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Overview of Particle Physics -- the path to the Standard Model 1 Topics historical flashback over development of the field o o o o o “prehistory” 19th century electron, radioactivity, nucleus cosmic rays spectroscopy era collider era standard model of particle physics 2 Atoms, Nucleus electron: first hint that atom not indivisible natural radioactivity understanding of composition of atom, nucleus atom = nucleus surrounded by electrons (Geiger, Marsden, Rutherford, 1906 -1911) hydrogen nucleus = proton, is component of all nuclei (1920) neutron (Bothe, Becker, Joliot-Curie, Chadwick, 1930 – 1932) 3 Cosmic rays Discovered by Victor Hess (1912) Observations on mountains and in balloon: intensity of cosmic radiation increases with height above surface of Earth – must come from “outer space” Much of cosmic radiation from sun (rather low energy protons) Very high energy radiation from outside solar system, but probably from within galaxy 4 5 Cosmic rays new “elementary” particles new detectors (cloud chambers, emulsions) exposed to cosmic rays discovery of many new particles positron (anti-electron) : predicted by Dirac (1928), discovered by Anderson 1932 muon (μ): 1937 Nedermeyer pion (π) predicted by Yukawa (1935), observed 1947 (Lattes, Occhialini, Powell) strange particles (K, Λ, Σ,….. 6 Cloud chamber Container filled with gas (e.g. air), plus vapor close to its dew point (saturated) Passage of charged particle ionization; Ions form seeds for condensation condensation along path of particle path of particle becomes visible as chain of droplets 7 Positron discovery Positron (anti-electron) predicted by Dirac (1928) -- needed for relativistic quantum mechanics existence of antiparticles doubled the number of known particles!! track going upward (has lower energy after lead) 8 Anderson and his cloud chamber 9 Particle Zoo 1940’s to 1960’s : Plethora of new particles discovered (mainly in cosmic rays): e-, p, n, ν, μ-, π±, π0, Λ0, Σ+ , Σ0 , Ξ,…. question: Can nature be so messy? are all these particles really intrinsically different? or can we recognize patterns or symmetries in their nature (charge, mass, flavor) or the way they behave (decays)? 10 The Particle Zoo! ± , 0 , ± , e, ± 0 0 K , K S, K L, 0 + , p, n, , 0 , , , … 11 Seeing = photon scattering experiment our eye is a photon detector; (photons = light “quanta” = packets of light -- see “photoelectric effect”) “seeing” is performing a photon scattering experiment: o light source provides photons o photons “interact” with object of our interest -- some absorbed, some scattered, reflected o some of scattered/reflected photons make it into eye; focused onto retina; o photons detected by sensors in retina (photoreceptors -- rods and cones) o transduced into electrical signal (nerve pulse) o amplified when needed o transmitted to brain for processing and interpretation 12 HOW TO SEE SMALL THINGS “seeing an object” = detecting light that has been reflected off the object's surface “visible light”= those electromagnetic waves that our eyes can detect “wavelength” of e.m. wave (distance between two successive crests) determines “color” of light if size of object is much smaller than wavelength, then wave is hardly influenced by object wavelength of visible light: between 410-7 m (violet) and 7 10-7 m (red); diameter of atoms: 10-10 m can’t see them with “ordinary” (visible) light generalize meaning of seeing: seeing is to detect effect due to the presence of an object quantum theory “particle waves”, with wavelength 1/(m v) use accelerated (charged) particles as probe, can “tune” wavelength by choosing mass m and changing velocity v this method is used in electron microscope, as well as in “scattering experiments” in nuclear and particle physics 13 Particle physics experiments Particle physics experiments: collide particles to o produce new particles o reveal their internal structure and laws of their interactions by observing regularities, measuring cross sections,... colliding particles need to have high energy o to make objects of large mass o to resolve structure at small distances to study structure of small objects: o need probe with short wavelength: use particles with high momentum to get short wavelength o Particles behave as if they had a wavelength = h/p mass-energy equivalence (E = mc2) plays an important role; in collisions, kinetic energy converted into mass energy; 14 About Units Energy - electron-volt 1 electron-volt = kinetic energy of an electron after moving through potential difference of 1 Volt; o 1 eV = 1.6 × 10-19 Joules = 1.6 × 10-19 W•s o 1 kW•hr = 3.6 × 106 Joules = 2.25 × 1025 eV mass - eV/c2 o 1 eV/c2 = 1.78 × 10-36 kg o electron mass = 0.511 MeV/c2 o proton mass = 938.27 MeV/c2 momentum - eV/c: o 1 eV/c = 5.3 × 10-28 kg m/s o momentum of baseball at 80 mi/hr 5.29 kgm/s 9.9 × 1027 eV/c 15 ACCELERATORS are devices to increase the energy of charged particles; magnetic fields to shape (focus and bend) the trajectory of the particles; electric fields for acceleration. types of accelerators: electrostatic (DC) accelerators o Cockcroft-Walton accelerator (protons up to 2 MeV) o Van de Graaff accelerator (protons up to 10 MeV) o Tandem Van de Graaff accelerator (protons up to 20 MeV) resonance accelerators o cyclotron (protons up to 25 MeV) o linear accelerators electron linac: 100 MeV to 50 GeV proton linac: up to 70 MeV synchronous accelerators o synchrocyclotron (protons up to 750 MeV) o proton synchrotron (protons up to TeV) o electron synchrotron (electrons from 50 MeV to 90 GeV) storage ring accelerators (colliders) 16 Van de Graaff accelerator use powersupply to deposit charges on belt; pick charges off at other end of belt and deposit on “terminal” now rubber belt replaced by “pellet” chain – “pelletron” 17 Cyclotron two hollow metal chambers (“dees”) with “gap” between them dees connected to AC voltage source - one dee positive when other negative electric field in gap between dees, but no electric field inside the dees; source of protons in center, everything in vacuum chamber; whole apparatus in magnetic field perpendicular to plane of dees; frequency of AC voltage such that particles always accelerated when reaching the gap between the dees; in magnetic field, particles are deflected: p = qBR p = momentum, q = charge, B = magnetic field strength, R = radius of curvature radius of path increases as momentum of proton increases time for passage always the same as long as momentum proportional to velocity; this is not true when velocity becomes too big (relativistic effects) 18 Synchrotron synchrotron Magnetic field B (to keep particles on circle) synchronized with electric field (for acceleration); magnetic field increases during acceleration, radius of orbit fixed. synchrotron is most common accelerator used in particle physics first synchrotrons: Cosmotron (Brookhaven), 3.3 GeV, 1953 Bevatron (Berkeley): 6.2 GeV, 1954 PS (CERN) 26 -> 28 GeV, 1959 AGS (Brookhaven) 30 -> 33 GeV, 1960 19 Bubble chamber Operating principle: Vessel, filled with (e.g.) liquid hydrogen at a temperature above the normal boiling point but held under a pressure of about 10 atmospheres by a large piston to prevent boiling. After passage of particles, move piston to reduce pressure boiling point lowered boiling starts along particle tracks bubbles develop. Let bubbles grow (about 3 milliseconds), then tracks are photographed (flash); provide stereo views of tracks by use of several cameras . Then move piston back recompress the liquid collapse bubbles before boiling all over. Invented by Glaser in 1952 (when he was drinking beer) 20 pbar p p nbar K0 K- + - 0 nbar + p 3 pions 0 , e+ e K0 + - 21 “Strange particles” Kaon: discovered 1947; first called “V” particles K0 production and decay in a bubble chamber 22 Particle spectroscopy era 1950’s – 1960’s: accelerators, better detectors even more new particles are found, many of them extremely short-lived (decay after 10-21 sec) 1962: “eightfold way”, “flavor SU(3)” symmetry (Gell-Mann, Ne’eman) allows classification of particles into “multiplets” Mass formula relating masses of particles in same multiplet quark model – three different kinds of quarks (u, d, s) Allows prediction of new particle Ω- , with all of its properties (mass, spin, expected decay modes,..) subsequent observation of Ω- with expected 23 properties at BNL (1964) ΩBNL 1964 http://www.bnl.gov/bnlweb/history/Omega-minus.asp eight-fold way quark model – particles made up of three different “quarks” – u, d, s p = uud, n = udd,… Ω- = sss refinement of these ideas, more quarks, “color”, gauge field theory Standard Model 24 Standard Model A theoretical model of interactions of elementary particles, based on quantum field theory Symmetry: SU(3) x SU(2) x U(1) “Matter particles” Quarks: up, down, charm,strange, top, bottom Leptons: electron, muon, tau, neutrinos “Force particles” Gauge Bosons o (electromagnetic force) o W, Z (weak, electromagnetic) o g gluons (strong force) Higgs boson spontaneous symmetry breaking of SU(2) mass 25 Contemporary Physics Education Project26 “every-day” matter Proton Neutron d u u u d Photon d Electron e Electron Neutrino e 27 Forces (interactions) Strong interaction 1 Binds protons and neutrons to form nuclei Electromagnetic interaction 10-2 Binds electrons and nuclei to form atoms Binds atoms to form molecules etc. Weak interaction 10-10 changes “flavors” (e.g. decay) important in stars’ energy “production” Gravitational interaction 10-39 Binds matter on large scales 28 Electromagnetic interaction Proton q1 Photon q1q2 F k 2 r Electron q2 29 The Strong Force d u g u Strong force caused by the exchange of gluons d 30 Weak interaction Beta decay Neutron u d Mean lifetime of a free neutron ~ 10.3 minutes Proton d u d Mean lifetime of a free proton > 1031 years! u W- Anti-electron Neutrino Electron e e 31 32 Testing the Standard Model want to probe small structures, create massive particles need more powerful accelerators – colliders more sophisticated detectors resources concentrated in large laboratories, effort international in scope 33 Fermilab Fermi National Accelerator Laboratory (http://www.fnal.gov/) Founded 1972 One of the top laboratories for high energy physics Near Batavia, Illinois (45 