU(2) x U(1). The first factor represents the strong interactions. is the number of different weak and electromagnetic interactions SU(2) x U( 1). The theory is called a unified factors cannot both factors but is not part of the effects of an interaction carry a charge, even though The Standard product: and types.) a particle do feel some residual are composites stituents colors) electromagnetic, interaction is as yet no well-established Weak charges, are explained strong, claims have been made of a need for a “fifth more than four interaction The three in SU(3) that in the particle here. There is no fixed rule about entirely the happens because it is very difficult without we will explain physics: are not color and flavor to redefine words probably a new word for a new concept in elementary from the one but is much more specialized. These names were chosen to convey sense. This tendency to invent for a word that is different Uforcen and “work” physics meaning meanings data. Model include type of fundamental (Occasionally certain requires carries The tendency Model. to explain by the Standard is the fourth In the Standard AND PHILOLOGY PHYSICS “a force due to the gravitational interaction” or “the 1 “a force due to the strong interaction.” The fundamental described Gravity Standard that meaning force” meaning interactions I be separated contain color charges for the quarks.* are described The by the second two factors, electroweak,theory because these two into one for weak and one for electromagnetic; rather, parts of each interaction. have been recognized, and some standard usage evolves. 2.2. FORCESAND INTERACTIONS Every force in nature interactions. electrical (in the sense of F = ma) is due to one of the fundamental For example, interactions the force of friction between the electrons the words “force” and “interaction” two surfaces and atoms in the surfaces. sometimes 14 between smears the distinction; is due to The usage of one speaks of * Again only for those who want more technical information: The three different colors of quark fields can be viewed as the three components of the fundamental representation of SU(3). The eight types of gluons correspond to the coefficients of the n2 - 1 = 32 - 1 = 8 traceless 3 x 3 hermitian matrices in SU(3). 15 . I i PARTICLE TYPES 2.3. anti-up quark opposite Spin - Bosons and Fermions. The first distinction major into two classes 1 fermions Enrico Fermi known as spin. momentum types and bosons. and ‘S. N. Bose. processes. is the separation of all particles carry an intrinsic to understand The quantum angular In the special case of bosons with and the antiparticle Classes = 6.58 x 10-25GeVs of angular momentum Leptons l one point mass rotating particles with possible unit of angular Leptons = 1.05 x 10-34Js. carry around half units another. of 6 as their a spin that is an odd number an integer number The major fermions that obey an atom. electrons the Pauli Exclusion cannot occupy momentum. in each level. is achieved of fermions of that some Any particle Any particle the same state with strictly the possible Bosons on the other hand are not governed the lowest available by this effect; m&y state. photons states The is to the electron limits is that principle at the same time. of this principle that and bosons The exclusion most levels in number of fermion, which corresponding electric). the particle, are simply electron (muon particle that exists in the Standard but The antiparticle Model, The antiparticle the opposite is usually The electron by noting is written or mu’minus) tau minus) a flavor sponding for its electric thus u denotes an up quark with electric (color, and mass to the flavor, and a bar over the name of teraction. This charge 2/3 and U denotes an charge. All matter example charge; thus, the positron. of a lepton. neutrinos of the of the ~1~ of the T- (tau or type there is a separate have no electric have the opposite has the anticolor, All color-charged object charge they do have sign for that the antiflavor flavor. particles and the opposite are confined mad& from three quarks and called baryons, and called mesons. quark sign by the strong in- only in those combinations These composite particles are called hadrons. due to their They the antiparticle The antiparticle and the antiparticle means they can be observed interactions is For every quark color and flavor there is a corre- and an antiquark from a quark Hadrons that are may be or bosonic, made All hadrons have residual constituents. Baryons Protons of three proton (n). For charged leptons the antiparticles For each neutrino Although which charge. fermionic, has an identical by writing antiquark either value for all charges denoted have color Quarks l is the most familiar is the p+ (mu plus), is the T+ (tau plus). In a laser, the coherence there also exists another (.!), and quarks, the positive e+ and called charge and the antineutrino’s strong is its antiparticle. written (e-) neutral. the same state. are leptons, that can be observed in isolation. color Antiparticles For every fermion is true for the photon have no color charge, which means they have no strong interactions. by this law and are in exactly are, the same object-this fermions which is the antineutrino. Principle. principle hence they tend to all occupy l angular the properties of the application It is the exclusion of the light intrinsic it has been found of half units of 6 is a fermion. between example Remarkably, in the usual situation of units of tL is a boson. difference two fermions familiar momentum zero value &r all charges, the and Quarks The fundamental is are particles It denotes the smallest equal mass but of Fermions made from these particles. h = h/2r Bosons also have antiparticles,,of momentum the conversation unit of angular charge -213. the Z boson. The names honor the famous physicists Particles Spin must be included in particle charge. particle in particle with and neutrons quarks are the most familiar have half integer has the quark content a down quark. consisting for example, the uud where u stands for an up quark and d stands for neutral 17 I All hadrons spin and are called baryons; These are color-charge 16 baryons. combinations made from one quark .n . I of each of the three possible ematics called arithmetic. the algebra qu’lk color charges. of S{/(3), it cannot For every baryon of the correspo’nding Classes This be explained made of three quarks, three antiquarks. rule is part of the math- Baryons 2’72 terms can have spin l/3, 3/Z,. the antiparticle of the r+ and the corresponding of ordinury there is an antibaryon Thus made .. the antiquark, is a v which I is rid. For qesops such as Q = TC, the particle made of a quark and the antiparticles same object. The following table summarizes the various particle types. of Bosom There Fermion are two classes of bosons. Fundamental , Boson q = quark g = gluon e = lepton y = photon 0 Force Carriers (as far as we now know) The fundamental photon is the quantum The W interactions. The bosons are the carriers quanta of the fundamental of the electromagnetic and 2 bosons of the strong field, play this interactions interactions. or the carrier The of electrical same role for weak interactions. are called gluons. All of these particles wz Composite (hadron qqq = baryon ) have spin 1. The quantum in contrast, difference of the electromagnetic the gluons between property that do carry strong strong Gluons holds the composite together. quarks In principle, and gluons. hadrons interactions are confined. Most hadrons important is responsible between among gluons for the are known Model types and antiglu- the eight where they provide in the Standard made only from gluons. This charge; There are eight possible there is some gluon exist only inside hadrons has zero electric color charges. There is no real distinction ons; for each of the eight gluons antiparticle. interaction and electromagnetic color charged particles of color charge for gluons. field, the photon, that is its the “glue” that to be composites of both there can also be some As pet there is no clear experimental evidence for such objects. l Mesons Hadrons are called consisting mesons. of a quark Mesons can have any integer The color charges of the quark state. The antiparticle and an antiquark and antiquark (for example, spin and thus they must be combined of a meson has the roles of quark 18 a+ which is ~2) are bosons. to a color-neutral and antiquark reversed. 19 qij = meson . I ‘8 I i 3. The Components of the Chart. system-the bution This chapter briefly explains all the tables and figures that appear on the chart. which, a certain for example, level are moving The chart Information is arranged about ored backgrounds with fermions the interactions is given in the central cloud in yellow. for those parts of the tables that for the figures depicting understand 3.2. electroweak the significance on the right. part of the chart. are used to emphasize related areas. For example, figure shows the area of the electron ground on the left and bosons Col- processes. Encourage interactions it is important your students to try to of the color coding on this chart. 3.3. were drawn then the electrons and quarks would be smaller particle figure charges physics now studies. Electron to the scale given by the nucleons, than 0.1 m m and the entire because that size is the present limit and quark of our ability there is no evidence of any size or structure for these particles. Students of an atom are familiar by electrons. neutrons. atom probably also knok that of this picture they are made of quarks. scales that as 5 lo-‘*n structure. as a nucleus surrounded the nucleus consists of protons is the structure From this starting within point So far and the neutrons the figure can be be stressed that not a picture. the problem with This this figure is a diagrammatic One cannot draw a sensible picture of scales, there is the problem 20 representation of the atom. that the atom is a quantum Apart of the from mechanical strong that assumes that of indirect colors into three sets called each generation contains opposite value for electric charge, flavor, of the five lightest masses of quarks pattern Model quite satisfactory. Model, this is strongly confirmed. All the hadrons and the pattern unexplained to seek further a single generation All stable matter by Each observed so Hadrons have yet to be found. (or lack of it ) of the by the Standard theories, 21 As far as the of quarks and leptons is made from particles Model. which must encom- but which can in some way go beyond it. is concerned, charges suggested flavors quarks and their antiquarks. is completely such as these lead physicists pass the Standard exists; and color. of the generations and leptons the electric that has the same mass and spin but the the top quark or its antiquark The repeating two leptons is in the masses, which evidence but not yet dire,ctly antiparticle far are composites do not. across the chart. has a corresponding Standard whereas leptons between the generations the top quark experimental into is that quarks have charges -1 and 0, and two quark flavors with fermion containing interactions, by background Notice The only difference chart a variety on the upper left of the chart is divided subdivided by physicists. the electric 2/3 and -l/3. Mysteries to the rest of the chart. It should atom, the description The new feature and protons; related They with small sizes are labeled to distinguish also applies to the nucleons and The basis of this separation and hence experience Each of the tables is further the idea of the extremely description fermions and quarks. increase as one moves to the right is used to introduce at every them. The table of fundamental two groups-leptons color would be about 10 k m across. This The constituents are aware of the electrons’ mobility; FERMION TABLES “generations” on the wallchart Students to stress that a similar to the quarks’ within and THE DIAGRAM OF STRUCTUREW ITHIN AN ATOM If the figure Such a system is always in motion. around each other. the top central This color is used as a back- relate to electroweak of such a system is in terms of a probability distri.!, gives the likelihood that an electron would be found at distance from the center of the atom if one were able to make an instanta- neous measurement. LAYOUT OF THE CHART 3.1. proper description would be in the first generation. .!, LEPTONS ! spin = l/2 (Antileptons) ? Electric charge Flavor Mass GeV/c2 Flavor -1 electron < 2 x 1o-8 neutrino neutrino e P 7 electron Electric 5.1 x 1o-4 Approx. Flavor UP q GeV/c2 spin = l/2 (Antiquarks Approx. Flavor charm < 3.5 x 1o-2 1.784, ij) Approx. mass GeV/c2 Flavor 1.5 top > 41 (not yet observed) d -l/3 down b S 7 x 1o-3 strange 0.15 bottom mass GeV/c2 t C 4 x 1o-3 tau 0.106 muon mass U 213 tau < 2.5 x 1O-4 neutrino QUARKS charge muon Mass GeV/c2 UT UP ue 0 Flavor Mass GeV/c2 4.7 The muon, discovered is said that of it!” Today in 1936, was the first the physicist I. I. Rahi By this he meant we still are trying more generations that asked “Who it was quite of t.he second generation. ordered unexpected to answer his question! exist? Are there further too heavy to have been produced further particle when questions and quarks in any experiment 1 x I to date? are simply The answers await are distinguished be isolated from quarks for observation. by the weak interactions electrically charged Neutrinos are leptons There that date is that that have zero electric neutrinos their is another and tau type. have zero mass. or even exactly conservation of leptons Each lepton is emitted by a lepton neutrino an electron and electron by emitting charged charge. flavor is the lightest particles into which from and named. and angular momentum with counting merely to find patterns is stable. to they “three characteristics particles (hadrons) such as mass, electric have been studied for each particle. have charge, Unsatisfied long lists, physicists tried in the information. Gell-Mann of quarks. quarks that is, protons (s). That could (“Quark” for Muster up (u) and down and George Zweig”’ They three flavors of quarks. interactions. measurements be composed Various (spin) interacting each new species and memorizing In 1964 Murray is obeyed in the decays of leptons. electron Lepton or absorbed. type, muon type, flavor When a transition does not change a charged W-boson between a charged Thus when a muon decays it becomes a virtual it been observed 200 strongly Mark” (d) quarks (uud) explain suggested all hadrons was a whimsical and neutrons (udd). then known might with only name taken by Gell-1\Iann from in James Joyce’s novel, are the constituents that. hadrons Finnegan’s of all common, The third quark \Vakc.) stable The matter was called strangcx zero. is emitted as illustrated can decay Hence they do not partici- All we know type is conserved. type. they can than the values shown in the chart; law which and an anti-electron-type flavor leptons The electron the electron the process always involves and its own neutrino charged has distinct.“flavor”-called a Z boson or a photon means that but only in weak (and gravitational) masses are not bigger could be much smaller Each generation are no lighter Since the 1930s approximately they do not have color This are unstable. charge is conserved, pate in strong or electromagnetic It is possible interactions. for the electron, and therefore particle. by the fact that strong Except could decay. Since electric There Quarks l charge and thus do not experience type .“I research. Leptons lepton After are: How many which 0 Leptons when 1 Before he heard and seemed superfluous. Related leptons that?” It W- boson which neutrino, rapidly thus conserving a muon decays to produce both muon flavor name was already or an S-quark, because when K-mesons seemed a “strange” was discovered Centernl or unexpected called in 197715’ A sixth containing Quarks upsilon quark, this quark National (T), with discovered, A fourth Laboratory!‘] “flavor” at Fermi top (t), has been predicted 25 Linear charm (c). Acrclcrato~ quark, National (as of January an .s- long lifetimes of quark, by the theories, color charge and hence, like gluons, contain their The bottom was first observed have not yet been observed have non-zero which in 1974 at the Stanford in the table below. 24 the K-mesons, were first property. in the rSr or J particle and at Brookhaven combination associated in a /,x Laborator! but pa.rticles 1989). they are confined .h . 1 I BO’SONS force carriers spin = 0, 1, 2,... Unified W- w+ z” 0 -1 +I 0 Mass GeV/c2 0 81 81 92 Strong or Color g spin = 1 gluon Electroweak spin = 1 Electric charge : 7 photon ‘. Electric charge Mass GeV/c2 0 0 : . objects. Each quark flavor type comes with any of three possible color charges. word “flavor” there is rrsed somewhat is one flavor separate flavor flavor for each generation; so ‘that the three is never altered wea.k (Z boson) in strong processes. charge of the final quark W-boson, e.g. a change between quarks that The on the table, occur between that any $213 quark replaced by an up or a down quark a Quark emits -l/3 processes involve would charged already Fermion to estimate estimate of heavy quark we can only give a lower limit If it were lighter have been produced than this limit, well measured. When particle mean the rest mass of the object neutrinos, neutrino figures particles, This means that model neutrinos have exactly transitions The tables on the upper the Standard concept quarks. is meant Unified mass they it always are not indicated.) upper limit that on the mass of each the neutrinos than For the the stated are all zero mass limit. The Stan- There are some extensions Theories. of the In some of these theories zero mass, while in others they have very small masses. We type of theory the columns be isolated, it is very difficult by quark is correct. are labeled mass. since most of the mass of protons but rather uncertainties on this matter. as Grand known these masses are experimen- use the word have any mass smaller do not yet know which For quarks, top quarks involves (All masses are here given to only a it is possible makes no prediction standard what physicist in question. and experimental or they could dard hlodel cannot leptons, all that can be given is an experimental type. containing in experiments. Model. to convey. right These of the chart are labeled carriers.” This is the quantum Each of these particles just as the photon shows the fundamental “force is the quantum bosons of is an important of the corresponding of the electromagnetic The masses of the W and Z bosons are determined few significant on its mass since it has not particles field. Masses The tables show masses for the charged tally masses. BOSON TABLES 3.4. interaction, l particle with that quark .!, , the mass differences between the a is, those in the same generation. and any -l/3 a heavy quark and a similar This gives an accurate For the top quark, quark transition quarks. containing yet been observed. the electric which to a charge given above this weak For Ieptons, or in the neutral charged, of the initial predominant a particle mass is called interactions makes a transition between of six flavors. are electrically By the definition flavor. each distinct give a total or electromagnetic 213 quark shown paired Rarer transitions for quarks generations will be different a charged of quark for quarks than for leptons. Since W-bosons a W + boson. by emitting differently The I This zero mass is a consequence to the exact conservation be very tiny, less than of electric to determine is especially mass.” Because well-defined quarks to define. concept; within The number it keeps changing the hadrons. exact symmetry, The SU(3) and formally way that exact electric Because of confinement, used by physicists, this requires that charge conservation it is difficult this mass is known to However, is not even a absorbed by the interactions is an the gluon mass is zero in the same that the photon this formal since this formal definition definition we show the gluon mass as zero on the chart. 29 and hence their in a hadron of the strong requires to relate masses I and is related be isolated, of gluons s y mmetry We can use the mass difference 28 Experimentally as gluons are emitted’and generation does not come from quark For the photon of the theory they cannot its mass or even to define fully true for the lightest confinement. a quark charge. The gluons are like the quarks in that masses are difficult experimentally. of a symmetry 10-24GeV/c2. mass to any mass measurement. “approximate and neutrons from the strong interaction an exactly mass is zero. of a gluon is the one - S a m p le B o s o n i c H a d r o n s I ;- - I Mesons a~ . 1 S y m b o l 1 N a m e 1 Q u a rk 1 E lectric / c o n te n t 7r+ I pion charge UZ G e V /c2 0 .1 4 0 +1 SE ( M a s s 1 S p in 1 0 I 0 .4 9I4 0 I I< - 1 k a o n I1 -1 P+ rho U2 +1 0 .7 7 0 1 D+ D+ ca +1 1 .8 6 9 0 - ‘lc e ta-c CT S a m p le Fermionic B a ryons S ymbol Name p r o to n P I F I a n tip r o to n n e u tro n I n I A I lambda I __ _f qqq I 2 .9 8 0 0 Hadrons a n d A n tib a r y o n s --q qq Q u a rk E lectric Mass c a n te n t charge G e V Jc2 uud 1 0 .9 3 8 E iz -1 0 .9 3 8 udd 0 0 .9 4 0 uds 0 I i.2 I omega sss 0 1 .6 7 2 S p in 112 112 112 112 312 c -- 3 3.5. HADRON TABLES The two hadrons tables labeled are just that, cles. Any combination neutral. quark sample of three quark By the rules of SU(3), with one can make a color-neutral an antiquark; the quark and antiquark The spin of a hadron one another. Different complete Particle listing them of quarks mesons are bosons. Any combination by taking one together. The angular momentum a spin of of flavors for the same quark example, the 7r and p mesons shown and their it contains of the quarks’ motions with particles is to combine meson. hadrons of all observed are color Since the total is made up from the spins of the quarks from the orbital -for combination are called mesons. makes a possible a contribution spin are possible Baryons parti- of flavors. such hadrons is an integer, bosonic observed object color charges and putting can have any combination the combination and sample makes a baryon. flavors The second way to form a color-neutral quark hadrons a small sample of the many experimentally of each of the three possible three quarks fermionic content properties but with on the table. and around different For a see the “Review of Properties”!’ 