• Forces: Theories of Mechanisms:
–Action at a Distance
–Action through fields
–Action through exchange of particles or
“force carriers”
• Four Forces:
–Strong, Weak , E&M, Gravitation
–“electroweak” (E&M and weak are linked)
• GUT’s: Grand Unified Field Theories (attempt to link the strong force to the others.)
• TOE ’s: Theories of Everything: attempt to link
Gravity to the others; “Quantum Gravity”

Standard Model of Particle Physics
Four
Forces
Strong
E&M
Range
~ 10
1/r 2
-15 m
Force
Carrier gluon
Examples
•Quarks forming nucleons
•Nucleons forming nuclei photon Already Seen
Weak
Gravity
(infinite)
< 10
1/r 2
-18
(infinite) m W, Z bosons
Radioactivity:
Ex: β decay graviton? (already seen)

• Every particle making up matter has an
“ antiparticle ”.
• The antiparticle has the same mass and opposite charge as the particle.
• “ Pair Annihilation ”: If the two meet; they annihilate each other giving rise to lots of energy: E = m tot c 2 where m the two particles together.
tot is the mass of
• “ Pair Creation ”: A photon can give rise to two antiparticles.

• Electron and Positron
• Proton and Antiproton
• Neutron and Antineutron
• Neutrinos and Antineutrinos.

Two types of Particles
“hbar” = h/(2π)
• Fermions:
– have ½ integer spin
– Compose all matter
– Have antiparticles
• Bosons
– Have integer spin
– If elementary: act as force carriers for interacting fermions
– Otherwise, can be composed of fermions
– Each is its own antiparticle.

Fundamental Fermions
• Quarks
– Compose heavier particles (the Hadrons )
– Have fractional charge: (+ ⅔ e, -⅓ e)
– Antiparticles have charge: (- ⅔ e, + ⅓ e)
– 6 types called “flavors” (plus their antiparticles)
• Up, down, strange, charmed, bottom, top
• u, d, s, c, b, t ; antiparticles have bars on top.
• (+ ⅔ e): u, c, t ; (-⅓ e): d, s, b
– Size: r ~ 10 -17 m
– Interact via the strong force
• Leptons
– Lighter particles, integer charge (ex: electrons, neutrinos) (Do not feel the strong force)

• Composed of quarks
• Quarks are never observed free.
• Baryons
– Composed of 3-quark combinations resulting in integer charge
• Proton: uud (charge = +e)
• Neutron: udd (charge =0)
– Baryon number is conserved in particle reactions
• Antiparticles have negative baryon number.
• Mesons
– Composed of quark-antiquark pairs ex: pion: π + : ud
– unstable