# Particle Reactions and Decays - I [Secs 16.1, 16.2 Dunlap]

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Particle Reactions and Decays - I [Secs 16.1, 16.2 Dunlap]

CAN IT HAPPEN ? Check B, L i, Qc No Yes IS IT WEAK? T, S, C violation ? Yes WEAK  W +, or W - involved.  Flavor change occurs with one unit of charge change. QUARKS LINK UP? γ in products Yes No REACTION DECAY enough energy Q>0 Q<0 No STRONG No ? Yes ? E.M.

LEPTON DECAYS (i.e. decay into leptons) LEPTONIC DECAY HADRONIC – LEPTON DECAY

In 1963 The UFI is back In 1963 theoretical physicist Nicolo Cabibbo gave an explanation in terms of quark state mixing and introduced an angle - The Cabibbo angle The quarks can exist either as eigenstates of the WEAK interaction or the STRONG interaction. Indeed it is best to think of the QUARKS as being fundamentally having FLAVOR STATES determined via the WEAK interaction. Then comes along the STRONG interaction which MIXES these flavors into.

How the Quark Mixing works θcθc θcθc After switching on the STRONG interaction – these are the new quark states. The Cabibbo Angle turns out to be ~15 °

Cabbibo allowed – Cabibbo Surpressed Cabibbo Allowed Cabibbo Surpressed ddsdds dusdus ddsdds dduddu

W+W+ u d W+W+ u s K+K+ MEASURING THE CABIBBO ANGLE

Example 1 BETA MINUS DECAY All the primary conservation laws (above the line) are ok, so the reaction should go. But is it S, W, or EM? There are two things that indicate the primary classification is WEAK. These are (i) the fact that this T of the final state is N.D (not defined) and (ii) the fact that leptons are present in the final state. Having established that it is a W (weak) decay we then make the inference: WEAK DECAY  INVOLVEMENT OF “W” PARTICLE

Example 1 BETA MINUS DECAY FEYNMAN DIAGRAM CLASSIFICATION = Semileptonic Hadron Decay

Example 2 ASSOCIATED PRODUCTION All the primary conservation laws (above the line) are ok. Also the secondary conservation laws (obeyed by the strong interaction, ones below the line) are ok. This means the reaction must be mediated by the STRONG force.  There will be NO Ws  All quarks will “link up”.

Example 2 ASSOCIATED PRODUCTION FEYNMAN DIAGRAM CLASSIFICATION = PURE HADRONIC

Example 3 K- MESON DECAY All the primary conservation laws (above the line) are ok. But here the secondary conservation laws (obeyed by the strong interaction, ones below the line) are NOT OK. This is a clear indication that this cannot be a strong process. The presence of leptons also confirms that this must be a WEAK interaction process.  the presence of W particles.

Example 3 K- MESON DECAY FEYNMAN DIAGRAM DECAY CLASSIFICATION = LEPTONIC HADRON DECAY

Example 4 LAMDA ZERO DECAY As with the K+ decay we see failure on isospin and strangeness. Again this is clear indication that the WEAK interaction is responsible. [We saw in the lecture on strangeness that a strong interaction could only occur if the strange quark that had been produced could find another strange quark for pairing up with – and annihilating with]  W PARTICLE involved in FLAVOR changing

Example 4 LAMDA ZERO DECAY FEYNMAN DIAGRAM DECAY CLASSIFICATION = Nonleptonic Hadron decay

Example 5 DELTA ++ PRODUCTION So all is OK for an allowed reaction and one going by the STRONG interaction. This a fully hadronic process.

Example 5 DELTA ++ PRODUCTION FEYNMAN DIAGRAM CLASSIFICATION = Hadronic Reaction

Example 6 D ZERO DECAY All is ok above the line – so the process is possible. Below the line we see a violation of strangeness and charm. Thus it looks as if we have a charmed quark changing into a strange quark.  Involvement of a W boson. This is a WEAK process.

Example 6 D ZERO DECAY FEYNMAN DIAGRAM DECAY CLASSIFICATION = Nonleptonic Hadron decay

Example 7 D ZERO DECAY We see again an allowed decay but one which involves flavor changing on a quark. Here, however the presence of leptons in the final state makes finding the Feynman diagram easy.

Example 7 D ZERO DECAY FEYNMAN DIAGRAM DECAY CLASSIFICATION = Semileptonic Hadron decay

Example 8

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