PX156 - G3 - basic rules

a photon can be emitted/absorbed by a lepton or a quark

everything can emit/absorb a $Z^0$ boson

gluons only interact with quarks or themselves

$W^{\pm }$ are quite complicated

• leptons:
  • transitions between charged and neutral leptons of the same family
  • eg: $e^{-}\to W^{-}{\nu }_e$ ; $W^{-}\to {\nu }_e{e}^{-}$
  • $W$ bosons preserve the family/generation of leptons
• quarks:
  • transitions between top to bottom rows
  • cross-generational couplings are allowed

there are no cross-generational couplings at the same level

• eg: $d\to s$ ; $t\to c$ are not allowed
• ie: no 'flavour changing neutral currents'

conservation rules apply at each vertex

crossing symmetries

• if there is a workable interaction, it can be changed into another interaction by moving a line from the initial state to the final state, and turning the line into an antiparticle, as long as energy is conserved
• eg: $u\to W^{+}d$

  • possible interactions
    
    
    

  • not possible: violates conservation of energy