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Particle Physics. PHY 3101 D. Acosta. Particle Physics. This field is about the study of forces and particles But forces and particles are equivalent! Recall that the electromagnetic field is quantized into photons

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Particle physics l.jpg

Particle Physics

PHY 3101

D. Acosta

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Particle Physics

  • This field is about the study of forces and particles

  • But forces and particles are equivalent!

    • Recall that the electromagnetic field is quantized into photons

  • In general, the “fields” corresponding to all fundamental forces are quantized into particles

  • Moreover, these fields (particles) obey certain conservation laws, which arise from symmetries in nature

  • Particle physics attempts to understand nature at its most fundamental level: to explain physical phenomena in terms the smallest set of conservation laws and symmetries

  • But, superficially it often seems like particle physics is like zoology!

PHY 3101 -- D. Acosta

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Beginning of Nuclear Substructure

  • Elements of the same group have nearly the same chemical property

  • Chemical periodicity depends on the atomic number Z

  • All elements composed of just electrons, neutrons, and protons

  • Any other fundamental particles?

PHY 3101 -- D. Acosta

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The Particle Zoo

  • Discovered in cosmic rays:

    • Positron, 1932, M = 0.511 MeV, anti-electron predicted by Dirac Eqn.

    • Muon, 1938, M = 106 MeV

    • Pion, 1947, M = 135 MeV, predicted by Yukawa to carry strong force

  • 1 MeV = energy electron gains when accelerated across 1 million volts.

  • Discovered in particle accelerators by 1960:

    • Kaon, rho, omega, … (Mesons, like pion)

    • Lambda, Sigma, Xi, …(Baryons, like proton)

  • Mesons and Baryons feel the strong nuclear force

  • Leptons (electron, muon) do not

PHY 3101 -- D. Acosta

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  • Hadrons:

    • Particles that carry nuclear “charge” andinteract with the Strong Nuclear Force

    • Mesons:

      • Integer spin, bosons (s = 0, 1, …)

      • , K, , , …

    • Baryons:

      • Half-integer spin, fermions (s =1/2, 3/2,…)

      • p, n, , , …

  • Leptons:

    • Particles that do not carry any nuclear “charge” and do not interact with the Strong Nuclear Force

      • electrons, muons, taus, neutrinos,…

    • Do not bind in nucleus

  • Both classes of particles can interact via the Electromagnetic force (if they have charge) and decay via the Weak Nuclear Force

PHY 3101 -- D. Acosta

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Conservation Laws

  • Energy and Momentum:

    • Conserved. In particular, Special Relativity applies

    • Example:

  • Electric charge:

    • Conserved. Must add up to the same charge on each side of the equation

  • Baryon Number:

    • Conserved. Baryon “Charge” must add up to same value on each side of the equation

  • Lepton Number:

    • Conserved. Lepton “Charge” must add up to same value on each side of the equation for each type of lepton (electron, muon, tau)

  • Note: anti-particles have the opposite “charge” of particles (electric charge, baryon number, lepton number)

PHY 3101 -- D. Acosta

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Lifetime of Particles

  • Fastest decays involve the Strong Nuclear Force:

    •   10-20 s

    • Example:

    • Compare to the time it takes light to cross 1 fm:

  • Next fastest decays involve the Electromagnetic Force:

    •   10-16 s

    • Example:

  • Slowest decays involve the Weak Nuclear Force:

    •   10-10 s

    • Example:

  • If a conservation law forbids a type of decay, then the lifetime will be significantly longer

PHY 3101 -- D. Acosta

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  • Some particles are observed to decay rapidly, others are not

    • Why doesn’t Kaon decay quickly like other strong nuclear decays?

  • Murray Gell-Mann and Kazuhiko Nishijima invoke new quantum number called “strangeness,” which is conserved in electromagnetic and strong nuclear interactions, but is violated by the weak nuclear interaction.

  • Kaon, Lambda, Sigma have strangeness = 1

  • Pion, Proton, Neutron have strangeness = 0

PHY 3101 -- D. Acosta

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The “Eightfold Way’’

  • Developed by Murray Gell-Mann and Yuval Ne’eman in 1961

  • Plot hypercharge Y (baryon number + strangeness) versus isospin

  • Observe patterns in multiplets

  • Omega predicted and observed 1964



Periodic table for “elementary particles”!

PHY 3101 -- D. Acosta

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  • Murray Gell-Mann and George Zweig propose in 1964 that mesons and baryons are not elementary, but are composed of smaller constituents: Quarks

  • James Joyce, Finnegan’s Wake: “Three quarks for Muster Mark.”

