…or really…

The Standard Model

CHAPTER 14ELEMENTARY PARTICLE PHYSICS

FUNDAMENTAL FORCES OF NATUREForce Particles

acted upon

Relative Strength

Lifetimes Range Mediating particle

Notes

Strong Hadrons and

Quarks

1 <10-20 s Short (~1fm)

Gluons(pions in nucleus)

Binds quarks into hadrons and nucleons

into nuclei

Electromagnetic All charged particles

~10-2 ~10-16 s Long (∞) Photons Unification of electric and magnetic

forces

Weak Quarks and

Leptons

~10-6 >10-10 s Very short (~10-3 fm)

W±, Zo Responsible for beta decay

Gravity Everything ~10-43 ? Long (∞) Graviton?

Unified in Electroweak Theory

Unified in the Standard Model

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

• Seen naturally in b-decay (pn+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+e- pairs results in…?

Every particle is now known to have an antiparticle

Another new fundamental particle!

• 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.

SEARCH FOR “FUNDAMENTAL PARTICLES”

Another new fundamental particle!• Pion discovered in 1947 by Cecil Powel and Giuseppe Occhialini

Comes in three charge statesp0 2gp+ m++nm

p- m-+nm

m-e-+nm+ne

m+e++nm+ne

More new 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.

SEARCH FOR “FUNDAMENTAL PARTICLES”

K0p++p-

• Associated production p-+pL0+K0 with 4 GeV/c pions in a bubble chamber.

K0p++p-

L0p+p-

• 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 PROPERTIESQ.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

• MesonsGuys like pions and kaonsSpin 0 or 1

• Leptons:• Spin 1/2• Appear to be point-like particles (no internal structure)• Come in three flavors (plus their antiparticles)

• Neutrinos can oscillate between flavorsimplies 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>1032 years.

• Lepton number

• Three versions: Le, Lm, Lt

Example:

• Tau particles are uncommon in nature. How do we produce them?

q 0 +1 -1 0

B +1 +1 0 0

Le 0 0 +1 -1

CONSERVATION LAWS• Strangeness Number

• S=+1 for particles with anti-strange quarks (K+) , -1 for particles with strange quarks (K-)

• Produced in strong interactions

• Forbidden reaction:

• Not conserved in weak interactions

q -1 +1 0 0

B 0 +1 0 +1

S 0 0 +1 -1

q 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

Spin ½ baryons Spin 0 mesons

same q, B, S, higher mass

Octet

PREDICTION OF THE EIGHTFOLD WAY

W- discovered in 1964 at BNL. Mass of 1680 MeV/c2

Spin 3/2 baryons

W-

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 quark-antiquark pairs.

QUARK COMPOSITION OF HADRONS

• 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

protonq=+2/3+2/3-1/3=1B=1/3+1/3+1/3=1

neutronq=+2/3-1/3-1/3=0B=1/3+1/3+1/3=1

p+

q=+2/3+1/3=1B=1/3-1/3=0

K-

q=-2/3-1/3=1B=-1/3+1/3=0S=0-1=-1

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-+pe-+p++n Example: e-+pe-+K+L

short lived, cannot been seen directly

missing mass (GeV/c2)measure energy/momentum of e-K+ and reconstruct mass of missing particle

L0

S0

QUANTUM CHROMODYNAMICSTHEORY 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 CHROMODYNAMICSEVIDENCE 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 L0

• us-bar combine to form K+

QUANTUM CHROMODYNAMICSHOW IT PERTAINS TO THE NUCLEAR FORCE

p-

Understanding this is one of the goals of the Jefferson Lab physics program.

PARTICLES OF THE STANDARD MODEL

mediating particles of the weak force (massive)

mediating particle of the strong force (massless)

mediating particle of the EM force (massless)

Higgs Boson: Needed to explain difference in gauge boson masses. Also explains existence of mass.Subject of physics program at LHC.Possible signal for it at ~124 GeV/c2

Gauge bosons