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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)

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## Particle Physics

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**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.)**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**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**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**Sizes and distance scales**• virus 10-7 • Molecule 10-9m • Atom 10-10m • nucleus 10-14 m • nucleon 10-15m • Quark <10-19m**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**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,….????)**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**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**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**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”**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**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.**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**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**“Strange particles”**• Kaon: discovered 1946; first called “V” particles K0 production and decay in a bubble chamber**Bubble chamber**- - - p p p n K0 K-+ - 0 n + p 3 pions 0 , e+ e- K0 + - -**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)?**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)**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-**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)**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)**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, …….. _ _ _ _**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**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**“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”**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**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 0e –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)**From Contemporary Physics Education Project**http://www.cpepweb.org/particles.html**From Contemporary Physics Education Project**http://www.cpepweb.org/particles.html**From Contemporary Physics Education Project**http://www.cpepweb.org/particles.html**From Contemporary Physics Education Project**http://www.cpepweb.org/particles.html**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**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)**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**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)**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)**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) - - - - - -**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**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**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-**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

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