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Particle Physics

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  1. Particle Physics • Particle physics – what is it? – why do it? • Standard model • Quantum field theory • Constituents, forces • Milestones of particle physics • Particle physics experiments • shortcomings of standard model • Summary • Webpages of interest • http://www.fnal.gov (Fermilab homepage) • http://www-d0.fnal.gov (DØhomepage) • http://www.cern.ch (CERN -- European Laboratory for Particle Physics) • http://cms.web.cern.ch/cms/ (CMS) • http://www.hep.fsu.edu/~wahl/Quarknet/links.html (has links to many particle physics sites and other sites of interest) • http://www.fnal.gov/pub/tour.html (Fermilab particle physics tour) • http://www.cpepweb.org/ Contemporary Physics Education Project • http://ParticleAdventure.org/ (Lawrence Berkeley Lab.)

  2. Topics • what is particle physics, goals and issues • historical flashback over development of the field • cosmic rays • particle discoveries • forces • new theories • the standard model of particle physics

  3. About Units • Energy - electron-volt • 1 electron-volt = kinetic energy of an electron when moving through potential difference of 1 Volt; • 1 eV = 1.6 × 10-19Joules = 2.1 × 10-6 W•s • 1 kW•hr = 3.6 × 106 Joules = 2.25 × 1025 eV • 1 MeV = 106 eV, 1 GeV = 109 eV, 1 TeV = 1012 eV • mass - eV/c2 • 1 eV/c2 = 1.78 × 10-36kg • electron mass = 0.511 MeV/c2 • proton mass = 938.27 MeV/c2 = 0.93827 GeV/ c2 • neutron mass = 939.57 MeV/c2 • momentum - eV/c: • 1 eV/c = 5.3 × 10-28kg m/s • momentum of baseball at 80 mi/hr  5.29 kgm/s  9.9×1027eV/c • Distance • 1 femtometer (“Fermi”) = 10-15 m

  4. Outline • what is particle physics? • Origins of particle physics • Atom (p, e-), radioactivity, discovery of neutron (n) (1895-1932) • Cosmic rays: positron (e+), muon (μ-), pion (π), Kaon (K±, K0) (1932 – 1959) • the advent of accelerators: • more and more particles discovered, patterns emerge (1960’s and on): • leptons and hadrons • Electromagnetic, weak, strong interactions • present scenario: Standard Model of electroweak and strong interactions • Formulation and discovery (1960’s to 1980’s) • Precision experimental tests (from 1990’s) • quest for new physics (beyond the standard model) • Open questions, possible strategies • Present and future experiments, facilities • Search for Higgs particle • outlook

  5. Sizes and distance scales • virus 10-7 • Molecule 10-9m • Atom 10-10m • nucleus 10-14 m • nucleon 10-15m • Quark <10-19m

  6. The Building Blocks of a Dew Drop • A dew drop is made up of 1021 molecules of water. • Each molecule = one oxygen atom and two hydrogen atoms (H2O). • Each atom consists of a nucleus surrounded by electrons. • Electrons are leptons that are bound to the nucleus by photons, which are bosons. • The nucleus of a hydrogen atom is just a single proton. • Protons consist of three quarks. In the proton, gluons hold the quarks together just as photons hold the electron to the nucleus in the atom   

  7. Contemporary Physics Education Project

  8. Goals of particle physics • particle physics or high energy physics • is looking for the smallest constituents of matter(the “ultimate building blocks”) and for the fundamental forcesbetween them (“interactions”); • aim is to find description in terms of the smallest number of particles and forces • Try to describe matter in terms of specific set of constituents which can be treated as fundamental; • With deeper probing (at shorter length scale), these fundamental constituents may turn out to consist of smaller parts (be “composite”). • “Smallest constituents” vs time: • in 19th century, atoms were considered smallest building blocks, • early 20th century research: electrons, protons, neutrons; • now evidence that nucleons have substructure - quarks; • going down the size ladder: atoms -- nuclei -- nucleons -- quarks – ???... ??? (preons, toohoos, voohoos,….????)

