Transcript decays
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 0 u 8 10 s 17 • 0 (uds) 0 (uds) 7 10 20 s ubar • 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 P461 - particles III 1 C-cbar and b-bbar Mesons • Similar to u-ubar, d-dbar, and s-sbar S 0 (cc ) b (bb ) S 1 J / (cc ) (bb ) • “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 ( ss ) K (us ) K ( su ) 4S (bb ) B (ub ) B (bu ) • But if mass <2*meson decays EM. “easiest” way is through virtual photons (suppressed for pions due to spin) c cbar m m P461 - particles III 2 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) BF ( m m ) 0.06 BF ( e e ) 0.06 BF ( hadrons) 0.88 • Some of the decays into hadrons proceed through virtual photon and some through a virtual (colorless) gluon) c cbar u u P461 - particles III d d 3 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 quarkantiquark 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 u u us su (etc) Mass(ee) ( Ee Ee ) 2 ( pe pe ) 2 e+ e- q qbar P461 - particles III 4 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 n p e e 0 e e U u d d d u W e e m u c t d s b • Charge current Weak interactions proceed be exchange of W+ or W-. Couples to 2 members of weak doublets (provided enough energy) P461 - particles III 5 Decays of Leptons • Transition lepton->neutrino emits virtual W which then “decays” to all kinematically available doublet pairs m e e m 100% m m e W e • 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. e e m m ( n ) P461 - particles III 17% 18% 65% 6 Weak Decays of Hadrons • Can have “beta” decay with same number of quarks in final state (semileptonic) K m m 0 • or quark-antiquark combine (leptonic) e u W d e e e or m m u • or can have purely hadronic decays s u u uu d K 0 K 0 0 • Rates will be different: 2-3body vs 3-body phase space; different spin factors P461 - particles III 7 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 • t b W e, m , c s u d • the t quark decays before it becomes a hadron. The outgoing b/c/s/u/d quarks are seen as jets P461 - particles III 8 Top Quark Decay • Very small rate of t-->s or t-->d • the quark states have a color factor of 3 • t b e e 11% t b m m 11% tt (be ) (b e ) 1.2% t b tt b(e or m ) b (e or m ) 4.8% 11% t b c s 33% tt (bqq ) b (e or m ) 29% t bu d 33% ( 2 * .22* .66) tt (bqq ) (b qq ) 44% t b W • e, m , , c , u , s, d P461 - particles III 9