CHAPTER 14
ELEMENTARY PARTICLE PHYSICS
…or really…
The Standard Model
FUNDAMENTAL FORCES OF NATURE
Force
Particles
acted
upon
Relative
Strength
Lifetimes
Strong
Hadrons
and
Quarks
1
<10-20 s
Electromagnetic
All
charged
particles
~10-2
~10-16 s
Unified in the
Range
Short
Standard(~1fm)
Model
Long (∞)
Unified in Electroweak Theory
1
Fµ
Mediating
particle
Notes
Gluons
(pions in
nucleus)
Binds quarks
into hadrons
and nucleons
into nuclei
Photons
Unification of
electric and
magnetic
forces
Responsible
for beta
decay
r2
Weak
Quarks
and
Leptons
~10-6
>10-10 s
Very short
(~10-3 fm)
W±, Zo
Gravity
Everything
~10-43
?
Long (∞)
Graviton?
Fµ
1
r2
SEARCH FOR “FUNDAMENTAL PARTICLES”
•
Atoms: From Greek word “atmos” meaning “indivisible.”
• Thought to be fundamental particle of nature until Rutherford’s discovery of the
nucleus.
•
Protons and neutrons form nuclei with electrons in orbit.
•
Positrons
Another new fundamental particle!
• Seen naturally in b-decay (pn+e++n)
• Antiparticle of the electron (see brief discussion of Dirac’s theory on p550)
• Observed in pair production event by Carl Anderson in 1932
• Requires
Eg 2mec2=1.02 MeV
• Annihilation of e+epairs results in…?
Every particle is now
known to have an
antiparticle
SEARCH FOR “FUNDAMENTAL PARTICLES”
•
Mesons
• Pion, p, hypothesized by Hideki Yukawa in 1935 as the mediator of the strong
nuclear force. The analog of the photon being the mediator of the EM force.
• Called it a Meson (Greek for middle)
• Anderson (again!) found a particle of mass 106 MeV in 1937.
• Weakly interacting…could not be the strong-force mediator
• Turned out to be the heavy cousin of the electron, or muon m.
• Pion discovered in 1947 by Cecil Powel and Giuseppe Occhialini
Comes in three charge states
p0 2g
p+ m++nm
p- m-+nm
More new
m-e-+nm+ne
fundamental
m+e++nm+ne
particles!
Another new
fundamental
particle!
SEARCH FOR “FUNDAMENTAL PARTICLES”
•
Discovery of “Strange” Particles
• 1947 Rochester and Butler discovered a “strange” neutral particle with mass
between that of proton and pion while studying cosmic rays in a cloud chamber.
K0p++pK0p++p-
L0p+p-
• Associated production p-+pL 0+K0 with 4 GeV/c pions in a bubble chamber.
• Strangeness is a quantum number that is conserved in strong interactions but not in
weak interactions. What are the forces in play in the above pictures?
SOME PARTICLES AND THEIR PROPERTIES
Q.N. for particle.
Antiparticle has
opposite sign.
CLASSIFICATION OF PARTICLES
•
Hadrons
• Have internal structure and finite size  made up of smaller particles (quarks)
• Baryons
Guys like protons, neutrons, L.
Spin 1/2, 3/2, 5/2, etc
• Mesons
Guys like pions and kaons
Spin 0 or 1
• Leptons:
• Spin 1/2
• Appear to be point-like particles (no internal structure)
• Come in three flavors (plus their antiparticles)
æ - ö æ mç e ÷ ç
ç ne ÷ ç n
è
ø è m
ö
÷
÷
ø
æ t ö
ç
÷
ç nt ÷
è
ø
• Neutrinos can oscillate between flavorsimplies they have (small) mass
CONSERVATION LAWS
•
Baryon Number
• B=+1 for baryons, -1 for anti-baryons, zero for everything else.
• Baryon number before reaction equals baryon number after reaction.
• Absolute conservation of baryon number implies that protons never decay
Measured lifetime of the proton>10 32 years.
•
Lepton number
• Three versions: Le, Lm, Lt
Example: n ® p + e + n e
-
q
0
+1
-1
0
B
+1
+1
0
0
Le
0
0
+1
-1
n ® p + e- + n e
• Tau particles are uncommon in nature. How do we produce them?