  9. Issues of High Energy Physics • Basic questions: • Are there irreducible building blocks? • How many? • What are their properties? • mass? charge? flavor? • What is mass? • What is charge ? • How do the building blocks interact? • forces? • Differences? similarities? • Why more matter than antimatter? • why is our universe the way it is? • Coincidence? • Theoretical necessity • Design? Why do we want to know? • Curiosity • Understanding constituents may help in understanding composites • Implications for origin and destiny of Universe

  10. 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

  11. Victor Hess’ Balloon ride 1912

  12. Positron • Positron (anti-electron) • Predicted by Dirac (1928) -- needed for relativistic quantum mechanics • existence of antiparticles doubled the number of known particles!!! • Positron track going upward through lead plate • Photographed by Carl Anderson (Aug. 2, 1932) • Particle moving upward, as determined by increase in curvature of the top half of the track after it passed through lead plate, • and curving to the left, meaning its charge is positive

  13. Anderson and his cloud chamber

  14. Neutron • Bothe + Becker (1930): • Some light elements (e.g. Be), when bombarded with alpha particles, emit neutral radiation, “penetrating”– gamma? • Curie-Joliot and Joliot (1932): • This radiation from Be and B able to eject protons from material containing hydrogen • Chadwick (1932) • Doubts interpretation of this radiation as gamma • Performs new experiments; protons ejected not only from hydrogen, but also from other light elements; • measures energy of ejected protons (by measuring their range), • results not compatible with assumption that unknown radiation consists of gamma radiation (contradiction with energy-momentum conservation), but are compatible with assumption of neutral particles with mass approximately equal to that of proton – calls it “neutron” • Neutron assumed to be “proton and electron in close association”

  15. Chadwick’s experiment

  16. Nuclear force – field quantum • photon carries the electromagnetic force. • Analogy: postulate particle as carrier of nuclear force (Hideki Yukawa, 1935) withmass intermediate between the electron and the proton • This particle also to be responsible for beta-decay. • potential energy between the nucleon field quanta has the form m = mass of the exchanged quantum; • from observed range of nuclear force: mass of the exchanged particle 200MeV

  17. More particles: Muon • 1937: “mesotron” is observed in cosmic rays (Carl Anderson, Seth Neddermeyer) – first mistaken for Yukawa’s particle • However it was shown in 1941 that mesotrons didn’t interact strongly with matter.

  18. Discovery of pion • Lattes, Occhialini and Powell (Bristol, 1947) (+ graduate student Hugh Muirhead): observed decay of a new particle into two particles • decay products: • muon (discovered by Neddermeyer), • the other is invisible (Pauli's neutrino). • muon in turn also decays into electron and neutrino

  19. Kaons • First observation of Kaons: • Experiment by Clifford Butler and George Rochester at Manchester • Cloud chamber exposed to cosmic rays • Left picture: neutral Kaon decay (1946) • Right picture: charged Kaon decay into muon and neutrino • Kaons first called “V” particles • Called “strange” because they behaved differently from others

  20. “Strange particles” • Kaon: discovered 1946; first called “V” particles K0 production and decay in a bubble chamber

  21. Bubble chamber - - - p p  p n K0 K-+ - 0 n + p  3 pions 0  ,   e+ e- K0  + - -

  22. 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)?

  23. 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) • “particle spectroscopy era” • Bubble chamber allows detailed study of reactions, reconstruction of all particles created in the reactions • find that often observed particles actually originate from decay of very short-lived particles (“resonances”) • 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 • Allows prediction of new particle Ω- , with all of its properties (mass, spin, expected decay modes,..) • subsequent observation of Ω- with expected properties at BNL (1964)

  24. Ω- • The bubble chamber picture of the first omega-minus. An incoming K- meson ( of momentum 5.0 GeV/c ) interacts with a proton of the liquid hydrogen in the bubble chamber and produces an omega-minus, a K° and a K+ meson: • K- + p ―› Ω-+ K+ + K0 • The omega minus then decays: Ω- ―> Ξ0+ π-, with subsequent decay Ξ0 ―> Λ + π0; • Λ ―> p + π- π0 ―> γ γγ ―> e+ e-

  25. Ω-

  26. Particle nomenclature • by mass: • baryons – heavy particles .. p, n, Λ, Σ, Ω-, ++…. • nucleons and their excited states: p, n, N*, ++, …… • hyperons: Λ, Σ, Ξ, Ω- , and their excited states • mesons – medium-heavy particles … π, K, K*, ρ, ω … • leptons – light particles e, μ, νe, ν … • by spin: • fermions: spin = odd-integer multiple of ½: ½, 3/2, 5/2,…… • leptons and baryons • bosons: integer spin 0, 1, 2, …. – mesons are bosons • by interaction: • hadrons: partake in strong interactions • leptons: no strong interaction • by lifetime: • stable particles: lifetime > 10-17 sec (decay by weak or electromagnetic interaction) • unstable particles (“resonances”) lifetime <10-20 sec (decay by strong interaction)

