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Syracuse Summer Institute Finale on Weak Decays

Syracuse Summer Institute Finale on Weak Decays. Oh, so many decays, and so little time. I’ve shown you one possible weak decay last time. A B - meson has 100’s of ways it can decay! Huh, why? Because it can! (Remember, tenet of Quantum Physics).

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Syracuse Summer Institute Finale on Weak Decays

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  1. Syracuse Summer InstituteFinale on Weak Decays

  2. Oh, so many decays, and so little time • I’ve shown you one possible weak decaylast time. • A B- meson has 100’s ofways it can decay! Huh, why? • Because it can! (Remember, tenet of Quantum Physics) This is a small fraction of the decaysknown for the B0 meson (bd) The number on the right is thefraction of time the B0 mesonundergoes that decay. For every one of these decays,one should be able to draw aFeynman diagram to represent it!

  3. Another B meson decay p-p+p- u u c d b d B0 D+ W- W+ p+ D+ K-p+p+ Happens ~9% of the time K- g u p+

  4. Weak Decays III II I d s b Width of arrowsgive a qualitativefeel for the decayrate. u c t • Top: tb >>> ts or td. • Bottom: bc >> bu . • Charm: (cs) ~ 25 x (cd); smaller total decay rate. m • Strange: su dominates in many cases, but not all.. e.g. K+ mndominant (no “u” quark in final state) n

  5. Comparison of “Branching Fractions” “Branching fraction” is just a fancy name that tells how often a particle decays to some particular final state. p- b d b d W- W- BF=2.68x10-3 c d B0 D+ u u p- BF=5.15x10-6 u d B0 p+

  6. New Physics in B decays • If in the decay of a particle, there is a virtual particle“loop”, there is the potential for a new particle tocontribute to the decay. • Since it’s emitted and re-absorbed, it needn’t conserve energy, as long as DEDt~ • Example coming on next slide. • Key point: Even though you may not see it directly, it will leave its fingerprints. • Deviations from SM expected rates or other observable quantities.

  7. “Penguin” decays g t t s b b s W- • Involves a virtual W boson loop – suppressed in the SM. • Potential for new, high mass particles to contributein these kinds of “loop diagrams”. • Just replace W and top quark with some new physicsparticles!

  8. Particle-antiparticle mixing b t d b d W- W+ The BB oscillation occurs ~200 billion times per second. Alternately said, the BBoscillate back & forth with a periodof ~5 pico-seconds. Recall, that the decay time is ~ 1.5 ps. So, about 20% of produced B0mesons will decay as B0 ! • Involves 2 W bosons – suppressed in the SM. • Involves a “loop” with 2 virtual top quarks and 2 W’s. • Potential for new, high mass particles to contribute Oddly enough, the weak interaction allows certain particles to transform into their antiparticles.

  9. Neutrino Interactions e- space ne W+ u d time • Here, a neutrino interacts with a d-quark. The neutrino emits a W+ and turns into an e-. • The d-quark absorbs the W+ and turns into a u-quark.

  10. The force carriers and what they couple to Quarks NeutralLeptons ChargedLeptons Carrier Force Mass u c t nm m nt ne e t d s b 0 EM Photon Yes Yes No Strong Gluon 0 Yes No No ~100Xprotonmass Weak W±, Z0 Yes Yes Yes

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