Weak Force

1
Weak Force

• Exchange particles are W, W-, Z0
• These particles have masses between 80,000 and
  90,000 MeV!!!
• Really short lifetimes by uncertainty principle
  of 3 x 10-25 seconds while the pions have
  lifetime around 3 x 10-8 seconds

2
Conservation Laws

• We pick up some new conservation laws by
  observing interactions of these particles
• Conserve baryons (nucleons others)
• Conserve particles (need anti-particles to
  explain)
• Conserve leptons (what are these?)

3
Table of the Particles

• Weakly Interacting Particles (Weak Force)
• Leptons
• Electron
• Muon (fat electron - 106 MeV - 10-6 s)
• Tau (fattest electron - 1777 MeV - 10-13 s)
• Electron Neutrino
• Muon Neutrino
• Tau Neutrino

4
Table of the Particles

• Strongly Interacting Particles (Hadrons)
• Mesons
• Pion (,0) negative pion is antiparticle of
  plus pion
• Kaon (,0,0)
• Eta (0)
• Baryons
• Proton ()
• Neutron (0)
• Lambda (0)
• Sigma (,-,0)
• Xi (0,-)
• Omega ()

5
More Conservation

• Strangeness arose from observations that some of
  the particles in the list were always produced in
  pairs
• Particles produced by a strong interaction but
  only decayed by weak interaction
• Quality called strangeness and a new quantum
  number

6
Still Higher Energy Probing

• Shoot very energetic electrons at protons
• Electrons always behave like perfect point
  particles with no clue whatsoever that they have
  a size
• Protons seem to have size!!! Why??
• Early experiments showed protons seemed to peel
  like an onion with layers
• Higher energies showed that there were three
  scattering centers inside a proton!!!

7
Quarks

• We arrive at the present
• We now know there are six possible smaller
  particles called quarks
• These guys along with their anti-quarks make up
  all the hadrons!
• Just like leptons, quarks come in generations
• We have six leptons and six quarks

8
Quarks

• The leptons are electron, muon, tau and their
  three neutrinos
• Quarks are called up, down, strange, charm,
  bottom and top
• Bound systems make baryons
• Bound quark-antiquarks make mesons

9
Quarks
10
Gluons

• The exchange particle carrying the strong force
  between quarks is called a gluon
• So we can now create sort of a grand table of
  what is called the standard model

11
The Standard Model

• Generations of quarks (u,d), (s,c), (b,t)
• Generations of leptons (e,?e), (?, ??), (?, ??)
• Generations of exchange (?), (W,Z), (gluons)
• Final problem is how to put two ups and a down
  together to make a proton for example
• The two ups cannot have the same quantum numbers
  by exclusion principle!!! (H2)

12
The Standard Model

• Need another quantum number which can be
  different
• Presume quarks have a property called color
• Color has three values, (red, green, blue)
• Hadrons must be white (one each of RGB) or
  (color, anti-color)

13
The Standard Model

• In the 70s, it was shown that the weak nuclear
  force and electromagnetism are two aspects of the
  same thing
• Now trying to unify the strong force into the
  picture

14
Quantum Electrodynamics

• A remarkably successful theory which agrees with
  experiment to 10 and more decimal places
• Lots of interesting stuff
• Shifts in hydrogen energy levels due to pair
  production and virtual particles

15
Quantum Chromodynamics

• Working to bring this to the level of success
  with quantum electrodynamics
• Much harder calculational problem
• Quark Confinement Issues and Asymptotic Freedeom
• Grand Unified Theories