CONSERVATION LAWS
•
Strangeness Number
• S=+1 for particles with anti-strange quarks (K+) , -1 for particles with strange quarks (K-)
• Produced in strong interactions
p - + p ® K 0 + L0
q
-1
+1
0
0
B
0
+1
0
+1
S
0
0
+1
-1
+
• Forbidden reaction: p + p ® K + n
• Not conserved in weak interactions
K0 ® p+ +pq
0
+1
-1
S
+1
0
0
More conservations laws to come
TOO MANY “FUNDAMENTAL” PARTICLES!
• The Eightfold Way (1961)
• Murray Gell-Mann and Yuval Ne’eman noticed patterns in the quantum numbers of
particles
same q, B, S,
higher mass
Octet
Spin ½ baryons
Spin 0 mesons
PREDICTION OF THE EIGHTFOLD WAY
W-
Spin 3/2 baryons
W- discovered in 1964 at BNL. Mass of 1680 MeV/c 2
Missing
particle?
QUARKS
•
Evidence for hadrons being made of smaller particles
• Substructure of hadrons
• Patterns of same spin particles
• Hadrons decay to other hadrons
•
Gell-Mann and George Zweig independently proposed quarks (Gell-Mann’s term) in 1963.
•
Original model had up, down, and strange quarks (u, d, s)
Never seen
alone!
Always bound
in 3-quark
pairs or quarkantiquark pairs.
QUARK COMPOSITION OF HADRONS
p+
q=+2/3+1/3=1
B=1/3-1/3=0
proton
q=+2/3+2/3-1/3=1
B=1/3+1/3+1/3=1
Kq=-2/3-1/3=1
B=-1/3+1/3=0
S=0-1=-1
neutron
q=+2/3-1/3-1/3=0
B=1/3+1/3+1/3=1
•
Baryons are made up of three quarks
•
Antibaryons are made up of three antiquarks
•
Mesons are made up of quark-antiquark pairs
•
Antimesons are up of the corresponding antiquark-quark pairs
CHARM, BOTTOM, AND TOP QUARKS
•
Charm proposed to account for discrepancies between experiment and predictions of the
quark model.
• J/Y (jay-psi) found by groups at SLAC and BNL in 1974
• M=3100 MeV/c2
• charm-anticharm meson
•
After discovery of tau lepton, physicist (who like symmetry) thought that if there are three
families of leptons, perhaps there are three families of quarks
• Bottom quark (previously called beauty) verified with discovery of upsilon meson at
Fermi Lab in 1977
• M=9.46 GeV/c2
• Top quark (previously called truth) verified with discovery of Y meson at SLAC in
1995
• M=173 GeV/c2
DISCOVERY OF HEAVY PARTICLES
•
How do we make them?
• Use very high-energy particle beams to convert energy into mass
•
How do we know we have made them?
• Look for “resonances” or bumps in the mass/energy spectrum
Example: e-+pe-+p++n
short lived, cannot
been seen directly
Example: e-+pe-+K+L
L0
S0
missing mass (GeV/c2)
measure energy/momentum of e-K+ and
reconstruct mass of missing particle
QUANTUM CHROMODYNAMICS
THEORY OF THE STRONG FORCE
•
Let’s look at the quark composition of the proton again…
• Quarks are spin ½ particles. So what is wrong with this picture?
• Violates P.E.P. How do we fix it?
• Introduce new degree of freedom that we call the color charge.
• Quarks carry one of three colors (red, blue, green) or anti-color.
• Together, they form a color neutral object.
• Two quark systems (mesons)
• The color force is mediated by gluons.
QUANTUM CHROMODYNAMICS
EVIDENCE FOR QUARKS
•
What happens when we hit a quark real hard?
•
As the u quark gets away from the other guys, it hadronizes, creating a ss-bar pair.
•
uds combine to form L 0
•
us-bar combine to form K+
QUANTUM CHROMODYNAMICS
HOW IT PERTAINS TO THE NUCLEAR FORCE
pUnderstanding this is one of the goals of the Jefferson
Lab physics program.
PARTICLES OF THE STANDARD MODEL
Gauge
bosons
mediating
particle of
the EM
force
(massless)
mediating
particle of
the strong
force
mediating
(massless)
particles of
Higgs Boson: Needed to explain difference
gauge boson masses.
theinweak
Also explains existence of mass.
force
Subject of physics program at LHC.
(massive)
2
Possible signal for it at ~124 GeV/c