32 1 II, 33 b I \t i . I ‘II PROPERTIES OF THE FUNDAMENTAL (interactions = forces) Electroweak Property Gravitational Interaction Weak Interaction INTERACTIONS Strong Interaction Electromagnetic Interaction Fundamental / ’ Interaction Residual Acts on: Mass-Energy Flavor Electric Charge Color Charge See note Particles Experiencing Interaction Particles with Mass Leptons Quarks (hence Hadrons) Electrically Charged Particles Particles with “Color Charge” (Quarks, Gluons) “Color Neutral” Hadrons (residual force) Particles Mediating Interaction Graviton (not yet observed) Gluons Mesons 25 Not applicable to quarks Strength Relative to Electromagnetic for two u quarks at a separation of 10-41 w+, w-, 0.8 ‘To 1 ‘. r x 10-'sm for two u quarks at a separation of 10-41 1o-4 1 60 Not applicable to quarks 10-36 1o-7 1 Not applicable to hadrons 20 r x 3 x 10-17m for two protons in a nucleus . I . PROPERTIESOF THE INTERACTIONS 3.6. that reflects fraction The first two rows of this table are self-explanatory. summary of the rdlative strengths The first lesson to be drawn of the several interactions from this is that of the int,eractions, since they even extreme conditions the effect of gravity strong interactions, the examples although presented A second point quarks at distance the table vary with shows that rows are a in various situations. there is no absolute the strengths in which The remaining the situation. is as strong this is clearly There as that to emphasize is the hypothetical cannot be isolated nature of the situation and pinned down. as they mo.ve around inside the hadrons. chart the smaller 10-18m , is chosen to illustrate the weak interaction the fact that is comparable of the weak interaction “two On the when they action for the strong is probably with d, as physicists d particle This W or 2 meson. is a consequence Electromagnetic of the fact that the photon Thus, as shown on the chart, when the quarks are at a separation only about already a tenth of their typical much weaker relative It becomes clear that one needs to know all practical the quarks purposes separation to the elettromagnetic to understand the probability that one can say that are at distances this effect. in a proton), of order already includes When nucleus, we must take a weighted interactions charged nuclei gluons and quarks, the exchange for the formation mean the fundamental force. (still the weak interaction is interaction. the rate of weak processes in a hadron, the quarks certain 10-‘sm are very weak processes happen or less. average of interaction for only when The last row of the table we discuss the interactions 36 close, because of two protons strengths with in a a weighting 37 charge. Atoms interaction is re- an electrical or the exchange interaction that of binds as due to exchanges between the nucleons. For takes place in the form of a incorporates the older view that of the nucleus. When interaction. It is important ‘. distinction. inter- case. We say It is clearly force, they mean the residual is massless. of 3 x 10-“m What strong of This distinction electric can be viewed view of this interaction interaction electrical constituents. the residual atomic of the process, is responsible of the as being due to the sharing Similarly, constituents, is the separation hadrons. of atoms to form compounds? refer to the strong physicists with to form the modern color-neutral because of their it is described range part Thus, between interactions objects is, the labeled resi,,dual strong to the familiar the atoms. and neutrons meson exchange v oc -,-m-W fall off as (-l/d). between the longer distance, meson. where m is the mass of the exchanged neutral .Fucleus, that ‘/ one for the fundamental by the analogy have electrical In chemistry, protons interaction for the binding electrons into two parts, best explained particles sponsible in a typical will have that separation. have net color charge, and another, of the color-charged to the electromagnetic decreases exponentially column which effect. The distance varying get very close together interaction objects that of a given separation the protons aspect of this table that needs to be explained are electrically . . .“. Quarks The strength of the not the case in between them is constantly one. are here. distance, Another strong way to compare the probability of time that nuclear However, to remember the I . t&l FRAME 1 Neulron Bets-Decay Flgure 1 ‘4,. it“:; L.’ c FRAME n -+ pe-iTe - 5 a- Time lapse pictures showing the sequence of events in neutron P-decay. ; 3.7. FIGURES These diagrams are an artist’s ezact and have no meaningful gluons or the gluon field, conception scale. of physical Green-shaded processes. areas represent red lines the quark paths, They are not the cloud of and black lines the paths of leptons. Neutron Decay The first figure on the chard represents cess, the decay of a neutron In the Standard quark Model and a virtual the electron-type The figure this decay occurs W-. The W-boson on the chart changes flavor unobservable once begun, pro- antineutrino: of a d quark creating when to represent The W-boson the leptons the entire to a u the electron and entire appears The process. It as the quark “time-lapse” The time of these frames is sequence of events occurs in a time of less that to actually observe that appear in the intermediate in this example, this are produced. this sequence of events. it is impossible particles as the W-boson then decays, is an attempt and disappears 1O-26 s! Thus and an electron by the transition of a sequence of events. shown here represent very short, an electron, weak interaction antineutrino. shows the history pictures to a proton, the most familiar are called “virtual” 40 i 1, the intermediate stages. The stage of such a process, such particles. 41 - e FRAME 1 ($-- FRAME 3 FRAME 4 FRAME 5 - _._--- -. __..~ _-.-- ~-. _--. FRAME 7 FRAME S __ -.----. - I -_.. - __ _. - .- .- ~--- ._ --..- .- c -- e+e- -+D+D- - Time lapse sequence for electron-positron resulting in production of D mesons. annihilation -On .. Electron Positron Annihilation The second figure on the chart beam experiment. Center then stored around opposite Sometimes or a virtual photon the energy of the original interaction them,which some additional combine an electron to form subsequently carry all moving apart. creates a region of force field The energy now in the force field creates and antiquark color-neutral they are produced points fast in the Because these particles color-charges down. particle and a bunch will annihilate The virtual antiquark. Thus going equally and a positron and positron, their slows them quark to form electron between At various to cross one another. Z-boson. Accelerator to high energy ring.” of positrons in a colliding- Linear are accelerated in a “storage are steered a quark and its corresponding The strong between bunches going one way meets a bunch a virtual produces and positrons t’he bunches direction. happens such as those at the Stanford electrons in counter-rotating the ring of electrons either In facilities in California shows a process that pairs. hadrons. The various quarks These are observed and antiquarks to emerge from the collision. In the diagram a particularly case is shown, where only one additional and axitiquark “time lapse” sequence of pictures actual process occurs on a very short time scale. In relatively where there a significant are created, simple quark is not sufficient fraction emerge, including here shows the process step-by-step. energy of events. sometimes and hence only a pair of mesons emerges. to make many In most higher baryons energy aqd antibaryons 44 j 1, mesons, Again low-energy this collisions will The the collisions happen many in particles as well as mesons. 45 I kFRAME 2 _ -_.. FRAME 3 ---- FRAME -~- 7 c 77,--f x+K”li’- - Time lapse sequence for the quarks in an Q meson to annihilate resulting in production of a pion and two I< mesons. . I ‘I I I Decay of 71~ 4. Accelerators In the third figure on the chart we see a possible The qC contains virtual gluons a charm quark annihilated a single gluon to produce because that and also by conservation gluons for separating quarks. Quark and combine that about 4% of such decays.6 with could The “time-lapse” This picture process The outgoing of a detector; on the basis of the Standard Model the next understanding as the color- In particle meters. in the strong field that final therefore is known possible we will experiments At such extremely No ordinary the probe and their via their interactions By observing is contained from such collisions, and interactions. that distances is sensitive as small nature It is amazing that Model. as lo-‘s of particles to these distances. of this process, but this is, in fact, the primary It is particle we learn about physicists In led to the as a probe, and shoot it into another interactions. study particles. in ‘the Standard it is the quantum object the outcome get information particles that distances, necessary to use one particle target, is used to study are done to probe small as a target. the intermediate world (human-sized) acting that some of the key experiments of the microscopic physics, reigns. the equipment describe here shows this process in a that to occur chapter we describe in mesons are observed the explanation momentum In this chapter case Here we show one of many an r]= decays. produce in the previous pairs are produced Here also the process occurs so rapidly be observed. components evolution described 4.1. INTRODUCTIONTO EXPERIMENTAL HIGH ENERGY PHYSICS in the previous The subsequent to that hadron. to produce They cannot of angular is similar hadrons. when and positron or Z-boson. by conservation and antiquark to form occur fashion. photon is forbidden begin to separate states stages cannot a virtual of color-charge. region step-by-step These can annihilate in much the same way as the electron example charged and its antiquark. decay of an unstable and Detectom, the are able to way that they of the process given here is made theory. Fig. 48 4-1. - A projectile strikes a target, is deflected, 49 and is observed in a detector. . Fixed Target b Experiments Figure 4-1 is a schematic particles, diagram of what one type of experiment the detector one can study that is the rate per incident through which the projectile reflects the details interaction with particles A setup of this kind is called a “fixed-target” as probes looks like. By moving final states which particle and the target. demonstrate convincingly that “hard” - the quark substructure was revealed, of the angle of this rate with the angle and the nature Using sufficiently physicists scattering of the scattered high energy at Stanford centers existed of the were able to inside the proton in much the same way that Rutherford comes. As an example, tal setup. Different can be gained by changing the parameters projectiles - electrons , pions, kaons, protons, ~ can be (and have been) used. Each projectile tum numbers, reflected eter in the experimental the projectile by the scattering. setup is,the projectile the smaller the size of objects muons, neutrinos possesses different many of which are conserved in the scattering in the final state produced of the experimen- intrinsic The virtual photon will materialize its antiparticle). The photon possible outgoing particles and its corresponding as long as twice virtual that antiquark. Beam There Another important through striking and antineutrino the energy of are very difficult these outcomes each other, allowing a “colliding-beam” positrons antiprotons DESY colliders) (pp colliders). laboratory proposed experiment. (called e+e- Colliding-beam and for protons An electron-proton in Hamburg, to be built experiments. some of the particles Germany. facilities and colliding-beam 50 of a single This is called exist for electrons on protons on (pp colliders) and on is being constructed at the The SuperCollider in Texas, will be a proton-proton For both fixed-target Instead beams are made to pass to collide head-on. collider (SSC) accelerator, collider. experiments the detector Model predicts when the experiment The or a quark flavors are possible collision to detect because the neutrinos passage. The Standard and energy. For the any neu- Such events (in which only neutrinos without the relative is repeated Since the electron directions with and positron interact so weakly leaving any evidence probability of each of many times. beam collide head-on (i.e. moving the same momentum), they have net momentum energy equal to twice the beam energy (2Eb,,, can be resolved in the target. two high-energy (that is as a particle 8 with electric charge. All quark and charged-lepton pair can be formed. and a Z-boson. and antilepton mass is less than the total they most often will pass through to-back), a fixed target, lepton or a virtual Z-boson all the states described above are possible and in addition of their param- Experiments is a second way to do high-energy beam of particles of an electron photon as a pair of particles are thus a charged the particle collision a virtual couples to all particles will travel out from the collision Colliding the high-energy to produce analysis of the various out- and hence are directly energy - the higher that quan- consider These can annihilate are produced) Much information To learn about W e call each individual particle .!’ a given process, we usually need to collect from many events and make a statistical positron. trino the atom. may result from any collision. an “event”. information experiment. as a function of the target and protons as the target, found the nucleus within dependence The variation structure the projectile electrons as projectiles projectile is scattered. of the internal between the angular collision I there are many possible p+p- each with point an energy ,!&,,,,. we observe them directly. 5 charged particles As explained increasingly in Chapter for example for example typically each materialize (neutrinos 6, when a quark-antiquark as they fly further Their particles energy materializes With (back- are e+e- or they decay rapidly 1, 3, or and sometimes pair are produced apart. therefore momentum particles are produced, particles the second figure on the chart.) available the produced of zero and total The possible decays of a T are into plus neutral strong attraction from the color force-field. When If T+ and r- and we observe the decay products. (rarely) ). The produced with equal but opposite in opposite A’S). they feel an They are unable to escape in the form of hadrons. a collision (See energy of 30 GeV, at the PEP storage ring at SLAG’, the quark and antiquark into 6 - 12 hadrons. 51 If the initiating quarks have high . 1 momentum, these particles ing in roughly v + ..__._..-.. l __._.______ * v Altogether Invisible! there the different e+e-+p+p- Among enew=E tracks beam possible of outgoing (back-to-back); final particles the arrows represent the possible outcomes r+ and r- the original states. E-E, NYS 1 quark and antiquark Almost back-to-back charged prongs energy < Ebeam (and possibly some photons) ‘(’ ‘) 4-2 is a depiction seen in a detector of the’final state particles. at PEP are events with two charged and each decay includes or p+p-. In the case when the only one charged particle, we see Also possible are events four charged tracks which can result from other decays of the r+ and T-. some cases the charged produce photons. particles (rr” + 7+7). of collisions. which can occur in the e+ethe directions these can result from e+e- are produced Fig. two charged tracks but they are not precisely back-to-back. A- with (74%) are many patterns In these diagrams, Back-to-back leptons with e+e-- centered about mov- directions. e+e - --c VV (only via the weak interaction) e+e-+e+e-or that is in groups of particles will appear in “jets”, the same direction, I are accompanied by A’S which in turn In decay to Events that result from the q7j production typically have many charged tracks and many photons. (24%) by<; (7s ) ;;;s~;;~;‘$$“&$“d We can now describe a typical to record e+e ---c qq --) Hadrons “snapshots” according to the different derstood Jet -Jet Multi-Jet average particles Events with on 12 charged and 12 photons. 6202A3 of millions scattering of collisions. patterns pattern, ple is the discovery they may point of the tau (r) the way to “new” at the SPEAR and a muon, served. only decays T+ + outcome e+u,P, could and T- result -+ urn?,,. and hence one detects only an e+ and Fig. positron 4-2. - This collisions the directions figure shows pictorially typical event patterns in electron- at a center of mass energy of about 30 GeV. The arrows represent of particles produced at the collision point. events) were unexpected. explain that them, it implied. from that collisions opposite T+T- The neutrinos a p-. The e+p- had to hypothesize this hypothesis This is how progress in particle We see therefore vide high-energy 52 Physicists and then confirm with to discover nature’s and detectors storage electric production A classic examring at SLAC. charge, was obfollowed by the are of course not detected, events ( and similar a new particle, by looking e-p+ the tau, to for other consequences physics is made. secrets requires to map the debris 53 must be un- In the case of an physics. e+e- is employed are then sorted These patterns governs the scattering. The final state of an electron This A detector Those snapshots of the particles. in terms of the physics which unexpected experiment. accelerators resulting from to prothose . collisions. d We will now separately look into prototypical accelerators and detectors. fields are used to accelerate the direction in which charged particles I and magnetic fields are used to steer they travel. 4.2. ACCELERATORS When Introduction to Accelerators Very high energy of particles small as matter distances, object that energies. probed. projectiles causes the particle are needed to probe small distances. waves, we know that we need small because a wave is essentially is smaller than its wavelength. undisturbed accelerator If we think wavelengths when Small wavelengths The greater the energy of the projectile, It is thus important a charged to study it passes by an correspond to high compensate become particle to lose energy. for this effect. relativistic. design. it radiates Accelerators Two The correct to minimize particles for momentum or as the particles when it, has a velocity of relativistic formula and this radiation loss grows rapidly “relativistic” properties photons, need to be designed The rate of energy We call a particle to the speed of light. accelerator is accelerated are important close for is the smaller the detail that can be to be able to increase the energy of the projectile. is a device that is used to increase the energy of a collision. An Several types of accelerators will be briefly discussed here. (At low velocity The earliest boiled and simplest off the hot cathode field then accelerators in the region accelerates the electrons. massive ions in linear accelerators circular accelerators important were cathode of a strong Early electric field. in the 1920s a method was developed. were invented. ray tubes; Late electrons The electric to acceleraten in the decade, These two types of accelerators are the first are the most for research today. Certain points an electric is the same. is 1 and this is the familiar when the speed is close to c you will see that change in v gives a large percentage celeration” means that the particles their speed is not significantly when they are accelerated change in p. gain momentum, altered. A charged review these points. particle experiences accelerators and in The basic principle a force parallel to field: circle, chrotron radiation problem FE = qE particle and hence also energy, “acbut by the particles as ‘u -+ c. If they are made to travel due to this radiation. towards This% the center of the referred to as syn- One has to make the radius of the circle bigger as the particles become more relativistic, the accelerator. If you examine a very small percentage The energy that is radiated also grows rapidly and hence losing energy form.) For a relativistic in a circle they are, of course, always being accelerated in designing of basic physics are important choosing which type to use. We will briefly of all accelerators this formula the denominator or they will lose energy faster than they can gain it from At the energies achievable is only significant for electrons. with present accelerator A linear accelerator techniques this avoids this problem altogether. A charged particle ular to both moving its velocity through a magnetic field experiences a force perpendic- and the B-field: Fe = Cockcroft-Walton qv x Two British B . the first ( This effect is also used in detectors to measure particle 54 momenta). Thus electric Walton Drift-Tube researchers, successful accelerator, linear Accelerator J. D. Cockcroft accelerator a supply and E.T.S. of the “drift of protons is obtained 55 Walton, tube” type. by ionizing designed and built In the Cockcrofthydrogen. The protons trode, are repelled by the positive . I ‘I ilectrode and ‘attracted by the negative elec- as shown in Fig. 4-3. particles (linear are accelerated accelerators). on voltage in a straight Cockcroft-Walton Cockcroft-Walton machine the tubes is alternating are accelerated the drift To avoid collisions between the protons beam, the whole accelerator objects between terminal (the negative spark formation by breaking keep the protons on a straight path. air remaining the beam particles and destroy used. Even without this problem, Cockcroft-Walton energy achieved de Graaff) accelerator with accelerators as “preaccelerators” protons modern which between the tubes The Cockcroft-\Valton Thus potential difference that of the positive reduce the risk of into stages. The AC power for the accelerator buffer. If the voltage delivered They to each tube is in the tube will ionize and then it will scatter the beam. This the maximum accelerators. “injected” also is rectified limits proton However, used today physics accelerator produces and a beam of It is easier to design such a beam. is a precursor 56 experiments of many accelerators particle when they so that the particles are in the cavities from the electric the time that the electric (spaces) field, because field is in the wrong to allow the tubes to be short enough which means about a meter or so in length. during (Clearly the time of one cycle for a fixed tubes changes with region when the voltage is designed leaves the drift so that tube at low energy. time (like a sine-function). is highest the voltage is still and enters the cavity get a maximum increasing region, The Particles push. as the average If one particle is too slow, it will be late coming from the tube and will get a slightly is too fast, it will leave too soon and get a smaller push particle If another particle As a result, the slow particle will be accelerated will continue to be bunched The practical tubes limitations means that will be accelerated less than the particles together in this way, it has a characteristic tend to stay in phase with the drift in which are field in the region between into the linac in pulses or “bunches”, the drift The accelerator than average. (and Van are injected are in the cavity from a The preaccelerator with which can be of the the particles In a linac based on the The phase is designed speed up they travel further between that the voltage into the larger accelerator. if one can start Particles voltage energies obtainable Cockcroft-Walton for nuclear as the particles the limitation frequency.) average. is only an MeV or so, which is a tiny fraction pre still a high energy accelerator tubes ( the cylindrical are at potentials for larger machines. is then The drift terminal). the total by use of a capacitive too high, the residual and air in the tube, which would deplete the the electric must be adjusted to construct, linacs, in a linac. they are shielded tube, during from small voltage. particles direction Then The frequency to be reasonable must be evacuated. the two electrodes) and ground and smoothed accelerator. in direction. tubes. they are inside the drift Fig. 4-3 The Cockcroft-Walton In more modern this means that in the desired suffered linacs are called .!’ in many stages, each of relatively One uses AC power to accelerate direction. Such accelerators accelerators at each stage of acceleration. accelerated between line. I When known as phase stability, the electric more and the fast in the bunch. automatically. bigger push than Thus the particles an accelerator is set up because the bunches field. on the maximum Cockcroft-Walton voltage, frequency, accelerators and length for can achieve about 50 MeV energies, but for higher energies other designs are needed. 57 . Electron Linacs Because of their small mass, electrons celeration from rest through energy by a factor energy in a circular accelerator After a potential of two. electrons that are accelerated they travel and energy. is simply The constant replaced substantial is very large. electrons constant to high energy. by a series of cavities separated pointing drift tubes of the Cockcroft-Walton cavity timed the accelerator in any cavity so the machine just as there machine. Bunches of electrons a little the force on the electron has the phase stability World power. It was developed War II. The klystron erators to supply power to them. above was developed Panofsky. The electron now produces erators treatment at Stanford accelerator radar, led by William at Stanford electric Linear of 50 GeV. the travel down the in the direction of microwave to linear accel- accelerator described Hansen and Wolfgang Accelerator Lower of the klystron intense and was adapted The design for an electron University, energy Center electron (SLAC) electrons with an energy have been built with this same design and are used in radiation-therapy accelFig. 4-4 centers. Schematic 58 i tt I e- I I e+ b above. is a device for generating to power wartime + Collider electron5 pmlt,““, before the field in that pointing described Linear down it at was in the spaces between Linacs have been built on a much larger scale since the invention during Stantord h ---- Electromagnetic and travel there is an alternating so that they always enter a cavity reaches its peak (with travel), few still gaining only by discs. The discs are fed into this structure This means that field accelerator, In electron speed allows all the sections to be identical. have a small hole in the center to allow the beam to pass through. along the linear in the first speed even though of cavities and drift tube regions in the Cockcroft-Walton fields, in the form of microwaves, the speed of light. Actotal amounts Thus, to close to the speed of light at effectively The sequence of accelerating at low energies. 500 kV increases their radiate unless the radius was the first device to accelerate momentum machine become relativistic of only about Relativistic accelerator linacs the electrons feet. I diagram of the SLC. (Drawing 59 .I by Walter Zawojski.) . 1 \/ I :J Electron-Positron Colliders An accelerator They simply direction. ring”. field B, the particle designed for electrons works equally have to be in the cavity Electrons and positrons The positrons travel when steered by the same magnets the electric accelerated the opposite particles at subsequent ring which is why accelerating way around as the electrons. it is called At SLAC a maximum facility do not interact crossings. the circle to the electrons, At several points around particles make ring. energy to compensate there rings (SPEAR electrons and positrons particles that but is not a storage did not interact. At collides to study reach the collision interesting positrons toward The particles region as is shown in the diagram test for the feasibility of building times. in Fig. 4-5 a) regions of strong with would spiral around He called this device a cyclotron, magnetic a region of electric field and pass through it is shown schematically 6 aD I at lmbmlJIR A third tiny collider bunches of ring because it does not recover the SLC the beam bunches physics. two D-shaped them back-to-back and P E P which must be only across when they collide in order to achieve a high enough collisions the gap many of the operates if he created them then the path of the particles are some short which that and arranged for that lost via synchrotron L inear Collider) SLC (Stanford between realized “dees”, interact. of circuits at 14.5 GeV per beam and has a radius of 350 meters). called the ring field, called the ring and may collide millions In the ring there are two large storage around some of the particles continue a storage is in the opposite of 3-5 GeV per beam and has a radius of 32 meters at SLAC, microns Most sections to provide radiation. operates which beam field Lawrence positrons. in the linac can be fed into a “storage the two beams are made to cross each other so that Those well to accelerate can be kept inside the region of the field as,it is accelerated. / two linacs which travel would rate of particle around of Fig. a few large arcs to 4-4. The design is a accelerate electrons each other to achieve very high energy center-of-mass b) and collisions. Cyclotrons At about linear accelerator, working netic the same time that on a different field direction. E. 0. Lawrence particle particles. on a charged moving its path bent into an arc of a circle. and Walton at the University way to accelerate can do no work A charged Cockcroft particle, at right of California He realized the first at Berkeley that, was while a mag- it can be used to change angles to a magnetic For a sufficiently 60 were developing its Fig. 4-5 The dees of the cyclotron b) Perspective view. developed There is a magnetic The path of a non-relativistic field perpendicular particle a radius field will have large region of strong magnetic by E. 0. Lawrence. R=z. 61 of velocity a) Top view. to the top of the dees. v, mass m, and charge q has . I As the particle despite their complete region speeds up the radius different come through the particle electric here, speeds, particles a semi-circle. between traverses the entire both for electrons As with must are all designed in the occurs each time with which the other accelerators be a vacuum. Cyclotrons (and other positive have been However so that the particles the required tube. magnetic travel require Modern circular in a circle of fixed Electromagnets lose energy from the beam by the radiation that keeps the beam moving It provides make silicon science research. microchips The evacuated by the proton is not practical. physics today. magnetic field accelerator The beam travels in a fixed of the electromagnets. quency (RF) cavities are inserted surrounding energy using cavities at regular the beam. intervals The machine too soon gets a smaller a greater push. RF frequency particles be controlled into the synchrotron. The proton able to stay in phase with steered the around in bunches. are already A particle synchrotron the protons must is greater large radii of curvature. than a few meters, using many small in a synchronized fashion, one large magnet magnets. which The magnets explains the name The RF tube coming just as it to the cavity Fig. 4-6 Various too late gets Many different can operate at a fixed when they are injected vary the RF frequency as they speed up. 62 through one coming electrons relativistic lead to designs with because the RF cavity traveling the average particle; circuits. .-... .-..w.. ...-... . . .... . .....m . ......... _. _.. ..........s..........._. .......................... ‘1 by radiofre- the circle into the evacuated the particles tool to w:.:.:,:.:,: :*:.‘a’ :,:, :::: s::: :::: M ,.*.,::$ ::;;::” JI’,’ by the variable linac. as a possible this around are accelerated synchrotron to work purposes synchrotron. used in high energy to those in an electron used to accelerate because the electrons circle is called a synchrotron travel push than A synchrotron radius The particles similar feeding energy into the beam synchronizes does in the linac-the most frequently radiation. the beam is called the beam line. The high energies The beam line is built Synchrotron is a circular to as synchrotron It has also been suggested very small integrated area around circle. The synchrotron with By the time the radius of curvature must This is referred a source of very high energy x-rays that can be used for medical and for materials attainable in a circle. due to the acceleration radius. placed around field to keep the beam moving Synchrotrons the studied ions). field and high vacuum. This then requires only a single evacuated tube then provide field to the particles to achieve very high energies because it would large areas of high magnetic accelerators by the time Thus acceleration and for protons the design is not well suited impossibly region take the same time the dees. The frequency is 2rm/qB. acceleration This means that, by the electric polarity direction. the region between be switched energies are accelerated switches again in the opposite the velocity. of various The particles the two dees, which field must designed increases with I to be sextupole magnets, used for a different to make the circular in a focussed magnet configurations. types magnets and bending purpose. orbits. beam bunch. a) dipole. are used magnets The bending Other b) quadrapole. dipole magnets, (see Figure 4-6). magnet magnets quadrupole magnets, Each type of magnet is bends the beam in one direction, are used to keep the particles If one looks at the effect on directions 63 c) sextupole. moving perpendicular to the beam line, a dipole magnet of the beam line toward the beam line along will bend particles repeated light the perpendicular the trajectories geometrical Magnets of particles in magnet design energy and sextupole The magnets The particle magnets are placed in a beams are bent by magnets act as lens elements. Accelerator in the beams using (essentially) technology The proton have led to ever higher synchrotron by use of superconducting superconducting as the equations the resistance of the coil. electromagnet coils are limited coil, and hence the magnet coil. Superconducting technology. is that to create mass. However of to study in a fixed-target beam plus the target outgoing particles to produce energy. Also the current, resistance So far magnets that become even wires out of these materials. CM frame. but they have allowed a laboratory are only built (SSC) will use superconducting magnets with Recently at higher, how to fabricate The proposed a fixed-target to guide the beam around can be converted momentum by conservation of the incoming of momentum, and hence they two beams of particles directions momentum particles into the must have is not available equal and opposite momenta. with of the two colliding is available are made to cross one partides for the production machine are described This is zero and hence all of interesting objects.* as being in the Center frame. of the total collision energy in a CM collision is most appropriately For very high momentum, beam of momentum with the energy available done by viewing this is a calculation p it yields the result both collisions in special relativity. in in the For that the new though large magnets Superconducting the total is the energy particles. in a colliding-beam of Mass oi CM the superconducting No-one yet knows The collisions Comparison of the is passed through set of problems, reliably. of conventional which requires very low temperatures. have been discovered temperatures. of a causes heating breaks down if too much current older type of superconductor, The advantage and hence the field by the fact that to be operated its losing energy due to in opposite in both that off a lot of momemtum experiments was able to double fields at experiment new and interesting traveling is the energy quantity The energy which goes into this kinetic’energy means that the total higher carry I the relevant is large. This means that, must another, the energy new particles new particles--that in one can obtain since one is not continually coils have a different higher field magnets sub-zero, available large kinetic physicists energies attainable at Fermilab magnet coil for an electromagnet lower cost in power consumption still Quadrupole directions. In an experiment In colliding-beam synchrotrons. materials from the center optics! Advances proton axis. in both sequence along the beam line. rays are bent by glass. study at some distance the beam line along one axis, and bend them away from can make the beam focus better . 1 \/ b or which can be compared with Supercollider E = 2pc its ring, which will be some 80 km in diameter. for collision Collision In a “fixed-target” target is better Kinematics experiment the beam is accelerated fixed in place. The total energy available in the incoming laboratory beam and the mass-energy frame and then in such an experiment of the target. Physicists aimed is the energy use the term to describe the frame of reference in which the target 64 at a is at rest. hitting of two beams of momentum to have two beams of particles a fixed target p. Thus, hitting if the aim is the maximum since by assumption one another possible p >> mc, it than to have particles total energy in the CM * This is precisely true if one of the beams is made from the anti-particles of the particles in the other beam. Even when both beams are protons and hence the colliding particles cannot annihilate it is almost true because the mass-energy of t.he protons is such a small fraction of the beam energy. 65 - . I frame. For the most energetic energies few experiments The fixed target machines. particles intense, small. colliding beams arc used. At very high using fixed-target measurement can occur. of collisions In colliding over the colliding the number of collisions Thus for experiments each time between empty particles in the beam fixed-target each other of events are required machines which is the Einstein The particles is in order forward trast, are preferred. 4.3. DETECTORS produced of the particles which are produced very high energies there are usually tens of particles each “event”). millions may appear and completely in high-energy produced as possible collisions. At in each collision particles We require typically of events in order to unravel photons, and generic are found must be able to record in a reasonable seconds). ability length The amount to analyze the recording rate. must If not, the analysis the final with time particle must the events becomes typical particles two characteristics 66 with decades or centuries of techniques hadrons must be built which (except to cover the are produced and neutrinos components by con- and hence the produced which - namely - are common can “track” to all these different for neutrinos). can be used to track any particular type choices are made based on the characteristics but constrained and the interplay of the different It is not possible to discuss our with rather describe by practical considerations components to our example of all possible some sample components of existing detectors form the overall detectors, of particle. of the physics like cost, geometry, (devices) to illustrate detector. making detectors and their function up the detector. cloud chambers here. Instead we will and then give a few examples how these components We do not try to present but will mention on the modern particle are combined a full historical and bubble chambers together perspective to on before focussing techniques. shown by fig. 4-2. To fully and trajectory charge and mass. are sufficient types it is. If we know the particle neutral detector to lo7 x at a rate comparable the momentum as well as its electric beam experiments, and the detector the types of particles placed at a situation. event patterns Colliding are therefore can be performed also be consistant computer states we need to measure in an event, the latter the known recorded The detectors are thrown the many a few years (1 year w 3 in fact measure ? Let us return the detector each of the final-state particle of information collisions experiments say not more than is, of course, an unacceptable electron-positron reconstruct of time, this data on a high-speed than years which What one per second) so that experiment at rest in the laboratory in every experiment process of interest, rate (typically These then are (in For each experiment Hence these devices particle. fixed-target region, in any direction However particles, experiments, A variety fast enough , for a moving of momentum). cone in the forward full 4n solid angle. is to record as faithfully relation in a high-energy have the center-of-mass charged outcomes. .!,, design. (by conservation cover a limited particles of a detector mass-energy the goals of the detector space even when they are the beams pass through where a large number to make a precise measurement, possible using E2 = p2c2 + m2c4 beam beams, the two beams pass through Since the beams are in fact mostly the properties the energy of the particle machines. do have one great advantage so a large number The purpose to calculate can be as long as desired and can be exposed to any number and those in the target one another. are performed machines The target of beam accelerat,ors, I to identify of For each which of mass we can use a momentum Cloud Chambers and Bubble In 1896, the cloud The chamber chamber Chambers was invented by C.T.R. is closed and contains (such as supplied by dry ice). When air saturated radiation 67 with Wilson alcohol at Cambridge. at low temperature ionizes the air or alcohol molecules, the alcohol after vapor condenses its invention, magnet poles, the tracks bent in opposite identified. photographs wanted happened chamber handle high particle had been through. chamber. liquid Bubble adequate hydrogen When clearing and provide Glaser chambers argon or some other contain liquid), ready for use, a bellows When charged boiling along the track before the entire liquid and measured. be charge could be fields inside them, evidence which of what liquid is just about go through of the particle. 