  27. Spin and statistics • Fermions obey “Fermi-Dirac” statistics: • multi-fermion states are antisymmetric with respect to exchange of any two identical fermions (wave function changes sign) |f1, f2, f3 > = - |f2, f1 , f3 > • Pauli exclusion principle is special case of this • Bosons obey “Bose-Einstein” statistics: • multi-boson states are symmetric with respect to exchange of any two identical bosons (wave function stays the same) |b1, b2 , b3 > = |b2, b1 , b3 > • consequence: bosons like to “stick together” (e.g. Bose-Einstein condensate)

  28. Towards the standard model • Quark Model (Gell-Mann, Zweig, 1964) • observed SU(3) symmetry can be explained by assuming that all hadrons are made of “quarks” • There are 3 quarks: u (up), d (down), s (strange); • quarks have non-integer charge: • u 2/3, d -1/3, s -1/3 • baryons are made of 3 quarks: • p = uud, n = udd, Λ = uds, Σ = uus Ξ = uss, Ω- = sss • mesons are made of quark-antiquark pairs: • π+ = ud, π- = u d, π0 = u u + d d, …….. _ _ _ _

  29. mesons qq + - 0 ++ uud ddd udd uuu p n us ds K+ K0 0  + - uus dds uds du uu,dd,ss ud - + 0   dss uss - 0 sd su K0 - K- sss The 8-fold way baryonsqqq

  30. color charge • problem with quark model • Quarks have spin ½, i.e. are fermions  must obey Pauli principle • Ω- = sss, has spin 3/2; spins of 3 s quarks must be aligned, i.e. Ω- has 3 quarks in identical state --- forbidden • similarly for ++ = uuu, spin 3/2 • way out: quarks have additional hidden property – “color charge” • 3 colors: green, red, blue • each quark can carry one of three colors • red blue green • antiquarks carry anticolor • anti-redanti-blueanti-green • observed particles are “color-neutral”; • only colorless (“white”) combinations of quarks and antiquarks can form particles: • qqq • qq

  31. “Elementary” particles? • “leptons” (electron, muon and their neutrinos) are fundamental, interact electromagnetically and “weakly” • “hadrons” (p, n, Λ, Σ, Ω-, ++ , π, K, K*, ρ, ω,…) are not fundamental particles – are made of quarks, interact electromagnetically, “weakly”, and “strongly”

  32. 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 •  (electromagnetic force) • W, Z (weak, electromagnetic) • g gluons (strong force) • Higgs boson • spontaneous symmetry breaking of SU(2) • mass

  33. Matter and forces • Fundamental forces (mediated by “force particles”) • strong interaction between quarks, mediated by gluons (which themselves feel the force) (QCD) • leads to all sorts of interesting behavior, like the existence of hadrons (proton, neutron) and the failure to find free quarks • Electroweak interaction between quarks and leptons, mediated by photons (electromagnetism) and W and Z bosons (weak force) • Fundamental constituent particles • Leptons q = quarks q = -1 e 2/3 u c t 0e  –1/3d s b • Role of symmetry: • Symmetry(invariance under certain transformations)governs behavior of physical systems: • Invariance  “conservation laws” (Noether) • Invariance under “local gauge transformations” interactions (forces)

  34. From Contemporary Physics Education Project http://www.cpepweb.org/particles.html

  35. From Contemporary Physics Education Project http://www.cpepweb.org/particles.html

  36. From Contemporary Physics Education Project http://www.cpepweb.org/particles.html

  37. From Contemporary Physics Education Project http://www.cpepweb.org/particles.html

  38. Strong quark interactions • quarks carry “color charge” (red, blue, green) and interact exchanging gluons, the carriers of the strong force • theory of strong interaction is “gauge theory”; form of interaction governed by invariance under local SU(3)c (“color SU(3)”) • 8 gluons carry color charge  interact with each other

  39. Electroweak interactions • leptons (and also quarks) carry a “weak charge” (in addition to usual electric charge) • they interact exchanging • neutral EW force carriers: photon , Z0 • charged EW force carriers: W± • theory describing EW interaction is gauge theory; gauge symmetry group SU(2)xU(1)

  40. Some milestones • Quantum electrodynamics (QED) (1950’s) (Feynman, Schwinger, Tomonaga) • electroweak unification – the standard model (1960s) (Glashow, Weinberg, Salam) • deep inelastic scattering experiments – partons (SLAC/MIT) (1956 – 1973) • Quark Model (1964) (Gell-Mann, Zweig) • Quantum Chromodynamics (1970s) (Gross, Wilczek, Politzer) • neutral weak current (1973) (Gargamelle, CERN) • Charm discovery (1974) (S. Ting, B. Richter) • Bottom quark discovery (1977) (L. Lederman) • gluon observation (1979) (DESY) • W,Z observation (1983) (UA1, UA2, C. Rubbia, CERN) • top quark (1995) (DØ, CDF, Fermilab) • Higgs (2012) LHC experiments