mi West of Chicago) presently (still – just barely) world’s highest energy accelerator: Tevatron = proton synchrotron, Emax=980GeV Operated as collider: proton – antiproton collisions at Ecm = 1.96 TeV Physics Program Collider experiments CDF, DØ, CMS neutrino physics: Minos, Mini-Boone Astrophysics: Auger Observatory, Sloan Sky Survey 34 …………. Fermi National Accelerator Laboratory 35 The TeVatron Collider Tevatron collider Colliding bunches of protons and anti-protons; bunches meet each other every 396 ns in the center of two detectors (DØ and CDF) (steered apart at other places) Each particle has ~ 980 GeV of energy, so the total energy in the center of mass is 1960 GeV = 1.96 TeV About 2,500,000 collisions per second 36 peak luminosity 1032 cm-1s-1 (5X1032 cm-1s-1 ) energy in c.m.s. 1.9 TeV, bunch crossing time 396 ns expect integrated luminosity 5fb-1 Turn-on March 1, 2001 First collisions April 3, 2001 37 Fermilab aerial view 38 Fermilab TeVatron tunnel 39 Modern particle physics detectors today’s particle physics detectors: combine many detection techniques “Russian doll” like structure -- many layers surrounding interaction region “general purpose detectors” – detect, identify and measure as many different kinds of particles as possible, (nearly) complete coverage of interaction region (“hermetic”) 40 Identifying particles 41 DØ detector Muon System 1.9T magnetized Fe, Prop. drift tubes 40,000 channels Central Tracking Calorimeter Uranium-liquid Argon 60,000 channels 42 the new DØ detector 43 DØ Detector in hall January 2001 44 The Discovery of Top Quark 1977 – 1992 Many null results 1992 – 1993 A few interesting events show up 1994, CDF First evidence mt ~ 170 GeV/c2 1995 – CDF, DØ Discovery! 1994, DØ mt > 131 GeV/c2 45 Creating Top Anti-Top Quark pairs b P t t b e uc -1/ 3 2 / 3 W e d s P -2 / 3 1/ 3 e uc - W - - - e d s 46 - Artist’s impression of a top event 47 What do we actually “see” _ t t e jets Muon Jet-1 Jet-2 Missing energy Electron 48 “event display” of a DØ top event t t e jets 49 Ωb (http://www.fnal.gov/pub/presspass/images/DZero-Omega-discovery.html 2008 DØ experiment at Fermilab: discover brother of Ω- , the Ωb Ω- = sss, Ωb = ssb, theory predicts properties, decay modes, .. confirmed by experiment 50 Particles of Standard Model Leptons -1/3 -1 0 u u u d d d e e c c c s s s t t t b b b g g g g g g g g I II III Z W± Bosons Fermions +2/3 Quarks 51 Summary we’ve come a long way …… technical breakthroughs in accelerators and detectors allowed new discoveries and new understanding Standard Model (theory of particle interactions) works embarrassingly well! Has been tested by many hundreds of precision measurements over last three decades – very few measurements differ by more than 1 or 2 standard deviations Even some amount of frustration – always hope to see experimental result in disagreement with theory But there are some open questions ………………… 52 53 Summary many different types of accelerators have been developed for nuclear and particle physics research different techniques suitable for different particles and energy regimes most accelerators in large research laboratories use several of these techniques in a chain of accelerators active research going on to develop new accelerating techniques for future applications many types of accelerators have found applications in fields other than nuclear and particle physics (e.g. medicine, ion implantation for electronics chips, condensed matter research, biology,….) 54 Summary Particle detection is based on interaction of particles with material in the detector; detectors usually have some “amplification” mechanism to render result of this interaction observable Many detection techniques have been developed over the last century breakthrough in detection techniques often led to breakthrough discoveries many of the detectors and/or techniques that were originally developed for basic research in nuclear or particle physics are now used in other fields; they often have led to advances in medical diagnosis (e.g. MRI, PET,….) 55 56 A Century of Particle Physics J.J Thomson Top quark 1995 Electron – 1897 57 Sizes and distance scales visible light: wavelength ≈5∙10-7m virus 10-7m molecule 10-9m atom 10-10m nucleus 10-14m nucleon 10-15m quark <10-18m 58 The Building Blocks of a Dew Drop dew drop: 1021 molecules of water. Each molecule = one oxygen atom and two hydrogen atoms (H2O). Atom: nucleus surrounded by electrons. Electrons bound to the nucleus by photons nucleus of a hydrogen atom = single proton. Proton: three quarks, held together by gluons just as photons hold the electron to the nucleus in the atom 59 Very early era (19th century) chemistry, electromagnetism discharge tubes, “canal rays”, “cathode rays” photoelectric effect (Hertz, 1887) radioactivity (Becquerel, 1895) X-rays (Röntgen, 1895) 60 61 What holds the world together? interaction strong electromagnetic weak gravity participants quarks charged particles all particles all particles 1 10-2 10-10 10-39 g gluon relative strength field quantum (boson) photon W± Z0 G graviton 62 The CMS Detector HF HE HB HO 63 Transverse slice through CMS detector 64