40’ K (or -230 the liquid, degrees Celsius). so the liquid too far, they can be seen reconstructed, three different This leads to the production cycle, the bubble Because hydrogen many to increase chamber of pulses per unit the only bubble detect to the bubble equipment chambers charged chamber, in the number necessary Also, the pictures 68 of this to keep the chamber were restricted particles. physics the tracks can be measured. detection Bubble I chambers were the mainstay in the 1950s and 1960s but they are not much used today. Modern Detectors We now turn to a general discussion described Actual below into detectors single, of modern used today multi-faceted techniques combine detectors They are quite large and complex., Among multiwire chambers, muon detectors, detectors, vertex detectors proportional calorimeters, Cerenkov data is collected and analysed The experimental Physicists Collider) in the future and LEP Center (the new e+e- for Nuclear cists from 39 universities the Soviet Research Particle plastic Union, Detection capabilities. chambers, scintillation counters, and very large magnets. will be even bulkier The and more depen- collider building the U.S., Italy, than now under construction at at the near Geneva in Switzerland a detector throughout East Bulgaria collaborations such as the SSC (the Superconducting (CERN) and laboratories Netherlands, have many than today because the goals are far more elaborate. interface France ). One of the collaborations China, that will have to band into even larger experimental European many of the components the parts one often finds drift present to be able to do physics at machines Super detection using computers. apparatus dent on the computer for particle and West and for LEP involves 440 physithe world Germany, (including France, India, Hungary, and Spain). and Identification Through Energy Loss Rate field sweeps ions Because temperature, particles of immense pulse out of the way of the beam. of the refrigeration Furthermore again, and a small electric was limited particle Switzerland, is made ready to use again by using the bellows the previous is ready are photographed data. from Liquid the ions they create trigger is aboil or the bubbles have drifted become liquid of particle (or sometimes views of the same event are photographed. The bubbles could the bubble on the verge of boiling. As long as the tracks of rolls of film to record particle that sorts of particles hydrogen reduces the pressure rapidly, particles detector in 1952 developed pressurized physicists cloud chambers numbers pressure. by taking for the sort of physics In order to get all three dimensions The chamber could no individual and identification. of Michigan boils at a very low temperature, to boil. would the experimenters was needed was some other (rates) Donald Soon between for later analysis. What fluxes particles records of events were obtained was not really reliable. charged track, track. was placed force, so a particle’s of the particle’s to do in the 1950’s. Even with were not terribly particle’s chamber and negative mass. Permanent of what The cloud if a cloud by the magnetic the density the particle’s that due to positive directions Also, from estimate on the ions along the ionizing it was realized . I ‘I 1 in their detection if the source To detect repeated time. at liquid volume. delivers become so full of tracks too that terial medium, ionization utilize particles they transfer or excitation the remnants mation. trajectory Two we must simple utilize the fact that energy to that medium. examples of a charged particle energy The particle loss in the medium are the use of a gaseous detector or a dense material 69 transverse a ma- This process occurs through of the atoms in the medium. of the particle’s when they detectors to extract must infor- to measure to absorb a photon. the In the first . I case, the charged liberated particle electrons) loses energy by creating along its trajectory. can be inferred. to segment and instrument the dense medium as well as the total a trail If the position sensed, then the trajectory be determined I of ionized atoms of the ionization In the second example, (and can be one attempts so that the site of the absorption can absorbed energy (that is the rise in temperature of detector components by considering of the absorber). We will begin the discussion tion of charged particles. the charge (in units The energy lost per unit of e) of the’ ionizing number, the amount does not depend on the mass of the ionizing While running as a function expectation is seen to fall to a common slow rise as the particles of different When formly mately I I11111 I llllll minimum be absorbed. momentum of energy particle species. The ionization species and then exhibit The curves corresponding against loss the momentum a to different variable, since have different velocities at the same momentum. charged slowly. to the point measurement it taken from a gaseous tracking for all particle when plotted and particle, The dots are the data and the curves become relativistic. mass relativistic and rather reduced of momentum for different species are separated particles to ionize its atoms, (v). Fig. 4-7 shows the actual at the PEP storage ring. are the theoretical particle of energy required on of the medium AE/Ax chamber I and properties depends its density. loss for charged particles I (AE/Az) such as its atomic does depend on its velocity I II particle length the detec- particles After sufficient where they will Most particles and will pass through traverse a medium, material they is traversed, lose energy unitheir begin to lose energy more rapidly produced velocity is and ulti- in high-energy collisions have high many meters of gas without significant reduction -1 Momentum Fig. plotted 4-7. - The energy as a function the theoretical muons, the TPC detector (GeV/c) length of the charged particle’s expectation electrons, lost per unit pions, at PEP. for the particle kaons, protons metal in a gaseous drift momentum (dots). types indicated and deuterons. 70 in their energy. 10 1 10 chamber will be absorbed, however, by a few meters of heavy such as steel or uranium. Electromagnetic Showers indicate Electrons or Photons The curves are on the figure This is Such particles namely data is taken from For all charged particles, above is the most important except electrons, energy-loss the ionization mechanism. 71 process discussed A second phenomenon dom- . inates the electron’s very light ier). energy (the next lightest In dense materials, inside loss in dense (solid) the atoms that charged electron they particle, paths and thus lose energy. typically converts This to an electron absorbed. rapidly, transverse a “shower” of interest takes place whenever from photons medium which by the electric fields Each photon along its path. absorber of high-energy a high-energy material photons; photon depth components in to measure counter particles the number calorimeter be about photon. to the initial photon particles, to record the showering by a detector of about 2-20 cm for To distinguish A sandwich cascade rather amount counter above is called followed usually by another needed to totally of this kind therefore a can the position or electron. charged particles the target trajectory travel plate, typically absorb the incident proportional option which of the and thereby is strips 72 the position of scintillating A typical is described but at the same time the struck of the shower, Another chamber the cascade. below. wire locations of the initiating plastic along along the wire in their amplifier in which magnet. in a high-energy, fixed-target in the figure. in a gas volume the direction As they (angle) in detector of their we can calculate to simultaneously the spectrometer measure charge. this device would particles produced spectrometer. provide in the collision. that which trajectory. of A and A’ are whose are produced counter will A measures From the difference in the angle of bend of to the ma&&ic field of the dipole of the particle. The deflection Thus if and the angle direction also As long as the devices A and A’ have the the directions has a large enough particles, of the 4-8, “downstream” Imagine to the momentum not the tracks over of the charged particle particles momentum. relativistic themselves we can measure in a laboratory), the particle measures the sign of their electric in Fig. A and A’, we can measure proportional field (which using a and the bending leave the target, This angle of bend is proportional (B) and inversely while direction aa shown The charged the spectrometer. direction The fact that energy allows us to follow experiment. capable of measuring and the direction particles, Such a device is called a magnetic a spectrometer, we know the magnetic of bend, field. now placing position magnet trail of their total of charged and hence measure both their initial towards ability used for sampling l@ht ,propagates Spectrometei device and a strong fraction is shown the particle. samples the development This is sensed by a light-sensitive and momentum leave an ionization losing a significant lead, followed should light. as the charge propagates - Magnetic the direction of a tracking the particle absorber (just Jts intensity Tracking We can measure A typical finely. could be a multiwire Particle Consider of showering can be measured. of each layer of absorber of material Such a device counts the ionization, measure Charged electrons hence if a calorimeter of dense absorber, There are several choices of detectors example energy, into scintillation counter track due to the magnetic since in a dense described is converted chamber). large distances This discussion The number the energy particles, etc.. The thickness 10% of the total showers calorimeter. is made up of a %andwich” by a detector followed the electromagnetic of showering the proportional gaseous detectors or an electromagnetic is proportional of the plastic combination The shower has a in the detector, the length the energy is the same shower phenomenon is present. energy called a “phototube”. it loses energy like lead. ionization then and positron the dense medium, is localized to radiate they leave the same signature. shower photon heav- This electron and a’penetration one needs additional A device count bent are 200 times causes the electrons pair. traverses and for a typical also applies to the detection is about and the cycle repeats itself until size of a few centimeters momenta because electrons bremsstrahlung. and positron Hence, as the electron producing This is called turn lose energy via bremsstrahlung fully the muon, are strongly pass through. photons materials, I geometrical the directions Indeed experiments. the 73 of all the produced coverage to “catch” and momenta particles and all charged of all the charged this is how it is done in fixed-target . I tional an appropriate Charaed Particle Tyajectory .’ counter. B A I Target Charged Particle Detector 1-89 6238Al n - Dipole Magnet Photon gaining q c potential liberated energy from processes results the electric Fig. 4-8. - A schematic of a typical as a solid line. It is measured the charged momentum. The wavy line emanating produced particle through in the target hadrons, behind and is detected like a neutron, would be absorbed source of photons by the magnetic C. If detector the positions hadron after It travels in the hadron Most the trajectory through in an avalanche their detector Tracking particle at the anode in the wire by a suitable tube or chamber of charged particles that A chain of such hire. electronic This circuit. avalanche The device because the signal is proportional passed through production.) Clearly calorimeter pions, photons large electromagnetic - Proportional proceed A further any neutrons stable (D), placed them. (The Fig. 4-9. - A portion undeflected the position 1, of a multi-wire of the anode wires which in the gas when a particle proportional collect chamber the avalanche is shown illustrating of electrons produced traverses the device. it can measure This same principle device called a torially produced. Counters devices used today j to the the tube. the magnet r” -+ y + y, which calorimeter produced. D, is needed to detect tracking to of is used in multiwire in fig. 4-9. A plane of many parallel planes. anode wires. are based on the propor- The cathode into strips strips will running detect 75 1f I proportional chambers anode wires is supported Each anode wire acts as an independent planes may be segmented 74 ‘I of electrons energy it from the (C). Neutral will not tell us about is from the decay of neutral and energies of all the photons charged gains sufficient 1-69 6238A2 The spectrometer C is a sufficiently Particle If the electron field. magnet cathode Charged field. electric the anode wire Cathode Planes is shown field and will arrive at the region where we have placed detector calorimeter, a radial towards calorimeter. But what of the photons? occurs very rapidly represents calorimeter producing process will drift to the particle’s (the line is wavy only to distinguish meaning). potential. The beam A and A’. A dipole from the target in the electromagnetic the electromagnetic primary chambers setup. trajectory an angle proportional other lines; the waviness has no physical undeflected detector A charged particle in the tracking (B) deflects a photon fixed-target on a target. metal tube, filled with .I1 A thin central anode energy of the gas, fresh ions are liberated. is called a proportional enters from the left and impinges is placed in the tube, can be sensed as a current number at a negative by the’ionization exceed the ionization Neutron Detector Photon Detector such device is a cylindrical gas and maintained wire at positive Any electron The simplest I in a direction currents detector. as shown picbetween The cathode perpendicular induced two on them to the from the . s drifting ions. These signals can also’be read out to provide information. planar In Fig. multiwire the horizontal proportional and vertical will leave a pattern chambers, direction. with are not detected, can then be used to reconstruct through the device. alternately in which traverse such a stack of signals in the wires closest to the particle Particle localization anode wires oriented Charged particles software Charged additional 4-8, devices A and A’ could be made up of many layers of the trajectories paths. Computer of the particles - Drift and momentum and hence no practical detector. However, proportional the inter-wire spacing, good enough. namely In addition, about they For many 1 mm. require an enormous to the wire, allows the inter-wire permitting spatial resolution is that very precisely. the drift field be quite needed, in addition of wires forming Fig. Another 100 microns 4-10 shows the design of a cylindrical target detectors; Other Neutral the direction drift chamber, distances) wires are design of the set like neutrons chambers are typical and vertical for a fixed of charged particles target experiment calorimeters thickness of the sandwich magnetic cascade, but by the multiple photons slices is governed, Considerably - hence hadron behind course all hadrons, more material and photons. 76 Neutrinos produced weak which can detect in the the electromagnetic from the magnetic gies, the calorimetric the magnetic considerably spectrometer. better steel and the not by the physics of an electro- is required too, are absorbed by which the neutron to absorb hadrons They are usually by such calorimeters. particularly However the charged particle directions ‘. how they and in the collisions 77 For to that obtained at higher ener- is more precise than that obtained by the spectrometer. than placed like device D in fig. 4-8. Of energy measurement In many situations, energy measurement spectrometer. used is usually calorimeter, charged hadrons they add a complimentary Neu- These are very much like elec- are more massive. in fixed- the position only exists in a general instrumentation. nuclear interactions calorimeters charged hadrons further the material layers are arranged illustrating because of the immense mass of material calorimeters. except’ that track coordinates. to measure detection have been built require which is the typical of tracks alternate measure horizontal of off. with Particles be combined momenta field-shaping To be inferred .,I Neutrinos’undergo for efficient detectors of neutrinos in hadronic are detected immediately 10 c m while still = 10m6 meters). large, specialized stable hadrons loses its energy. in which carry method chamber is shown in fig. 4-10(b). W e have now discussed the main elements of a detector might chamber, (due to the long drift Stacks of planar drift to reconstruct to have their wire directions of instrumented sense wires, A typical the basic unit or “cell” of a drift detector. (1 micron To achieve this, to anode and cathode shape for a collider this is not time of the beam (the moment requirement uniform. by to travel from the point of origination measure this time we must measure the arrival the collision) number spacing to be increased to about of about determined applications called a drift anode wires to cover a large area. A refinement we measure the time taken for the ionization resolution they may sometimes detector. trons Chambers chambers will have a spatial that the effects of the collisions passing production the energy tromagnetic Multiwire their interactions Neutral, Tracking though I from are measured l 1* ’ e . I I . . . .!I . . /. A : . . . * . Beam Pipe-/ 888 Fig. chamber 4-10. - b) A quadrant used in the MARK of the end-plate II detector. A blowup . . . . 4494KU of the large, / cylindrical region shows the position drift of the field wires and the sense wires for a single “cell”. . . . . . . . . . . Fig. 4-10. - c) The paths taken by the electrons as they drift wires are shown. Electrons liberated along the particle lines to the sense wire. 80 81 trajectory towards the sense follow these drift 8 f 2 Q-4 . Collider Detectors The devices which we have described as elements of a fixed-target are the same as those used in a colliding-beam is different sphere) since the produced subtended at SLAC, anti-particle in fig. around beams. which chamber provides 72 radial AT SLC II in cylindri- the particle produced and are tracked in a large gas-filled measurements of the ionization to the beam direction) of a solonoidal of all the charged particles. system without particles to the beam direction). The combination and is provided and energies are measured. to the earlier explanation Mark II does not have hadron calorimetry not detected in it. If a hadron calorimeter the electromagnetic calorimeter, drift by magnetic by the coil as in- field and the cylindrical the sign of the electric Photons in the deposited A solonoidal drift charge and traverse the charged particle leaving any signal and enter the electromagnetic ters where their positions outside the Mark devices are arranged Charged allows us to measure the momentum, in a manner similar detector, “beam pipe” which contains the beampipe (The wires are parallel in the figure. the trajectories tracking The detector MARK II the geometry the full 4~ solid angle (the entire A prototypical are also instrumented. field (field lines are also parallel dicated However experiment, The beams are made to collide in the center of the detector. travel out through the particle. 4-11. experiment. populate point. the evacuated The ends of the cylinder chamber particles by the collision is shown cal symmetry collision I calorime- These counters are segmented of “sandwich” type calorimeters. like neutrons The and therefore particles are were included it would have to be placed but inside the muon counters. Fig. 4-11. components 82 - An drawing of a typical are discussed in the text. 83 colliding beam detector. The major . I I ‘. 1 More on Particle We have not Identification’ discussed The only charged yet how we measure identity of the charged particles. separate all the charged particle In general the particle mass, it is hard to use a single momentum be inferred lost by the particle tracking chamber. of the energy However one sees that power is lost for most species. Electrons are identified absorbed while showers are identified calorimeter matches There description phenomena of Cerenkov particles provided this angle can be combined measured This radiation occurs important travels The Hence measurement for the particle of Those addition they steel because traverse muons do not large quantities repeated “feel” nuclear the strong of steel before their processes and, in fact, they leave the detector collisions force. energy (12 inches per layer) steel plates 84 mass. in a dense absorber is depleted of the Mark interspersed with ‘new’! physics measured to the track muons by the trajectories are identified. with ,electrons) The play a very as we saw with the example chart) have sufficiently time (about 0.2 mm). detectors”. The MARK constructed, to 30 microns capable an important long lifetimes to permit To do this requires devices as close to the production device via the weak interaction far enough before decaying point of the T. (1 micron placing (fig. drift role in understanding (about lo-l3 high-resolution seconds) of their life- measurement “ver- Such devices are called 4-11) has two such devices; chamber = 10e6 meters) positions (such as the D+ a measurement as possible. II detector pressurized of measuring Event capable and a three-layer to 5 microns. or isolating of measuring Long-lived unexpected one track silicon objects strip can play physics. Pictures Fig. 4-12 shows several “event storage muons Computer (digitized) and momenta like those final states which will near the beginning II there are 4 layers tubes. Contrast tracks, their full traverse energy has been applied and find the energy and posi- of the events correspond Beam Experiments” the drift to the we can reproduce Fig. 4-12a is the e+echamber except that but in fact penetrate to section final st,ate, is a back-to-back in the electromagnetic looks identical in the calorimeters II data at the PEP In this fashion The patterns (see fig. 4-2). fig. 4-12b which cles are not absorbed devices. we discussed in the “Colliding charged Mark software of the charged tracks of the photons. and deposit this with taken from reconstruction of this chapter where two oppositely configuration pictures” hits in the detector tions in the calorimeters due to ionization proportional rings. In before all of their energy is gone. In fig. 4-11 then one notices that around the periphery of thick greater So high-energy way since they (along on the wall the trajectories do not undergo In this decay only measured because of the muon’s chamber. which they travel e+e- do not shower in calorimeters in the drift hadrons that to find its Detection Muons trajectories all The angle mass, and hence its identification. Muon The track which can penetrate Detectors positions medium. muons. point the steel plates can be linked role in discovering employ the at the collision of muons is very important is a carefully of travel of the particle. an energy measurement made and kaons, a particle in that detection tex protons when than the speed of light by the tubes between the muon shown system. pions, proportional be used. in the drift produced four steel layers are high energy Vertex Hence localized track the energy to the speed of the particle. with must other than to say that they typically radiation. related on. in the tracking in a cone about the direction of the cone is directly the separation where they are totally to a charged high momentum can in the gas of the methods continue that measured at a speed greater is emitted other to of interest. its identity 1 GeV/c, calorimeter can be linked for separating a detailed radiation which the momentum are methods a medium momenta charged as electrons, but we omit through For higher energetic in the calorimeter chamber above about by the electromagnetic all other is known, is the device types over the full range of momenta Fig. 4-7 shows how once a charged particle’s by a measurement that particles calorimeters. the charged through parti- the steel of 85 - the muon system. This is a pL+p- final state. Following 4-12a,b we deduce that fig. 4-12~ is an event with from r+r- production as discussed events, both charged and neutral, clear “jet” structure the quarks follow In Fig. drift “boxes” taken relatively - the larger at the periphery by the charged of-flight scintillation combination numbers in GeV. track of the drift particle of this flight next appear Finally quark from For particles deposits extrapolated Notice qq. materialize trajectories the time the production area delineates by the charged which momenta is sufficient the from in the central The small (in nanoseconds) point below to the time1 GeV/c, to distinguish the electromagnetic tracks in the calorimeter towards to the hits in the muon proportional pions, (those lines them). are the energy is shown with the calorimeter and the photons which do not have a charged track pointing to the energy are multiparticle -+ the momentum. measure with comes directions. as curved In figs. 4-12 b) and c) the muon system trajectory 4-12d,e The hadrons time and the momentum deposited fig. the smaller chamber to travel The hexagonal the energy in the calorimeter tracks the curvature counter. kaons and protons. in which well the original of fig. a e+pL- final state, which from the process e+e- at these high energies. 