  41. Brief History of the Standard Model • Late 1920’s - early 1930’s: Dirac, Heisenberg, Pauli, & others extend Maxwell’s theory of EM to include Special Relativity & QM (QED) - but it only works to lowest order! • 1933: Enrico Fermi introduces 1st theory of weak interactions, analogous to QED, to explain b decay. • 1935: Hideki Yukawa predicts the pion as carrier of a new, strong force to explain recently observed hadronic resonances. • 1937:muon is observed in cosmic rays – first mistaken for Yukawa’s particle • 1938:heavy W as mediator of weak interactions? (Klein) • 1947:pion is observed in cosmic rays • 1949: Dyson, Feynman, Schwinger, and Tomonaga introduce renormalization into QED - most accurate theory to date! • 1954: Yang and Mills develop Gauge Theories • 1950’s - early 1960’s: more than 100 hadronic “resonances” have been observed ! • 1962 two neutrinos! • 1964: Gell-Mann & Zweig propose a scheme whereby resonances are interpreted as composites of 3 “quarks”. (up, down, strange)

  42. Brief History of the Standard Model - 2 • 1970: Glashow, Iliopoulos, Maiani: 4th quark (charm) explains suppression of K decay into  • 1964-1967: spontaneous symmetry breaking (Higgs, Kibble) • 1967: Weinberg & Salam propose a unified Gauge Theory of electroweak interactions, introducing the W±,Z as force carriers and the Higgs field to provide the symmetry breaking mechanism. • 1967:deep inelastic scattering shows “Bjorken scaling” • 1969:“parton” picture (Feynman, Bjorken) • 1971-1972:Gauge theories are renormalizable (even when symmetry is spontaneoulsy broken) (t’Hooft, Veltman, Lee, Zinn-Justin..) • 1972: high pt pions observed at the CERN ISR • 1973: Quantum Chromodynamics (Gross, Wilczek, Politzer, Gell-Mann & Fritzsch) : quarks are held together by a Gauge-Field whose quanta, gluons, mediate the strong force • 1973:“neutral currents”observed (Gargamelle bubble chamber at CERN)

  43. Brief History of the Standard Model - 3 • 1974:J/discovered at BNL/SLAC; • 1975:J/ interpreted as cc bound state (“charmonium”) • 1976:t lepton discovered at SLAC • 1977:discovered at Fermilab in 1977, interpreted as bb bound state (“bottomonium”)  3rd generation • 1979:gluon – jets observed at DESY • 1982:direct evidence for jets in hadron hadron interactions at CERN (ppcollider) • 1983:W±, Z observed at CERN (ppcollider built for that purpose) • 1995:top quark found at Fermilab (DØ, CDF) • 1999:indications for “neutrino oscillations” (Super-Kamiokande experiment) • 2000:direct evidence for tau neutrino () at Fermilab (DONUT experiment) • 2005:clear evidence for neutrino oscillations(Kamiokande, SNO) - - - - - -

  44. e e Feynman diagram    Feynman diagrams • Feynman Diagrams • In our current understanding, all interactions arise from the exchange of quanta • The mathematics describing such interactions can be represented by a diagram, called a Feynman diagram

  45. Present scenario • Most of what’s around us is made of very few particles: electrons, protons, neutrons (e, u, d) • this is because our world lives at very low energy • all other particles were created at high energies during very early stages of our universe • can recreate some of them (albeit for very short time) in our laboratories (high energy accelerators and colliders) • this allows us to study their nature, test the standard model, and discover direct or indirect signals for new physics

  46. Study of high energy interactions -- going back in time

  47. Homework set 5 • HW 5.1 • go to Particle Data Group website: http://pdg.lbl.gov • find masses (in MeV or GeV) , principal decay modes and lifetimes of the following particles: π±, π0, K0, J/Ψ, p, n, Λ0, Σ+ , Σ0, Ξ, Ω- • give the quark composition of π±, π0, K0, J/Ψ, p, n, Λ0, Ω-

  48. Summary • Particle physics was born during last century, grew out of atomic and nuclear physics • huge progress in understanding over last 50 years, due to revolutionary ideas in both theory and experiment • intense dialog between experimenters and theorists • precision tests of standard model ongoing, looking for hints of new physics • next: • symmetries • tests of standard model, experiments, accelerators • problems and shortcomings of standard model • new projects, outlook