EM Decay of Hadrons

1. EM Decay of Hadrons u g • 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 g ubar P461 - particles III

2. 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) m+ c g m- cbar P461 - particles III

3. 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 charge2(EM coupling) and mass (phase space) • Some of the decays into hadrons proceed through virtual photon and some through a virtual (colorless) gluon) c g cbar P461 - particles III

4. 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 q e+ g qbar e- P461 - particles III

5. 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) U d d u d u W e n P461 - particles III

6. 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 P461 - particles III

7. 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 P461 - particles III

8. 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 P461 - particles III

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