4-12 the charged chamber earlier. coming from the explanatiou the drift The measured chamber counters. Fig. 4-12. - a) An eiewith momenta measured energy in the calorimeter. final state. Two high-momentum, as 14.5 and 14.2 GeV/c !’ .I each depositing tracks 14.6 GeV The beam energy in this run was 14.5 GeV. 87 86 back-to-back of r c ;-- . . I I I I I 1 h I L I I. 3 3’ L . I 5. Experimental Like the rest of physics, egant and eminently experimental Basis particle successful results of Particle physics Standard accumulated I Peysics ‘/ is an experimental Model stands over the past half century. because the particles science. on a firm The el- foundation of This achievement especially remarkable that are the objects of investigation incredibly small and in most cases persist for only tiny fractions is are of a second. 5.1. THE NUCLEUS How is it possible to study nuclear ment to do this was an archetype. and Hans Geiger, ester, England. is, ionized They helium deflected were working atoms. suggestion cles, instead of deviating quently. by a microscope. screen, light See Fig. was emitted ments - beam, target, event resulting 90 from q?j how much the alpha particles were of all, they found carried a thin metal foil. that path, actually this happened At parti- bounced fairly fre- a large charge L the nucleus. an experiment in the foil, 5-l. that When a beam, the alpha which the alpha particles was visible and detector with and a detector, through production. 91 parti- was a screen viewed struck the eyepiece. - are present in nearly experiment. Fig. 4-12. - d) A two-jet that also checked to see if by chance some alpha performed the atoms in Manch- to propose that at the center of the atom there must very small that Geiger and Marsden cles, a target, Ernest Marsden alpha particles, a few degrees from their original This led Rutherford The first experi- Rutherford path when they passed through they To the astonishment be something of Ernest source that emitted They were measuring from their original matter? In 1909, two young researchers, the laboratory used a radioactive Rutherford’s backward. and subnuclear the zinc sulfide These same ele- every particle physics Fig. 5-l. particles Diagram emerged from the tube AB. the zinc sulfide through of the apparatus screen, S. a microscope, M. This of Geiger and Marsden. They produced The shield, bounced (o) off the screen RR and struck scintillations P, prevents The alpha that could be observed alphas from striking the screen directly. 5.2. RAYS COSMIC Fig. 5-2. positron The alpha particles they that Rutherford have energies of only a few MeV began to study in outer collisions atmosphere and create than those of emitted be controlled, upon nor their time Technology a cloud chamber discovered of the curvature electron volts). Other The cosmic rays are generated They collide with have very high energies, bn the other researchers hand, the cosmic atoms in the much higher a positive From charge. opposite charge. magnetic field. He observed See Fig. a particle a track just in his cloud 5-2. like an electron, chamber that Institute of but with the curved in the because curvature exist whether the particle The positron lost field the particle the same momentum was known could to have not have traversed unknown Rev. 43, 491 (1933)] Anderson field. knew not be a proton. in ordinary chamber he had placed the particle had entered lost momentum matter. 92 1, Phys. of the cloud more in the magnetic could i with a path in the magnetic so the track must have been made by a previously [C. D. Anderson, In the middle of the California A proton a lead plate. a more curved showing rays cannot anticipated. in 1932, C. D. Anderson the direction such a great distance, particle. by C. D. Anderson from below and passing through afterwards. and place of arrival the positron, taken (million him to determine Using picture followed that alpha particles. chamber energy in the plate and therefore our atmosphere. more particles entering cloud used for his beams have a severe limitation: of cosmic rays with targets. space and rain The From in passing it was positive. The positron I .’ its ionization elementary is not found 93 the plate of the particle’s From the first plate. This enabled from above or below the plate through the direction He had found a metal motion track particle ordinarily and curled and its he knew that because up it does not when it ,* . I i 11 collides with an electron an important medical enables researchers the two annihilate application, positron to locate precisely can be introduced to form two photons. emission artificially into the body in specially tomography produced prepared This process has (PET), positron radioactive which emitters that with a mass a few hundred War II, further particle particle, compounds. cosmic times as great as that of the electron. ray research because it did not interact the pion, revealed strongly was discovered Anderson’s discovery a new particle, the muon, by its ability of material. nearly so far because they lose large quan- tities of energy they undergo do not penetrate by emitting nuclear Protons photons. do not penetrate large thicknesses collisions. 5-3. A picture rest (from upper off to the right left corner). (continued an e+ and neutrinos. the e+ (and neutrinos et al., Nature When predicted binding emulsion showing The ?r+ -+ p+ + v reaction above) a r+ meson coming to Fig. occurs and the p+ goes emulsion The n+ also decays to entered and *also comes to rest. This emulsion was, however, are not observable not sensitive enough in such experiments). to record [C. M. G. Lattes 159, 694 (1947)] was discovered by Hideki Yukawa of neutrons it was mistaken of Kyoto and protons University. in the nucleus for a particle Yukawa that had shown could be explained had been that the by a particle 5-4. exposed along al., Nature t, Yukawa’s The true Yukawa and it was found that it 5-3. labeled War, many perplexing particles to &X+X-. k. The x - interacted with very short lifetimes. The picture is of a photographic was sensitive At the point with discoveries to charged A, the K+ particles. decayed a ,nucleus at the point The I(+ into three B. [R. Brown et 163, 82 (1949)j as we cannot pion has a lifetime lifetimes predict particle when a radioactive nucleus will decay, we cannot will decay, only what its life expectancy say is. The charged of about 2.6 x 10-s s. Cosmic ray studies revealed new particles from lo-’ to 10-l’ s. These included the A, which decays either into 95 94 i All involved to cosmic rays that the path when an unstable with cosmic rays. The decay of a I(+ charged particles. Just the muon the nucleus. ray events In the first few years after the Second World were made studying of a photographic was not 5.3. STRANGE PARTICLES very far because _ Fig. see Fig. the muon Just after World in cosmic rays. The muon is distinguished Electrons to penetrate was found that with in cosmic decayed into a muon and a neutrino, Five years after I I - . . rr-p or 7r”n, and the K+ which decays in many ways, including K+ + rT+rT+~- and I<+ --+ ZL+v. See Fig. 5-4. These new particles The particles particles that nucleus. It was possible sible for their new particles lifetimes a value of “strangeness,n conserved a A with strangeness. (The interactions, decay A + everywhere.) Each of the new particles could like electric that interactions. together which would No experiments the a factor be assigned this charge is Thus the collision one and in a process that changes not change strangeness, cannot Because This flavor-changing the weak interactions than they would process occurs through the weak throw the most of the law conservation says that cannot would if an experiment determine appear which to follow does not distinguish experimenter would conservation of parity Laws, the Maxwell between right event in particle of parity. physics stated, history of a perfect was the real experiment and which all physical laws. right-handed Sometimes and left-handed,” in the mirror. the Schrijdinger (The occasional Why 96 of parity an observer is the image. Both as “Nature since a right-handed did people believe that laws of physics equation, “right-hand” mirror this is phrased was correct ? All of the known equations, was the over- the conservation front appear left-handed and left. Simply is performed,in between of parity rules conservation. e- decays. Fig. remarkable convention. more slowly 5.4. PARITY VIOLATION Perhaps had ever shown a violation with interactions. are in fact weak, the decays proceed if they were strong is defined like forces if we use left-hand e- take place because the mass of the A is less than the sum of the masses of the Zt’and the p.) quantities P F 7’ s of The strangeness The A decays to r-p K-p, other The magneticfield but then the force law also has a right-hand or between bind charge except of convention. We get the same results for physical by about and p can create a I<+ with minus one. convention, force was also respon- have been shorter and electromagnetic x- which matters seemed too long! was given by A. Pais and M. Gell-Mann. a quantity particles, strangeness and protons, if this same strong should in pairs. only in the strong of two nonstrange of neutrons that to the paradox were created because their lifetimes of the strong to estimate decay, their 10-13. The solution confusion in collisions feel the effect are always a right-hand brought were created magnetism I - Newton’s etc. - made no distinction rules that afflict the study of 5-5. A nucleus spin and magnetic with momentum is a correlation between of the outgoing electrons spin in front pointing the direction from Thus it will be possible its mirror image. that opposite of the magnetic to distinguish C. S. Wu and her co-workers parity number test whether parity of experiments S. Wu (of Columbia of the real nucleus. moment If there and the direction in the mirror between found image has its will be the the real experiment such a correlation and and thus is not conserved. T. D. Lee and C. N. Yang realized could that Thtsmirror /3 decay, the correlation opposite. demonstrated of a mirror. was conserved that no experiments in weak interactions. that could and the first one was carried University) and her collaborators. 97 had been done that They proposed out by Madame a C. Most nuclei have magnetic . \/ moments, which .d point along the spin (angular this sense act like spinning tops. The direction defined by the rule that if the fingers motion, then your thumb points the mirror pointing downward. but it will be the left thumb!) the electrons a strong emitted magnetic was correlated its magnetic What with the direction moment. moment was whether of the magnetic points upward will were polarized the outgoing moment. upward, was the direction nuclei electron of with direction studies of/Z decay were prominent the preponderance with of particle high-energy particle rect a beam on a target soon as the accelerators they supplanted high-energy secondary ’ See Fig. 5-5. Though of has its magnetic be pointing 5.5. MORE AND MORE HADRONS is Now this means that Wu measured Co 6o . The cobalt The question moment and in hand curl in the direction the thumb Madame by radioactive field. of your right (In the mirror of the nucleus, the magnetic along the magnetic image of a nucleus with moment momentum) protons posite the direction exactly that of the magnetic along the magnetic experiment in the laboratory moment from the mirror image. the magnetic moment. tion opposite moment. the electron In the mirror always emerged it would always emerge and it would be possible to distinguish In fact the emitted Parity electrons was not conserved op- favored the real the direc- handed investigations could particles that particles like an electron, measure the component just undergo as we can consider principles of quantum the spin parallel be summarized When weak interactions. muon, as follows: and neutrino, mechanics to the direction and ‘spin-down” tell us that of motion we call it left-handed. left-handed? of the particle’s for electrons such a measurement or anti-parallel right- left-handed have “spin one-half,” of the spin along the direction “spin-up” between it is mostly is a particle which With achieved and with by 0. Chamberlain, of the prediction in pairs, the discovery For we can motion, in an atom. The will find either to it. If it is anti-parallel workhorse after 5-6 shows a picture its varez’s bubble permit a detailed chamber, analysis of bubble chamber lifetimes less than speed of light, measuring invention of high-energy pictures would infer their existence. the known s quarks, volts), produced to generate of Pais and Gell-Mann that at the Beva- and detailed in a photographic studies emulsion annihilation. by physics. filled Fig. the bubble liquid hydrogen, event. led to the discovery of the particles distance into which interacting The analysis was no evidence that mathematical 99 Alcan of large numbers new particles with at nearly the However, by they decayed were found it was possible to and they expanded to this vast collection by and G. Zweig in 1964. All be accounted quarks L. W. even moving model by M. Gell-Mann could the before decaying. Order was brought particles from became of how such a picture of a numberof More and more such hadrons particles. chamber 5-7 shows an example of a complicated of the quark but there with Glaser, not go a perceptible list of known strongly D. seemed at the time more a convenient 1, As par- of the antiproton of an event to di- high-energy 1O-23 s. Of course such particles, the tracks the introduction ,j with E. Segri: and co-workers, gave the proof of antiproton electron The accelerators the early results out location. these beams it was possible Among were confirmation tron in Berkeley the enormous 98 it was possible in an appropriate research for the most part. or electrons Fig. accelerators an energy of a few GeV (billion are produced which 1950s was carried in weak interactions. of p decays showed that the inequivalence and left-handed accelerators. weak interactions, in the early and to place a detector strange particles Soon Further starting beams of pions or kaons. of I< mesons. in understanding physics cosmic-ray ticle accelerators Suppose for simplicity I for by the u, d, and as such actually construct. existed. They lx . ‘I 5.6. STRUCTUIXE OF TIIE NUCL’EON During toward the 1950s and 1960s proton accelerators played the end of the 1960s a new and very powerful at Stanford. The Stanford of reaching about the electrons Linear Accelerator 15 GeV. When remain large spectrometers. intact and their The surprising scattering through objects. The conclusion earlier. Feynman the basis of constituents was similar of the probability for In fact it turned explained It looked likely that these results on with tained evidence had originally They only weakly tities been proposed momentum chargeless. model came from comparing with results by Pauli conservation with matter. in /3 decay. As a result, before undergoing number of neutrinos from neutrino to explain have no electromagnetic of material enormous for the quark scattering or strong they usually a’collision scattering. an apparent Pauli’s with were massless and interactions pass through a nucleus. protons using a nuclear running, direct detection reactor of neutrinos at Los Alamos 5.8. NEUTRAL electrons Laboratory. the scattering 102 and Steinberger and Experiments at similar measured the scattering of neutrinos and verified the apparent the rates for electron Because the electrons interact depends on the charges of scattering and neutrino charge assignments scattering for the quarks. CURRENTS Neutrino experiments in 1973 revealed Events were seen in which the neutrino events because no charge was carried Only When neutrino at the large accelerators rate for the electrons the fractional instead were similar and neutron. enormous quan- by using an to observe and Cowan the reactor from the p decays of fission products. Schwartz, at SLAC The results a muon or electron, by Reines is turned later). in Geneva, Switzerland. so they interact is it possible was achieved National there is a large flux of neutrinos 1962, a team led by Lederman, and at CERN By comparing when they the neutrino an electron beams became available and neutrons. strangeness, for example used an accelerator is currents, something that scattered but still as a neutrino. searches before for neutral were extremely first kind discovered Very high energy neutrino from In the interaction pion from the beam. in the beam come from /3 decay, they turn (and a third to the ones done with The charged in the rare instances is, there are two kinds of neutrinos, near Chicago intense enough Neutrinos lack of energy and neutrinos and a very large detector the results ob- them. The in the detector. it was possible to confirm EXPERIMENTS from electron angular That beam. and the muons are removed can then be detected If the neutrinos a muon neutrino the quarks. Important remain material electromagnetically, NEUTRINO to make a beam of neutrinos presence of quarks inside the proton these constituents were quarks. 5.7. that into electrons. Fermilab by Rutherford Laboratory decays into a muon and a neutrino, out not there must be small charged a model that the proton. were reminiscent to the one drawn and neutron developed within small. The beam was made by using a pion into a muon. in that to be detected. interact and neutrons, can be measured were obtained be rather Inside the proton and Bjorken and direction National The neutrinos beam was capable off protons It had been expected a large angle would to be so infrequent. half a century energy role, but began to operator electron scatter results that the results of Geiger and Marsden. accelerator Center’s the electrons a dominant the Brookhaven I more surprising. off the target emerged not as Such events were called neutral to or from the neutrino. but in processes that the decay K” + ~+Z.I-. rare. However, .much There also involved It was known current had been a change of that such processes the new results showed that neutral current process were not rare when there was no change in strangeness. This discovery reflected In studied weak interaction at neutron into a proton, on the basic nature was ,0 decay. an electron, of weak interactions. The simplest @ decay is the decay of a and an antineutrino. 103 The earliest Yukawa, in his work of 1935, had already products proposed of a particle \, s that now called I the electron and antineutrino More the W boson. t were the decay precisely, the neutron I > so00 wa.s (a)- . 1000 r 500 . 2000 viewed as decaying Up until into a proton 1973 it appeared were charged. Thus antineutrino. experimentally in /I decay In muon-type and becomes a p-. and a “virtual” The virtual The results from the CERN was especially L. Glashow, S. Weinberg, the W bosons, decays scattering, experiment exciting W- into by the target, done with if they existed, an electron the neutrino W+ is absorbed showed that there must be another This that a virtual neutrino W boson. emits a virtual increasing the Gargamelle and an W+ 3 I; had been developed so. 20 chamber - largely and A. Salam - that called for just such neutral Moreover, using the results to predict the masses of the W and 2 bosons. from neutral current experiments of 80 GeV, a maSs too high to be reached with I i .----_-Y *... ‘3. ----___ -c; * IO- I a 3.10 by S. 3.12 3i4 E,,.(Gev) bosons. it was possible These predictions ! : t i ::, 100 r weak boson (now called the Z) that was neutral. because theories ii . 200 its charge. bubble rl :: then were in the range any accelerator that existed discovery. groups, at the time. 5.9. J/?c, PARTICLE The next year produced different time, kinds of experiments, with at SLAC When that another a mass of about using a newly the two collided converted stunning discovered constructed ring they occasionally into various a particle three times that particles with of a proton. that Two collided annihilated doing a surprisingly One group electrons with in the detector. the rate for such events varies only slowly with the energy of the colliding However, the team at SLAC found that a total the rate suddenly previous which value. increased This by a factor is shown in Fig. center-of-mass long life- was working into electromagnetic that were observed “‘A-[G*V] very Fig. positrons. energy Typically particles. energy near 3.1 GeV of 100 and then gradually fell back to its 5-8. The peak was evidence for a particle, e+e- 5-8. a. The cross section annihilation of the J/+. observed by B. Richter Note the logarithmic 33, 1406 (1974)]. and co-workers (which scale. b. The mass spectrum in collisions of protons the same value as the peak observed Lett. is proportional and co-workers [J.