  • u, d, and s quarks (up, down, strange)

  • These quarks have spin 1/2, and have fractional electric charge (2/3, -1/3)

  • Proton: u u d

  • Neutron: u d d

  • Pion: ud, uu - dd, du

  • Kaon: us, ds, sd, su

  • Not clear if this is a mathematical convenience, or reality

PHY 3101 -- D. Acosta

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Rutherford Scattering

Experiments by Geiger & Marsden in 1909

PHY 3101 -- D. Acosta

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Rutherford Model of the Atom

Conclusion: the atom contains a positive nucleus < 10 fm in size (1 fm = 10-15 m)

PHY 3101 -- D. Acosta

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How we “see” particles

The smaller the wavelength, the smaller the features observed.

Recall that the deBoglie wavelength of a particle is  = h / p

So we need high energies to probe quarks

PHY 3101 -- D. Acosta

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Particle Accelerators

PHY 3101 -- D. Acosta

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Size of Nuclei

  • Robert Hofstadter performs experiment at Stanford using new linear accelerator for electrons in 1950s

  • E = 100 -- 500 MeV

  •  = 2.5 fm

  • The proton is not a point! (Deviation of elastic scattering rate from Rutherford Scattering)

  • Proton and nuclei have extended charge distributions

  • Nobel prize in 1961


PHY 3101 -- D. Acosta

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Deep Inelastic Scattering

  • Stanford Linear Accelerator Center (SLAC) constructs 2 mile long accelerator in 1966

  • E = 20 GeV

  • Experiment by SLAC and MIT (Friedman, Kendall, Taylor -- Nobel in 1990) cracks the proton (inelastic scattering) in 1967

  • Surprise when scattering rate follows Rutherford formula for scattering between point particles!

  • Richard Feynman dubs proton consituents partons

  • Quarks are real!



PHY 3101 -- D. Acosta

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Quarks within the Proton

  • Scattering rate is independent of resolving power of incoming electron

    • Quark are fundamental (Bjorken Scaling)

  • Total scattering rate consistent with spin 1/2 quarks

    • Callan-Gross relation

  • But momentum fraction carried by a quark is not 1/3

PHY 3101 -- D. Acosta

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Proton is a Complicated Object

  • Total momentum fraction carried by quarks is only 50% ! Rest is carried by gluons, the carriers of the strong nuclear force

  • Total spin carried by valence quarks (uud) is also not 100%

PHY 3101 -- D. Acosta

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November Revolution

  • New quark discovered in 1974: charm

  • Sam Ting leads discovery of a resonance in proton-nucleon collisions at Brookhaven National Laboratory: calls it the J particle

  • Burt Richter leads discovery of a resonance at an e+e- collider at SLAC: calls it the Psi particle

  • Mass of J/ is 3.1 GeV

  • Richter, Ting share 1976 Nobel prize

  • Example of charmonium

    • Bound state of c c

    • Like positronium (e+e-) but for strong force

    • In other words, like H atom

  • Higher mass excited states exist also, like  ‘

PHY 3101 -- D. Acosta

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The Bottom Quark

  • In 1977 Leon Lederman leads discovery of a higher mass resonance at Fermilab

  • Mass of  (Upsilon) is 9.5 GeV

  • Example of bottomonium: b b

  • Again excited state resonances exist

  • Now studied in great detail in e+e- collider experiments:

    • CLEO: studies B mesons (b quark + u,d,s,c)

  • Soon there will be a round of many new experiments doing B physics

PHY 3101 -- D. Acosta

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Discovery of the top quark

The top quark was discovered in 1995 with a mass of 175 GeV, as much as an atom of gold!

PHY 3101 -- D. Acosta

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CDF and DØ

PHY 3101 -- D. Acosta

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Particle Physics Detectors

PHY 3101 -- D. Acosta

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History of the Discovery of Quarks

  • SLAC, 1968

    • Discovery of quarks in electron-proton scattering

  • SLAC and Brookhaven, 1974

    • Discovery of the charm quark in electron-positron annihilation

  • Fermilab, 1977

    • Discovery of the bottom quark in proton collisions

  • Fermilab, 1995

    • Discovery of the top quark in proton-antiproton annihiliation

PHY 3101 -- D. Acosta

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  • Despite all this evidence, no one has actually seen a single bare quark!

  • Instead, we observe clusters of known particles (Jets) which travel in the direction of the scattered quark

  • These jets behave as if they originated from a spin 1/2 quark

  • Cannot calculate this effect exactly in theory because series expansion does not converge!


PHY 3101 -- D. Acosta

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Where’s the Glue?

  • Only indirect evidence for gluons

  • But many calculations are confirmed by experimental measurements

  • Gluons couple to the 3 “color” charges of quarks (red, green, blue)

  • Scattering rates in e+e- colliders show this factor of 3:

PHY 3101 -- D. Acosta

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Do Quarks have Substructure?

  • Next generation of Deep Inelastic Scattering experiments in Hamburg, Germany

  • HERA: World’s only electron-proton Collider:

  • Spatial resolution is 10-18 m

  • Extends reach by several orders of magnitude over fixed-target experiments

PHY 3101 -- D. Acosta

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CERN Particle Accelerators

PHY 3101 -- D. Acosta