Particle Physics
PHY 3101
D. Acosta
Particle Physics
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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!
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Beginning of Nuclear
Substructure
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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?
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The Particle Zoo
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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
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Classification
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Hadrons:
– Particles that carry nuclear “charge” and
interact 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, , , …
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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
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Both classes of particles can interact via the
Electromagnetic force (if they have charge)
and decay via the Weak Nuclear Force
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Conservation Laws
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Energy and Momentum:
– Conserved. In particular, Special
Relativity applies
– Example:  0   
but not  0  
e  e     but not e  e   
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
p     
B: 1  0  1
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)
   e   e   
Le :
L :
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0  1 1 0
1 0 01
Note: anti-particles have the opposite
“charge” of particles (electric charge, baryon
number, lepton number)
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Lifetime of Particles
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Fastest decays involve the Strong Nuclear
Force:
–   10-20 s
  p   
– Example:
– Compare to the time it takes light to
cross 1 fm:
1015 m
24
t

3

10
s
8
3  10 m / s
Next fastest decays involve the
Electromagnetic Force:
–   10-16 s
0  
– Example:
Slowest decays involve the Weak Nuclear
Force:
–   10-10 s
– Example:
K      
K    
If a conservation law forbids a type of
decay, then the lifetime will be significantly
longer
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Strangeness
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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?
K      
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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
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The “Eightfold Way’’
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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
Charge
Isospin
Periodic table for “elementary particles”!
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Quarks
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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
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Rutherford Scattering
Experiments by Geiger &
Marsden in 1909
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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)
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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
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Particle Accelerators
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Size of Nuclei
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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
proton
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Deep Inelastic Scattering
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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!
e
p
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PHY 3101 -- D. Acosta
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Quarks within the Proton
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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
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PHY 3101 -- D. Acosta
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Proton is a Complicated Object
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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%
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PHY 3101 -- D. Acosta
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November Revolution
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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 
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The Bottom Quark
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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
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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!
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CDF and DØ
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Particle Physics Detectors
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History of the Discovery of
Quarks
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SLAC, 1968
– Discovery of quarks in electron-proton
scattering
SLAC and Brookhaven, 1974
– Discovery of the charm quark in electronpositron annihilation
Fermilab, 1977
– Discovery of the bottom quark in proton
collisions
Fermilab, 1995
– Discovery of the top quark in protonantiproton annihiliation
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The Standard Model of Particle
Physics
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Jets
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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!
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quark
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PHY 3101 -- D. Acosta
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Where’s the Glue?
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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:
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Do Quarks have Substructure?
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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
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European Center for Nuclear
Research (CERN)
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CERN Particle Accelerators
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