-E. of e+e- at SLAC. 33, 1404 (1974)] Another research team working measured electron-positron at the accelerator pairs produced 104 at the Brookhaven National in the collisions of protons 105 rate) for at energies near the mass Augustin et al., Phys. pairs observed on a beryllium the group named the $. Laboratory to the reaction target. [J. J. Aubert Rev. Mt. by S. C. C. Ting The peak occurs at et al., Phys. Rev. . t with nuclei. fully The produced electron and positron using magnetic and positron the pair. fields. From the momenta it is possible to calculate momenta the mass of the object convention we now call the particle the narrowness to perhaps AEAt 2 li/2 be used here to interpret The narrow width the width, and a proton The 4 decays into KR, this decay is allowed. Thus the Jill, quark, This explanation curve and measuring combining of the state. the curvature the momenta the particles K-d are about lo-l2 determined G. Goldhaber The detector the momenta of the particles. the mass of an object to a mass of about the charmed particles s so they travel and was able field caused the particles could be calculated. pointed Just as the strange particles, lifetimes I Detector. A magnetic of the particles decayed into this configuration with accomplishedby that An accumulation would to By have of events 1.86 GeV. are produced only a short distance in pairs. Their before decaying. . See Fig. 5-9. by the strong interaction. proposed. The particle and its antiparticle, in a hydrogen atom. and each K contains barely greater For the newly discovered cannot was formed from a bound together Another much such meson, the #J, a strange than twice J/+, quark or antiquark. In the mass of the K, the mesons that turn into DE. Instead, the charmed contain be equal numbers u and d. was especially quark, this interpretation quark, This convincing of the J/G. of quarks with The charmed that quark, I, must run into each other and annihilate. made before the discovery quark. can Mark of times longer than would takes so long (10s2’ s) and accounts for the narrowness charmed Principle) was first to track the passage of charged particles. for a hadron of such a mass. Uncertainty This using the SLAC-LBL so the quark are called D and the mass of the J/t+b is less than the mass of two anticharmed to verify aspect was processes like Do -+ K-n+. his collaborators as being formed from the strange quark and its antiquark. the mass of the 4 is just charmed decaying pair was to be 70 keV, compared 79 keV, in terms of the lifetime quark (the charmed quark) like an electron found (a form of the Heisenberg was immediately was already understood that decayed into The most astonishing have been expected for a particle An interpretation Ds. that which they named the J (by meant the state was iiving thousands be expected new, fourth the J/G). of the peak, which was ultimately 500 MeV that might The relation fact, of the electron They too found a peak when the mass of the electron-positron near 3.1 GeV. This showed the existence of a particle, usually were measured care- and the directions I when it decayed through was achieved Model 106 that that there of the s. In order it was necessary to show that weak interactions, when the charmed a prediction required produced meson, called D, Fig. 5-9. A bubble chamber picture particle and charge 213. The first pair was c, was needed as the partner was correct of the peak. because it was actually The Standard charge -l/3 c, and the It is this process the and an anticharmed splits into three tracks particle. One particle at the decay point. and decays into two charged particles. directing of the production The particles and decay of a charmed is charged and makes a track The other particle were produced a beam of 20 GeV photons at the bubble chamber. is neutral at SLAC were both created at the left end of the track of the charged charmed particle. Abe et al., Phys. Rev. Lett. by The charmed particles [K. 48, 1526 (1982)] a strange was observed in 107 - . I II 5.10. T LEPTON quarks. The partner of the b, the t quark is the subject of intensive searches at /1/ ’ this time. While the story at SLAC, another, of the J/lc, and the charmed less anticipated phenomenon turn decays, sometimes sometimes into into a neutrino into hadrons (strongly an electron A new lepton, and muon was observed. and a virtual interacting and a neutrino, was being unraveled was observed. analogous to, but much heavier than, the electron as it is called decays by turning particles 5.12. The mounting W boson, which in particles and sometimes into including a muon called for an all-out and a neutral evidence like the pions), current The discovery SLAC-LBL of the r was made by M. Per1 and his collaborators Mark I Detector sults in the creation in studies of e+e- of electromagnetic annihilation. The annihilation energy, which then materializes, as hadrons, sometimes as leptons. T produces an electron and the other a p. The rest of the produced neutrinos and are not observed. with e+p- (or e-p+) When a T+T- observed that first re- sometimes pair is created occasionally It was the occurrence CERN using the one particles are demonstrated that something experiments them, within a fast-moving of the energy of the proton. A better antiproton, constituents Rubbia had been produced. and the discoveries since the primary approach proton, to create and store a beam of antiprotons at the CERN No existing laboratory an accelerator and quarks, of the charmed two families: quark appeared to complete (I+, e, u, d) and (v,,, /J, c, s). two sets of leptons However, the discovery the T suggested that there ought to be two more quarks, which were dubbed expected are antiquarks. 5.11. FIFTH QUARK for their quark. of the beams. The neutrino, In an experiment similar J/t+h, p+pL- pairs were found that resulted mass of about nearby, showing spaced energy levels. Ultimately found evidence of to one of the two that from a narrow the system was rather like an atom the discovery 108 the a mesons were found with A search was begun for mesons containing the search succeeded with discovered meson, this time with 9.4 GeV. As was the case for the $, additional that creation a single electron course not observed. the fifth under the direction in Switzerland. was found With and to collide it with a of S. van der Meer and C. each beam having an energy by looking with large momentum which had the balancing to produce the electrons and and a neutrino, transverse both of which were observed, and similar The observed events indicated the predictions produc- to the direction transverse momentum, was of Similar events were observed with muons in place of electrons. of the Standard that events with and a positron, p+ and pL-. See Fig. 5-10. the W and 2 had masses in agreement with Model. the new quark. contain at outgoing There were as well events in which a 2 decayed into an electron a series of closely of B mesons, which Thus the two beams in opposite directions The W s decayed some of the time into an electron ing events with led by Leon Lederman from an W s and 2s. Evidence b and muons. at Fermilab and an of t. In 1977, a collaboration a quark at but they carry little of 270 GeV there was enough energy in the quarks and antiquarks The discovery machine is to get the antiquark of the antiproton This was done by circulating interactions, of the c and b quarks, but it was possible to modify in the same ring, a feat accomplished new of electroweak a W or 2 it is necessary to collide There are antiquarks it was arranged Model to find the W and 2 bosons. to do so. To produce antiquark. beam of protons. of these very unusual events for the Standard attempt was capable of producing neutrino. just AND Z BOSONS ’ W The r b 109 . I 5.13. I FUTURE ‘!h The successful perimental conclusion challenge and much more. unanswered, While questions of the Standard of the search for the W and 2 does not end the ex- to the Standard the Standard that Model demand tied to the existence of mass. Others of other is correct without periments about annihilation. further is a success, it leaves many questions experimental Some other We cannot high energies at Fermilab Super Collider ulate on the new kinds of leptons, to be found, ‘In the simplest which, and antiprotons energy collisions (SSC) to be built have have a may come from ex- of 2 bosons are created of protons The highest quarks, Model but instead if any, of these models Such results millions version is intimately of the Standard Higgs bosons, know collisions and CERN. answers. versions results. in which may come from a t-quark called the Higgs bbson that experimental to be carried the Superconducting Model have no ordinary new particles. They There remains there is a particle several Higgs bosons. plethora Model. in Texas. by e+eat very will come from We can only spec- Higgs bosons, 2s that may appear when we reach this new domain. 5.14. REFERENCES 1. E. Segre, From X-rays W. H. Freeman, New York, 2. A. Pais, Inward Fig. 5-10. A display of an event containing figure shows tracks of many of the particles and antiproton. transverse events, [UA-1 The lower figure to the direction experimenters Collaboration, Whys. Lett. produced in the collision shows only those tracks of the colliding at CERN a Z that decays into e+e-. beams. By observing were able to demonstrate 126B, with The top of the proton large momentum a number the existence of such 3. R. N. Cahn Physics, 4. Cambridge M. Riordan, to Quarks: Modern Physici&hnd Their Discoveries, 1980. Bound, Oxford and G. Goldhaber, University University The Experimental Press, New York, The Hunting Press, New York, of the Quark, 1987. of the 2. 398 (1983)] 111 1986. Foundations of Particle and Schuster, New York, 1989. Simon \I . I 6. F’urder Explanations quantized; e, can take integer 5 e, 5 4 (’including values 4 2E + 1 values for e, for each e. The z-component This chapter viously provides introduced. of these subjects-we and provide a more detailed discussion of some of the concepts The scope of this book does not allow a full discussion leave the interested a bibliography as a starting reader with the advice pre- two fermions principle s, gives the component to search further the z-axis. point. can occupy system the word “state” electron bound these concepts each possible of charge 2. takes the form the energy of the electron set of quantum in that (n,!,e,,s,). called we can calculate The spin can be either of the system. case of the the possible mechanics, the “wave function”. states, Now let us imagine For as well as until Th e meaning labeled of these quantum by the numbers n is a positive number. It is related to the radial structure states increases with is of the wave adding states of a given n an “energy electrons However, principle, up) with level”. For that there are 2(21+ 1) one by one to a nucleus of charge 2. The by radiating energy in the form of photons, we start filling these two electrons now this level is fully at n = 1). By the exclusion both i.e.spin n and, to a lesser extent, it reaches the lowest energy state (n = 1). The second electron the periodic quantum the z-axis (T= = +1/2, along (sr). but, because of the exclusion Next momentum) down). will work its way down, orientations. in an atom can be completely angular because there are 2t! + 1 choices of e, and, for each of these, two choices of spin orientation first electron the solution the wave function allowed principle, occupied one cannot the states at n = 2, which then we must begin filling n is the principal i.e.spin aligned with e. We call the set of nearly-equal-energy states for as follows: function. (sr = -l/2, spin (internal each choice of n and e we can see from the above definitions state. states of an electron numbers In quantum No mechanical to the familiar for example, of a quantity state in our example The possible In a quantum as they apply We can calculate, to the nucleus of such a problem of matter. is used to describe any allowed configuration states in an atom. electrons to the structure the same state of a system. Let us begin by reviewing of the electron The energy of the electron is fundamental zero). Thus there are /a angular momentum is e,h. 6.1. THE STRUCTUREOF ATOMS Exclusion of orbital of any or anti-aligned The Pauli I the n = 3 states. table of the elements. will do likewise will have different spin (since only P = 0 is allowed add any more electrons to it. can take up to 8 electrons, This is the reason for the pattern and of Note that the energy of the state increases with n and e. integer. For n > 2, the states of higher e for a given n require more energy than the low e is the orbital angular angular momentum can have integer momentum of the electron quantum about number. The square of the orbital the nucleus is !(e+ l)tL2. For a given n, e e states for n + 1. This is the source of the complications region of the periodic of the transition element table. values of 0 5 e 5 n - 1. eZ gives the component of e along the z-axis.* In a quantum theory * The choice of axis is of course arbitrary. We can choose any axis and classify set of states by projection along that axis. This is called a choice of basis. The basis can be re-written in terms of the states of another basis. By convention classify states with respect to the z-axis. (You can choose any direction to be 112 this also is the complete states of one we choose to that axis.) Suggested Explain Student Exercise why there are 8 elements in the second row of the periodic 113 table. . elsewhere. Solution one must build are as small as, or smaller The second row of the table are atoms n= Clearly 2 states. where the outermost electrons are in be localized. inversely Let us count such states: proportional such a state mostly out of wavelengths which ‘/1 , the size of the region in which the particle is to than, For all particles, a contribution I just as for photons, to the wavelength (p = h/X). the momentum of a wave is Thus the kinetic which grows as the region in which the particle energy gets is localized shrinks. n = 2,P = 0 has 2 spin states The spatial n = 2, e = 1 there are 3 z-projections e, = +l, 0, -1, each with 2 spin states. spread of the lowest energy state is that the sum of kinetic Thus (2 states) + (3 x 2 states) = 8 states. and potential Since the probability size is not as simple For an electron the wave function in any given state, represents the relative position. Your students are probably orbitals. Such pictures attempt various quantities of an electron in that probability familiar magnitude of finding the electron at any of electron this probability distribution, or at Given the wave function of a state, one can such as the average position state. of the square of with some kind of picture to represent least the regions where it is largest. evaluate the relative The uncertainty or the average momentum principle, 2 tL/2 distribution momentum square) that in any wave function values about is very peaked momentum their average values. in position values and vice verse. The potential energy is inversely related is to view the localized as talking’of the radius is closer to the nucleus but even with localizing to the size of the region. in the small 114 average size. This is because a particle in a region. but there is also a tiny contribution is actually less than have some kinetic region and to interfere mechanical probability as the average radius define the size distributions. of the electron from adding the electrons. This contribution Even though the is less than the mass-energy potential of a energy due to the electrical W e would have to add energy to remo,ve an elec,tron from the atom, so of energy the electrons are bound to the positively they cannot leave it unless they obtain is what object energy from some outside source. that that when we talk of a composite is charged, energy is quite arbitrary. well defined. additional charged nucleus; we mean by a bound state. in this discussion elementary potential m ,c2(that at rest) because of the negative by conservation That the mass of the nucleus, energy in the localized states, the energy of an electron the separation The total W e choose to say that energy. wavelengths, such as the proton or the electron destructively (electric) surrounds this W e must the sum of the masses of the electrons. have zero potential One way to understand of waves of different This of a ball. Most of the mass of the atom of course comes from Notice state as a superposition chosen to add constructively its 2 state has a non-zero energy associated (or rather values, then it must be more spread out in energy is lower when the electron so, the lowest energy electron there is a kinetic If a wave function and the distribution. free electron there is a spread in the position does not have sharp edges, specifying W e choose to define the size of the atom interaction. is the statement for of the atom using the language of quantum electrons , gives a minimum energies. in any of the bound states is less than ApAx which field that energy in a given situation define the to include or even an of energy into mass-energy the separated Thus we object, positive mass-energy and negative and is always charges of-a charged particle the energy stored in the Coulomb it. Now consider what happens if we bring a proton 115 . and an electron together to form a hydrogen cancel one another, so the total of the atom, is just which to define the potential zero for the separated energy differently potential fields Thus the mass by c2, is less that and electron. In other energy as negative words the sum of we are forced in the atom once we have defined it as W e are not then free to define the zero of potential in the two cases. There is real content energy is negative Coulomb energy divided proton objects. Some of their energy of the system is reduced. this total the masses of the separated atom. in the atom. to the statement It is the reason that atoms that the are stable objects. the residual strong (Note the same form appears here as in the weak ‘/1 case, the only difference is the mass of the exchanged particle.) For the interaction interaction. protons there is also a correction this interaction STRUCTURE O F THE NUCLEUS The same kind of quantum mechanical factor of twenty). The kinetic contribution object so we treat interactions between color-charged potential the problem nucleons-that constituents. than the Coulomb description and the exclusion principle Now there is no massive in terms of positions interaction is, the protons Th is interaction potential relative to the center is due to the residual strong and neutrons-because provides of the previous The wave function of their a much more complicated problem. dynamically determined widely separated closer together, nucleons. tending Again, potential, although we define the potential Here m is the mass of a pion. interaction potential This energy to be zero for energy is negative zero for large separation V(r) a when they are r as is that which minimizes energy in a because exchange of pions provides The size of the system between the lowering energy as the system gets smaller. the total for is again of the potential The stable size energy. solve for the energy levels of a system of nucleons. due to the Coulomb repulsion sets of states - one set for neutrons, 2 protons and (A - Z) neutrons, between Ig- the protons, we and one set for protons. In nuclei the lowest are approximately protons the residual strong the longest range part of they fill these levels starting from the lowest, in the same way that the electrons fill levels in an atom. energy for a given number, equal numbers occurs when and protons. However, of neutrons term falls off more slowly with distance are at somewhat than the residual correction. of the exclusion principle 117 there since the strong interac- Hence the levels for higher energy than those for neutrons e. Thus larger stable nuclei have a small excess of neutrons The working For small A, of nucleons tion term, for larger nuclei it becomes a significant mass enters in defining distribution Again we can define the size of the nucleus by the competition W e can (in principle) Coulomb -e-mcr/h r . 116 potential we do know some less than the sum of the masses of its Then the potential towards to the negative of the distribution. energy and raising of the kinetic have two identical the nucleons. to the mass of the nucleus, however, as in the atom, of a nucleon in that state. a nucleus containing constituents, in the nucleus In fact we do not of its properties. The mass of the nucleus is somewhat and neutrons for each nucleon state provides a probability noring the small correction yet know how to derive the nucleon-nucleon energy of the protons a stable nucleus. inside the nucleus. Here the attractive nucleus (as the chart shows, it is weaker by about is small compared as some average property central This is a small potential strong positions of mass of the nucleus. more spread out than the neutrons. than is the residual the location and neutrons interaction; term is much weaker at the range of the typical apply again when we go to the much smaller scale of the nucleus and ask about the of the protons due to their Coulomb effect because the Coulomb also gives a small contribution THE to this potential is repulsive because they all have the same charge. Thus the protons in a nucleus tend to be a little this positive 6.2. I at the same n and over protons. at the level of nucleons explains the . patterns of stable isotopes and thi Consider, protons for example, are filled, a nucleus in which decays of one element all the states cannot decay via P-decay. A free neutron but in this nucleus one more proton heavier object. at a given energy principle, have to be put into a higher energy state than that occupied would make a would by the neutron. Thus at least the nucleus formed by adding one more proton be heavier than that formed is heavier than a proton. lower-energy by adding one more neutron, This is because the additional state in the nucleus than is available Now let us consider higher energy than a nucleus in which the lowest-energy can become a neutron by emitting inverse P-decay or p+-decay. proton a nucleon. Again there is some wave function size of the proton something that even though neutron state available proton that these constituents This decay is allowed This proton cannot and negative elsewhere. the fundamental strong is lighter than a isolated neutron type of potential. infinity 0 but, unlike them, as T + How can this be? For energy to be zero for two infinitely This definition makes sense because we and define their masses in that way. For we must make a very different Like the previous choice. strong interaction This we have two cases it does go to minus as r -+ 00 the color-potential energy behaves as switched to a neutron. V(r) the ar . An energy grows without limit as we attempt to separate and hence, by energy conservation, because of the energy in the color-force field between them. There is decay via inverse P-decay no way we can define the potential Other radioactive latter is greater yet ,we have said elsewhere the nucleon. is forced on us because for the fundamental a very different the quarks, cannot objects interaction In other words, the potential isolated proton quarks, escape from can indeed isolate the constituent now we notice The mass of the proto,n or neutron cases we defined the potential objects However The and a v,; this is called because the nucleus with is heavier than the same nucleus with the proton separated difference proton. is in a state of for a neutron. (anti-electron) a neutron can be put into a for the additional the outermost a positron to this nucleus will is some average of this distribution. seems very strange. of quarks within that describes this distribution. than the sum of the masses of its constituent the two previous against P--decay. Furthermore, Now we go to a yet smaller scale and consider the distribution In this the proton inside such a nucleus are stable, and so is the nucleus itself, 6.3. THE STRUCTUREOF NUCLEONSAND OTHER BARYONS for is heavier than a free and one less neutron This is because, by the Pauli Exclusion the neutrons to another. but not all the states at that same energy for neutrons. nucleus, a neutron proton radioactive I is actually decay processes are spontaneous fission too, where one of the resulting which is called an a-particle in this situation. fission and o-emission. nuclei is a helium Spontaneous The nucleus fission occurs because typical separation contribution the major of quarks within so that it vanishes at’infinite a nucleon is such that to the mass of the nucleon is small. contribution comes from the kinetic Unlike separation. the potential The energy the two previous energy of the constituents. This the energy per nucleon in large nuclei is larger than the energy per nucleon in small is because the nucleon is much smaller than the nucleus, and its constituents nuclei in which all the protons light. and neutrons can be in low n levels. By rearranging the many nucleons of, for example a uranium form of kinetic energy of the fragments between the Coulomb residual repulsion strong interactions and photons of the photons determine nucleus, energy can be released in the (y-emission). and the attractive The competition The mass of the proton proton and hence the typical 3h/r are where r is the radius of the wave length of a quark confined inside a proton. factor of 3 is because there are three quarks, each with average kinetic The energy h/r. forces due to the the rate at which nuclei undergo fission. Having the quarks. 