1. P461 - particles III1 EM Decay of Hadrons If a photon is involved in a decay (either final state or virtual) then the decay is at least partially electromagnetic Can’t have u-ubar quark go to a single photon as have to conserve energy and momentum (and angular momentum) Rate is less than a strong decay as have coupling of 1/137 compared to strong of about 0.2. Also have 2 vertices in pi decay and so (1/137) 2 EM decays always proceed if allowed but usually only small contribution if strong also allowed u ubar  

2. P461 - particles III2 C-cbar and b-bbar Mesons Similar to u-ubar, d-dbar, and s-sbar “excited” states similar to atoms 1S, 2S, 3S…1P, 2P…photon emitted in transitions. Mass spectrum can be modeled by QCD If mass > 2*meson mass can decay strongly But if mass <2*meson decays EM. “easiest” way is through virtual photons (suppressed for pions due to spin) c cbar   

3. P461 - particles III3 C-cbar and b-bbar Meson EM-Decays Can be any particle-antiparticle pair whose pass is less than psi or upsilon: electron-positron, u-ubar, d-dbar, s-sbar rate into each channel depends on charge 2 (EM coupling) and mass (phase space) Some of the decays into hadrons proceed through virtual photon and some through a virtual (colorless) gluon) c cbar 

4. P461 - particles III4 Electromagnetic production of Hadrons Same matrix element as decay. Electron-positron pair make a virtual photon which then “decays” to quark-antiquark pairs. (or mu+-mu-, etc) electron-positron pair has a given invariant mass which the virtual photon acquires. Any quark- antiquark pair lighter than this can be produced The q-qbar pair can acquire other quark pairs from the available energy to make hadrons. Any combination which conserves quark counting, energy and angular momentum OK e+ e-  q qbar

5. P461 - particles III5 Weak Decays If no strong or EM decays are allowed, hadrons decay weakly (except for stable proton) Exactly the same as lepton decays. Exactly the same as beta decays Charge current Weak interactions proceed be exchange of W+ or W-. Couples to 2 members of weak doublets (provided enough energy) UddUdd uduudu W e

6. P461 - particles III6 Decays of Leptons Transition lepton->neutrino emits virtual W which then “decays” to all kinematically available doublet pairs For taus, mass=1800 MeV and W can decay into e+nu, mu+nu, and u+d (s by mixing). 3 colors for quarks and so rate ~3 times higher. W e

7. P461 - particles III7 Weak Decays of Hadrons Can have “beta” decay with same number of quarks in final state (semileptonic) or quark-antiquark combine (leptonic) or can have purely hadronic decays Rates will be different: 2-3body vs 3-body phase space; different spin factors W e

8. P461 - particles III8 Top Quark Decay Simplest weak decay (and hadronic). Mtop>>Mw (175 GeV vs 81 GeV) and so W is real (not virtual) and there is no suppression of different final states due to phase space the t quark decays before it becomes a hadron. The outgoing b/c/s/u/d quarks are seen as jets t b W c u

9. P461 - particles III9 Top Quark Decay Very small rate of t-->s or t-->d the quark states have a color factor of 3 tb W