Download teachers` resource book on fundamental particles and

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
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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
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1. Robert
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CA 94720.
144
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for particle
t,
Berkeley
physics
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in the “Review
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appears
without
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of
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Berkeley,