118 is approximately cases said all this one might well wonder W e can compare two otherwise 119 what similar we mean by the masses of hadrons that contain different . I quarks; these hadrons have the samlk potential and kinetic their masses. Hence we can use the differences the differences lightest between quark quark such hadron out that individual quark acceleration masses can be defined of the quark by (immediate) the chart are defined actually measured response of the quark. because this response masses given on in this way. The way in which the values of these masses are is quite technical, but it corresponds (You may find elsewhere that a quite different the strong The quark interaction potential to the definition definition energy and the kinetic given here. is used, one which includes of physicists and the masses that the masses we defined are called “current” include the interaction energy are called quark masses, “constituent” quark The proton the d-quark, is made of three quarks uud. the proton three the proton. principle quarks students particle protons that u-quarks, is the lightest has spin 312 and is, in fact, Since the three quarks in a baryon symmetry requirement a u-quark three quark object. alone is not enough to explain fundamental Although The A++ @(Xl have different heavier than which than colors the exclusion this fact, but it is a consequence of the same that is the reason for the exclusion for two fermions 9 X2) particle considerably * The exclusion principle is a consequence of a property required the wave function of a system containing more than one of the property is that the wave function must be a,ntisymmetric under the same type of fermions. This means that the wave function all the coordinates of the two fermions. (Boson wave functions Now consider a wave function one separately is lighter = fh(~lM?2) 120 as a product by quantum principal.* mechanics for same type of fermion. This the exchange of any two of changes sign when we swap must be symmetric.) of the wave functions of each physicists and. other regard quarks as very real objects hadrons. can never be observed are likely to be bothered is never seen except However in isolation. by this. How something apart predictions that as their electric Rutherford depend in detail discovered the nucleus within so high energy experiments today The second answer is perhaps a little quarks is a matter improbable. of entropy-the Let us consider rapidly by scattering probe the structure deeper-the situation purposes, zero. collisions, Between a can makes many quarks, such Just as a-particles off within the nucleons. unobservability of separated but merely extremely for comparison-say a room room where all the air of the ceiling, The state is so highly apart. Model are borne out in nature. one centimeter is, for all practical moving else? This question state is not impossible In high energy electron-positron an antiquark can we call something there exists a possible state of that observe a room in such a state. quarks. among of the individual the atom a more familiar molecules just happen to be within its occurrence The philosophers you do not always have to take on the properties charges, and these predictions can several The Standard to observe its constituents. that it has been stated as a part of something be answered on two levels. The first answer is that full of air. In principle masses. ) contains your inside that atoms, energy due to confinement as part of the quark masses. This gives much larger masses for the u and d quarks. In the jargon be found times disturbance. happens on a slower time scale. Hence the mass of the quark itself is what controls the instantaneous By now it is clear that the of the probe means that we can ignore the response of the color- force fields to the instantaneous 6.4. CONFINEMENT masses to find (in the sense of F = ma ) to a very high frequency The high frequency to masses. This still leaves the puzzle of defining mass. It turns how they respond between energy contribution I ordered but you will never that the chance of The same is true of separated we often produce a quark and them there is a region of color force The rule for fermions is that Clearly this cannot be tr,ue in the example above if $1 is the same function two fermions cannot be in the same state (i.e. have the same individual as $2. Thus the wave functions) because antisymmetry of @ requires +i to be different from $2. In a three-fermion state this same rule of antisymmetry applies when any pair of fermions is swapped. The color-neutral three-quark state is antisymmetric in the three color labels. When the three quarks have the same flavor it is clearly symmetric in the flavor labels and the lowest state is spatially symmetric (! = 0). To maintain overall antisymmetry this requires it to also have all three spins aligned the same way so that it is also symmetric in the spin part of its wave functionhence the 3/2 spin. 121 . field. There is enough energy densi& and antiquarks. additional There are many in this field to produce further more possible pairs than there are states for the system with the color-force-field. more improbable The further no additional always observe is the system rearranged and antiquarks, room. (that Highly is, hadrons,) improbable such are separated pairs are formed. into color-neutral just with all that energy stored in apart the quark and antiquark it becomes that pairs of quarks states of the system Thus what combinations we of quarks as we always observe the air filling states of a system are simply the the not observed. While for each pos.sible spin combination antiquark pair state, the light neutral quark and antiquark. For example that some probability that There fj or an 9’. In the standard and gluons. describing When model, every hadron we say a proton the simplest can emit combination and absorb gluons, internal composition basic constituents complicated however, object is all that strangeness. quantum just a color-neutral is required three at any instant made from quarks quarks, will is forever changing; we are merely Because the quarks contain quarks and the corresponding some gluons antiquarks. The simplest content its basic properties of the hadron gluons,or the overall quantum The rules for mesons are similar is a more of a hadron, such as charge and the charge and strangeness, For color charge, the rule of combination is that one of each of the three possible quark-antiquark numbers pairs are always formed are unchanged. to those for baryons. mechanical by observation and are not easily explained, situation Any color-neutral is a little complicated. for neutral For example, 122 for each meson or even fully understood. in this way, thus the nc is almost pure cE, and, likewise, the nb is almost 6.6. COLOR AND COLOR NEUTRALITY The SU(3) interactions. factor in SU(3) which objects that object. This analogy chose the term “color” of a quark plus mesons such as the x0, the there is no meson which is just UE. of the group SU(3) One often hears an analogy colors which can be combined for the strong that it is a very limited color neutral objects. is no fixed the greek letters are not written convention moment that call the three quark also explains between the fact to make white color charges combine (no to a color interaction charges, but it must analogy that does not extend to explain for naming a, p, y, so that at all-so strong may in fact have been part of the reason the Gell- remembered There re f ers to the fundamental is the reason that there are 3 quark colors and and the fact that three quarks of different neutral Mann primary x U(1) The mathematics can be color neutral. there are three color), x SU(2) The 3 in this formula 8 = (32 - 1) colors of gluon. One can find the electric combination pairs pure b8. and so on. More often, in fact, the color-charge and antiquark. be an masses, the heavier quarks do not get significantly constituent is a possible meson. However, it will is not one to one. The precise probabilities write quark sense, there is equal probability if you form a spin zero ?su state, there is charge and the strangeness of a meson by adding the charge and strangeness of its an antiquark of uzi, dg and SS. These mixtures The are given by combining For electric can be made by taking mixed of ~21 and d;i find a ?r’, but also some.pr,obability you will Because they have such different only the flavors of the three In other words, a real proton to explain numbers The additional object can give a proton. of its basic constituents. baryon colors of quark. that three quarks. means adding. in such a way that is made from are unchanged. than just The quantum numbers combining of a proton (quarks) is a composite the proton and perhaps even some additional are found than one pair of pion, ?y”, is a mixture are mixtures Conversely ofmore are three spin zero states for the three quark-antiquark but the correspondence 6.5. THE QUARK CONTENT OF HADRONS the neutral in the quantum the a0 is a ?iu or a zd. there is one meson state for each quark- mesons are mixtures while the n (eta) and n/ (eta prime) are to be understood I we say q” ijq the quark colors. Physicists means a quark of one color, labels are suppressed-that is a color singlet colors red, yellow 123 qfl and blue. object. be other often another is they Let us for the Then the color-singlet . I d quark anti-quark anti-red), combination is aciually and (bl ue and anti-blue). (yellow and anti-yellow) is equal probability at any time an equal quantum that the color neutral mixture of (red and That means that there object is any one of these In order to explain of virtual three combinations. particles, Field Theory When a quark emits or absorbs a gluon its color can be changed. emit and absorb gluons very frequently within the color of an individual quark-that is why we generally denote the color changes. Imagine red and anti-red. we start with Since quarks there is no way to observe do not even bother a quark-antiquark This cancels its anti-red Repetition of similar proba.bility of being charge. Now the anti-red cha.rge and leaves it with process explains (yellow a gluon that quark absorbs this an anti-yellow how we can have an object (red and anti-red), t,o pair tha.t are The red quark becomes a yellow quark by emitting carries both a red and an anti-yellow gluon. a hadron, and anti-yellow) that charge. has equal and (blue and ant.i-blue). To explain the eight gluon charges we can say that a gluon carries a combination anti-yellow However, as we have just of (red and anti-red), in fact color neutral. (yellow stated, the combination and anti-yellow) that is a equal and (blue and anti-blue) There is no gluon that is color neutral, is not a possible gluon charge; this explains there are nine so that is combination why there are only eight, rather than nine types of gluon. The quarks, baryons obj&ts whose basic quark (If the color group were SU(n) structure is three instea.d of SU(3) then be n types of color charge for qua.rks and one would need n quarks to form a color-neutral baryon). Color-neutral objects can also be formed from any number of gluons except one. One can add any number of gluons and any number of quark-antiquark are a verbal theory is not whether predicted t,his test the standard the calculations pairs to any color-neutral system to be color-neutral. object and still arrange the overall thinks new ways to calculate is matched The tiathematics physics student, students find this description special to write relativity a theory that contain gravity. to find a formalism Standard They The real test of the the behavior of the experiments. area of current research is to learn in the regimes where the usual Model tests of the theory against is too complicated for the so this book can only give the verbal description. peculiar or hard to understand descriptive belongs to a class of mathemati&i for physics. mechanics. The aim of much current will allow theories theoretical that incor- particle a theory that how is gravitation-and is why the standard us to write approach. No one has yet learned general relativity-that That When it may help to remind more than an intuitive, also incorporates In successful, at least in the regimes where of the Standard model does not physics research is includes both the theory for Model and general relativity. The aim of the game in particle which by the outcome field”, formalism. the test is whether of the theory and quantum that are due to the of the electric the calculations. made. ( Another them that it is based on something Model model is written. mathematical and hence to develop further beginning porate about the predictions is not reliable, The Standard is the quantu; and called field theory. says “inter’&tions of this model is extremely can be reliably the predictions in all experiments. match physics is to find one particular the observed The mathematical least under certa.in conditions) 124 or “the photon interpretation by the calculations and annihilation of the formalism the words sound plausible; yields sensible predictions are color-neutral one of each color. there would particles”, creation language in which the standard to recognize that when a physicist describe the way the physicist experiment.) above we had a gluon wit.h red and charges. Since there are three colors and three anti-colors combinations. mixture as in the example we need to review a little exchange of virtual these words the key concepts of particle is the mathematical It is important method of a color and an anti-color, .!/, 6.7. WORDS AND PICTURES FROM EQUATIONS particles formalism and their contains behavior as found rules for calculating (at the expected rates of various physical processes such 125 . I as particle decays or scattering into this formalism, example, physicists interpret (that us explain various to explain field of various this further. that In a field theory, and bosons. energy For of position and time. can result particles and pictures that emerge each field at each time. We begin by depicting from in the outcome are viewed his- observed. Let as excitations the history use energy and momentum Of E total = m4c2; For ts E tota, = (r&c4 process. has two di- + p:c2y2 processes involve tures display represents one. Fig. a process in which conservation. Pl =- p2; Doing a little algebra IPll = (7g4/4 - m&2 ‘DC Pofmon Fig. 6-1 back. This process only be allowed conserves in a theory in which A, sitting ~0, decays at time transfers its energy to two particles B which as well as energy. This process IPll = to would do not carry any Histories involving Paul Dirac first the electron, found he found fermions Student (first Exercise 126 (r-n;/4 = to interpret that to his surprise the positively by Oppenheimer) are governed an equation one but two types of particle first Suggested -p2 - rni) w c to and move off back A and B are bosons that charges. at rest at position of type momentum of type one finds: a A particle + p;c2p2 Ptota1= Pl + P2 Use energy and momentum of space. The pic- + (n&c4 and and the other for time. here only Ptotcd = 0 were in of very simple real of each B particle. Before to: we use one of those to dis- course, to find the momentum Solution three dimensions 6-l conservation to the fields as our page only play position - , of the type. show where the excitations mensions to ‘!h mass- the all possible field for each particle Because Time and the relativistic E2 = m2c4 + p2c2 as a way of calculating which traces out what happens The diagram the mass of B = mg, relation After a function the mass of A = mu, for the wave function contributions and there is a separate we can draw a diagram Given are built principle. introduced processes by summing fields of the theory, each history Exclusion Feynman is, sequences of events) fermions by field theory some of the words theories. laws of physics that distinguish required of the Pauli now try probabilities tories properties the explanation We’will The conservation as are the properties the symmetry provide eveits. I that with by a different could describe that the theory equal and opposite charged particle the equation 127 a spin l/2 particle automatically electric as a proton. represented set of rules. When such as contained charges. He tried not at It was soon realized two particles of the same . mass, which ruled out that interpretation. The remarkable of the electron, even more kind prediction the positron, striking of fermion, partner, The equations of the equation was discovered in its implication. the equation an antiparticle. was later just a new pa.rticle! the prediction for the electron, an equal mass but There were only two possibilities; the production was but for every oppositely charged either Now let us redraw our picture Time The antiparticle in 1932. However Not demanded predicted confirmed. the equation charged the wrong motion one or every fermion have confirmed of fermions, that and that must Dirac’s for every have an antiparticle equation fermion partner. is the correct type there is another one line ,, A of the particle Furthermore the opposite it has been found equal mass but opposite complex conjugate is its own complex charge. This all charged conjugate; We call this Such bosons are represented similarly in field theory is a boson with by a single real field. one can say that particle by a of the A- creates a B particle and its Now consider a slightly more complicated history. Here is a pro- Any cess where initially there are two B moving right and a B real field between particles-a particle moving left with equal energy. that is, they disappear all their transfers Fig. 6-3 now, in this example, can say that A-particle. initial the ?? is moving right the B and B have scattered (Scattering particles the momenta error on the wall chart makes it read incorrectly on this point. If you change the outer parentheses in the paragraph titled “Matter and Antimatter” to commas it reads correctly; i.e. only the specified neutral bosom are their own antiparticles. A neut.ral meson which has non-zero flavor charge, e.g. Ii’ = &, does have a separate antiparticle. ing interact; is the particle the resulting are changed). carefully about It should satisfy E2 - p2c2 = the energy anew B particles to. B back by creatparticles, but left. Thus, we with the for any process in which the may be the same or different, sensible until A photon 0 for a photon) 129 time interaction of the A as a photon. m2c4(= and and the B moving physics jargon at en- late’r’,‘at tl, the A-particle because of their All this looks perfectly the interpretation and transfer to an A-particle Sometime Positi& They meet and “annihilate”; ergy 128 now be thought In the picture zero value for they are the same object.* * Note that an editing The B. such a boson is its own no charges, there is no distinction could B. Po#lMon Fig. 6-2 of that is ‘that we have labeled as a photon. its antiparticle. bosons also have antiparticles The one exception can be represented For a boson with and antiparticle, that pair of fields. all types of charge. antiparticle. value for all charges. are The pic- B and the other A-particle I of =0 equal mass with or are fermions. has changed Subsequent description the B particles ture has not changed much-all was of of a pair of particles in case in which to experiment I you think and more has zero mass. . I (This is Einstein’s in this diagram, mass-energy relationship for moving Fig. 6-3, has zero momentum a photon? We will agree that to go with the picture. and momentum it cannot be a real photon It is a “virtual” to be a real photon, photon. particle but it is somehow A excitations is a real photon. and hence travel long distances between really its energy, means is that very tiny, that such a history written zero, which would in terms of energy occur. In this context and time instead right to -- that time. What principle is virtual can be photon, energy sufficiently to determine process. These objects real particles. is simply of the overall precisely They that are called histories one cannot measure there is a difference contribution from between energy, momentum, “virtual” involving rate of processes; At that between that diagrams are possible. to different particles. these calculations In Fig. choices of photon, are part give results that which contribute In fact, here is a third agram, E2 to because to the Fig. the virtual A:,y,article sorbs This it. type of di- 6-5, where the B emits and the B ab- contributes to the same process as the others. such things Particle physicists of the calculations these diagrams that agrams are confirmed simply tion of quantum distorted mechanical into one another processes. by changing Notice use a version called Feynman as an aid to calculating scattering. Fig 6-5 Now, let us now draw a couple more diagrams: t, 6-3- the there are many cases of each, corre- probability n 6-4 the net re- to form a virtual to and tl. All are histories by experiment. 130 its direction scattering. and mass to be The sense in which ‘these objects rate of BB Furthermore its history do not last long enough to be observed, they do not have the right relationship exist that 2 c 4, then there can be a reasonable andpZc2+m the overall accurately time and is ab- B and the B have scattered. Here . they have done so by exchanging a whereas in Fig. 6-3 they annihilate 1 h/2 lasts for such a short !L? particle into the later it creates new B and B particles. total the particle later bumps and momentum sponding When mov- A particle sult is the same as in Fig. Fig 6-4 Both AEAt - this for such a particle moving of motion. Podtion it is very improbable particle sorbed by it, bhanging with the wrong relationship the uncertainty of position The A particle h -- Thus the ?he’B and itself bounces back to the right. which can last a long time from long histories is the same as saying it is possible scatter- ing left creates an virtual energy in this theory. and mass only lasts a very short the net contribution essentially ing process. a set of words If E2 - p2c2 = 0, then is a stable object Here is another can it be to the photon; the photon in space. An excitation momentum and invent related into two types. Such a particle The A-particle energy-how It does not have the right a lump of energy stored in the field that represents we have now classified particles.) but non-zero I Dithe of a process such as this Feynman diagrams are a short hand for the calcula- the Fig. 6-4 and Fig. 6-5 can be the order in time of the processes. 131 of (In 6.4 . I 11 < whereas in 6.5 to tl > the histories that well-measured positions in spa&. process discussed to the process. is simply momenta, Fig. diagrams and not their position contribute ciple in this context Feynman to). energy and of the particles, that, specify only the momentum at each time. The meaning when we calculate the histories which of the uncertainty all prin- the rate for a process with contribute 6-6 shows the Feynman and They represent come from many different diagrams for the BE also show why can view these pictures the B particles. the Einstein of Fig. 6-6 in- choices of 5. The second all choices of Student are rules that lation of a quantum each Feynman Show that ability Fig 6-6 Feynman formula photon contribution to mc we can rewrite + terms of order (p/mc)4 which gives the usual (low energy) version of E = rest energy they each other-but annihilate to some other field. of electromagnetic diagrams * of Fig. their For example, fields particle and antiparticle energy energy and momentum and momentum and create a charged particle rewrite the expression as .,.: series expansion of the square root. (1 + Z)l’2 = 1 + x/2 - x2/s... say be transferred can be transferred and its antiparticle. out The 6-4 and 6-5 The diagrams All the peculiarities of quantum behavior come from the fact that this is the square of the sum and not the sum of the squares. The latter would correspond simply to a separat.e probability for each history. However, because we add the contributions and then square, there is interference between the various histories. 132 energy. to all possible can disappear-we must + kinetic Solution then make a Taylor we have seen that this formula E = (m2c4 + p2c2)lr2 g mc2+ p2/2mc First for the process.* In this section, For a moving is E2 = m2c4 + p2c2. E = mc2(1 + (p/mc)2)1’2 diagrams famous for The prob- of a given process such as corresponding the B and of Einstein’s at rest. One represents Diagrams is given by the square of the sum of the contributions between a generalization is only true for a particle when p is small compared 6- define a calcu- diagram. of a virtual exchanges. Exercise to and tl in Fig. 6-3. There as par$icle to and by Fig. diagram interactions as the exchange equation tl, both those represented by Fig. 64 and those represented about scattering Suggested cludes all possible talk We have a!so introduced E = mc2. The well known particle, above. The first diagram physicists I in a field that also introduce the notion does not have the right energy-momentum only gives contributions from very short-lived virtual as an intermediate particles appear of a virtual 133 histories. particle. It is a disturbance relationship and hence In the Feynman diagrams stage of processes. This explains the W-boson, how a neutron which heavier than the mass difference decays to a proton electron travels energy faster than which between the speed of light distance Fig. even though weak field is much and the neutron. 6-7(a) shows two The neutron shows possible Udd the time Feynman histories Diagram represented uud for tra quark Since the W is very heavy compared to hadron. Furthermore, travels particles a very short since nothing distance. have a corresponding udu of these uud uud udd of an exwithin this quark combination When the the other hadron annihilated by one and another leave the hadron; son. udd Each and antiquark antiquark dud interaction r like 6-7(b) The antiquark quark This diaglam: Fig. can be seen as production an time. process. by this produces short I this and then the W-boson it only by massive the charged the proton neutrino. falls off with with W-boson it lasts an extremely is why processes mediated potential decay via a weak interaction associated and a “virtual” and an anti-electron the available tin is a particle . I L, forms a me- meson the arrives at antiquark is one of the quarks -mcr/h V(r) Note that for a massless particle the electric and gravitational massless (or very light) we know that particle the gravitational a Te in that . r jj!, this corresponds to the familiar We know potentials. even though potential that l/r the graviton behavior must it has never been observed, behaves as l/r of ,i;i,, dud udu be a RESIDUAL STRONG INTERACTIONS between them. they interact hadrons This is not so strange with one another occurs because of residual the charged constituents molecular two atoms. swapping are color-neutral tions, binding there are residual - atoms are electrically and bind to form molecules electrical interactions, are distributed within neutral or crystals. objects we can view the residual the hadrons. but the binding atom. strong interaction as meson exchange. forces in the nucleus still applies. we can describe between there is a significant the interaction between meson exchange However, overlap them when the hadrons between their wave func- more directly, in terms of their constituents. 6.9. DECAYS OF UNSTABLE PARTICLES We can view the between as due to quark In the standard proton are stable proton is not explained model, stable, earlier, In some extensions though it has a half using Feynman diagrams. model predicts life of greater that happen state compared unless the initial 135 to that even the 1O25 years. neutrons. As For all are also calculated of these are the type of interaction of excess mass in the initial a decay cannot than they contain decay patterns and the model, The rate of any decay process is controlled the two most important the amount plus the electron of the standard nuclei can be stable, even though the standard Clearly only the massless particles particles. other particles factors; 134 the same thing that is exactly This binding which in turn result from the way the neutral hadrons be- interactions as being due to the exchange or the sharing of electrons Similarly between objects, separated The old view that explains we see that dud Fig 6-7 interactions Even though Thus the process of quark interchange because to a very high accuracy. hadron. tween udu come so close together 6.8. Cb) by several involved and in the final state. state has more mass than the final . . state. When the excess is small dependence is complicated, and their individual on all the other momentum, Let us start strong decay processes This interactions. in about of quarks allowed decay, is equal provided the branching the standard model fraction of about to the number energy, angular fractions should level. The example There Any 4% of all 71~decays. decay are processes involving any Quark flavor other by the of a strong occurs are many 4%. final decay when possible That state means that in which of antiquarks momentum At present for each flavor gives an and all other we do not know how to predict such predictions. heavy baryons Some other in principle examples such as the A++(1232), Nuclear emission fission, + p(uud) heavy of a processes generally hadron, but no matching antiquark, then that flavor No quark of quark Electromagnetic the pion the quark so that and then the quarks there is enough the initial baryon and antiquarks energy into a lighter available baryon find a way to rearrange to make the virtual plus a meson. must be 136 the original themselves pair real by splitting The half- strong of energy. Such strong decays, partly ones and partly Because of this it is very difficult strong to pion decays to two photons and antiquark within is no possible process allowed. by an electromagnetic the pion annihilate is not an allowed or angular strong transition to produce in which the photons. decay because that momentum.) decay for it. cannot (The conserve The rate of this decay depends on it is proportional to the square of the number In a model with no color this process would,be life of the vc is than in a model with expected three colors of quark. 77 is one of the key pieces of evidence for the standard to proceed Thus the rate model. since it is nine times bigger. A more familiar An excited ground hadron, (Y involved. meson there the quark charges, and furthermore in the gluon within (including of the nucleona in the residual in cases where there is no competing is the lightest energy and momentum, virtual field nuclei allowed by conservation fast on human scales). decay into a single photon than is produce’d mass differences still extremely The neutral In each of these processes an addit,ional pair very rapidly and weak decays proceed much more slowly than strong decays observe them except found in one or other of the final hadrons. quark-antiquark into two smaller strong forces are weaker than the fundamental because of the smaller flavors change in any strong decay process, so if there is a quark of a given flavor in the initial proceed occur much more slowly than fundamental because the residual (although a nucleus splits and occurs whenever nine times less rapidly to two pions. interactions 10Pz3 . These e.g., + ,+(,a) meson decays such as a p decaying by strong is due to rearrangement potential of ?ro + and similarly in which processes) of quark colors. A++(uuu) decays mediated of apprqz$mately the conservation, decays, although has a half-life indeed. the c final 10wz2 , of the A(1232) show that Because of all the various provide numbers interaction any decay mediated each other. has a branching rules can be satisfied. accurately at the quark means that if it violates occur. is the decay of the 71~which the qc annihilate The one shown number It also depends approximately in the decay, such as angular cannot does not change qu,ark flavor. The precise in the final state mass difference. decay is one which by analyzing within it is observed of particles must be conserved on the wall chart and Z quarks strong A forbidden interaction conservation which more slowly. charge, and so on. We call a decay forbidden in strong process shown states. masses as well as on the total law. is conserved it depends on the number quantities electric conservation the process proceeds I atom is one with the lowest state a photon. example allowed when its outer electron state for that the electron Because each electron (and hence frequency) particular of electromagnetic states occurs. decays occurs in atomic in a state which electron. The excited makes a transition state has a definite Conversely, energy, the atom can absorb 137 has higher atom and emits the photon between a photon energy decays to its to the lower state will be the same any time a transition processes. energy these two only when it has the correct This is what energy to excite an 4ectron is responsible from one allowed for the characteristic emission . I I 6.11. PMWICLE THEORV IN ASTROPHYSICSAND COSMOLOGY state to a higher one. and absorption spectra Particle of atoms. physics trophysics Weak decays ,of hadrons, bosons as virtual the chart emitting intermediate quark particles. is the classic weak process. a virtual other quark When in which is changed, The example Heavy quarks always of neutron involve p-decay the initial hadron the W decays it can produce The diagrams causing either that all decay to lighter a lepton quark W- quarks by by some also to change its flavor. and an antilepton or a quark of Fig. 6-7 show some weak decay processes. requires some spin zero particles weak and electromagnetic mechanism for giving and Z bosons. They who proposed them). particles, no direct new accelerator their are necessary masses to the fundamental are known as Higgs bosons (named In the standard which model go beyond fermions evidence the Higgs bosons the standard proposed for Higgs bosons. for construction to provide a and to the W universe. Both cosmology at early in the evolution of the universe energetic particles observable observable Thus, scales becomes objects. number of good popular Cosmic Code: quantum tam, present. important inside stars or there way, the physics to understand books have been written prop- on a full are so hot and dense that in a fascinating We do not attempt physics the evolution observed depend The conditions processes. scales. Asor even clus- and tries to explain and astrophysics particle of the behavior of here to discuss this subject. A including as a language of nature, the following: by Heinz R. Pagels (Ban- 1984). Longing Devine for the Harmonies: and Frank A Short History are fundamental model they fermions. are com- So far there is One of the goals of the SSC-the in Texas-is what but do have goes a step further baaed on the laws of physics and the presently of the underlying times ‘/, on the tiniest we can observe such as galaxies understanding the smallest The of the universe themes and variations Wilczek (Norton, from modern physics, by Betsy 1988). after one of the theorists of some new types of fundamental and, we hope, to learn much more about studying have no color charge nonzero composed experimental which we have not yet mentioned. These particles properties. in some extensions posite objects model which objects and cosmology erties of the visible the largest There is one part of the standard the structure the largest of the entire universe are many 6.10. THE HIGGS BOSON model studies ters of galaxies shown on W. The W may then itself decay or it may be absorbed within and an antiquark. flavor studies to find these Higgs is beyond the Standard bosons Model by of Time:jrom (Bantam, 1988). The First Three Minutes: Weinberg (Basic Books, the Big Bang to black holes, by Stephen W. Hawking a modern New York, The Big Bang: the creation Silk (Freeman, San Francisco, view of the origin ojthe Universe, by Steven revised edition, by Joseph 1977). and evolution Calif., ojthe universe, 1988). properties. The Left Hand of Creation: John D. Barrow 138 the origin and Joseph Silk (Basic and evolution Books, 139 of the ezpanding New York, 1983). universe, by 7. Bibliography 6.12. BEYOND THE STANDARD MODEL While most now understand this theory today of this book through methods would of the predictions and find by investigating theory. The second standard model, remaining where that by a single grail that comparisons which with do not it in some way that sought a unified physics. It is level of the some of the the most important We want A classical data to probe incorporates George: 1965, Cambridge predictions answers theory Gamow, improving fit correctly. which General that research one finds the clues to the next of particle unified Better Model. is to seek a theory Einstein physics with 7.1. physicists to recognize is concerned are these questions ? Perhaps What the holy any, free parameters are details that Particle of the Standard goes beyond masses and gravity. remains completely there such details questions. understand it is important to further area of research but that the things as yet unanswered. allow the model to be subjected always that questions model, into two main areas. The first of calculation its accuracy to explaining the standard leaves many can be divided is devoted 7.2. contains many the world preferably one that are adjusted arbitrarily to fit the data. for example Model. Although are independent we can accommodate not in any sense understand generations are still by further Standard of quarks. further Model The Standard generations experiments the pattern with contains Close, Frank: The Cosmic Onion Institute of Physics 1986, American A clear and succinct few, if The standard not predicted cannot Model, tell us whether quarks will and charged we do or not there leptons. allow us to go beyond and find answers to these and many other Close, Frank; Martin, i 1, Michael; The Particle Explosion 1987, Oxford University An excellent introduction and Sutton physics. Christine: Press to particle many color photographs. physics, Contains with an attractive good material layout that on experimental physics. questions. Only the Crease, Robert P.; and Mann, 1986, Macmillan An accessible joining Publishing history Ne’eman, Yuval; 1986, Cambridge Charles C.: of particle physics, successf~~ll~~ physics to the lives and work of its practioners. and Kirsh, Yoram: University Press to particle The Second Creation Company of the development the ideas of particle An introduction 140 to particle mechanics. by the Standard of masses or even why there are repeated can we find the clues that introduction and quantum to described very quark masses in the Standard yet heavier to the ideas of relativity the masses of all the quarks parameters Model introduction Press Level particle and the charged leptons University in Paperback of all interactions- theory, such parameters, Tompkins Introductory includes model Mr. The Particle Hunters physics for the general reader. 141 . \I 7.3. Intermediate s Level Grifhths, Harper Cahn, Robert N.; and Goldhaber, The Ezperimental Foundations 1988, Cambridge A description University Gerson: of Particle Physics Press of how particle developed, with emphasis on crucial Duff, Brian Fundamental 1986, Taylor ’ Bound: An Introduction to Quarks and Leptons in particle Of Matter University and Forces in the Physical World Press presentation of the history of the reader. decades of elementary physics. particle of modern physics The last third demanding consid- of the book deals with four physics. and Francis An excellent containing Pais, Abraham: erable background Particles: Particles ‘/1/ textbook An erudite G.: to Elementary & Row 1986, Oxford experiments. Introduction An introductory Inward physics David: I survey of particle the essentials physics for undergraduate of the field in a brief physics but readily majors, understood pre- 7.5. Periodicals Two of the best sources are the monthly Scientific American New Scientist; published in England, and the weekly sentation. Riordan, Michael: The Hunting 1987, Simon and Schuster A historical survey of the Quark (Touchstone of particle Books) physics, with special emphasis unfortunately available in the USA. Material relating be found in two journals Teachers, The Physics the latter, to the teaching of physics, including particle is not widely physics, can on the role of published by the American Association of Physics SLAC. 7.4. Advanced Material Level at a more popular cover, National Cooper, Necia Grant; Particle Physics: 1987, Cambridge A collection A Los Alamos University of articles ratory containing Dodd, James: 1984, Cambridge An Introduction and West, Geoffrey Geographic, Science Digest, Primer Press by particle physicists a wide range of topics The Ideas of Particle University Press to particle physics at Los Alamos National Labo- in the field. Physics: An Introduction for Scientists designed to meet the needs of the profesConsists of a large number short chapters. 142 Journal level will be found B. (eds.): sional who works in some other field of physics. of well-organized Teacher and American 143 of Physics. in publications Science ,Fws such as Dis- and Smithsonian. c \, s REFERENCES 1. Robert P. Grease and Charles Publishing Company, 2. M. Gell-Mann, TH401, Phys. 412’(1964) 3. J. E. Augustin 4. J. J. Aubert 5. S. W. Herb 6. This Particle C. Mann, Letters 8, 214 (1964). et al., Phys. Rev. Lett. such numbers Group Properties.” Physics Letters, m, Particle Data Group, Macmillan G. Zweig, CERN Preprints (unpublished). et al., Phys. Rev. Lett. Data The Second Creation, (1986) p. 169. et al., Phys. Rev. Lett. and other Particle N.Y. 1406 (1974). B2, 1404 (1974). a, 252 (1977). are tabulated at Lawrence The 2, most 1 (1988). Berkeley recent Laboratory version of this C o p ies can be obtained Bldg. 50-308, Lawrence CA 94720. 144 i for particle t, Berkeley physics by the in the “Review review appears without Laboratory, of in cost from: Berkeley,