Why Does Z Boson Only Decay to Fermion-Antifermion Pairs?

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Discussion Overview

The discussion centers on the decay processes of the Z boson, specifically why it predominantly decays into fermion-antifermion pairs and the implications of flavor conservation in weak interactions. Participants explore theoretical frameworks and mechanisms that govern these decay processes.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions why the decay Z → anti-down, strange is forbidden, noting that while charge conservation is maintained, strangeness is not conserved, which raises further inquiries about the weak interaction.
  • Another participant suggests that the decay is restricted due to quarks and leptons being organized in SU(2) doublets and mentions the unitarity of the CKM matrix, referencing FCNC and the GIM Mechanism as relevant concepts.
  • It is noted that the Z boson is flavor-diagonal, with only charged weak interactions violating flavor, as they connect different types of quarks, while the Z boson couples to the same type of antiquark.
  • A participant elaborates that at tree-level, the Z-boson couplings to quark-antiquark pairs remain flavor-diagonal, and discusses the implications of this in the context of the recently discovered Bs → muon + anti-muon decay, which involves a Z penguin process.
  • Another participant raises the question of whether the Z boson could have flavor-changing decays to strange + anti-bottom at the one-loop level, expressing an expectation for such decays despite a lack of experimental evidence.

Areas of Agreement / Disagreement

Participants express various viewpoints on the mechanisms behind Z boson decays, with some agreeing on the flavor-diagonal nature of the Z boson couplings while others question the potential for flavor-changing processes. The discussion remains unresolved regarding the existence of certain decay channels.

Contextual Notes

Participants reference specific theoretical frameworks and experimental observations, but there are limitations in the discussion regarding the assumptions made about flavor conservation and the implications of the CKM matrix. The absence of certain decay processes in experimental data is also noted.

copernicus1
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Why does the Z boson only decay to fermion-antifermion pairs? I'd just like to understand the basic reason why something like Z --> anti-down, strange wouldn't work. This would conserve charge. It obviously wouldn't conserve strangeness, but the weak interaction doesn't, so I'm just wondering why this decay is forbidden.

Thanks!
 
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In a nutshell, because quarks and leptons occur in SU(2) doublets and because the CKM matrix is unitary. Key words to google are FCNC (Flavor Changing Neutral Currents) and the GIM Mechanism which suppresses them.

For a simple explicit demonstration that they exactly cancel in tree order, see pp 144-146 here.
 
Because the Z is flavor-diagonal. Only the charged weak interaction violates flavor, because that links a quark to a different antiquark. The Z (and the photon) link it to the same antiquark.
 
Indeed at tree-level the Z-boson couplings to quark-anti-quark are flavour-diagonal even after you rotate the quarks to their mass eigenstate basis. This is because the neutral Z must couple to two down-type quarks or two up-type quarks and the combined unitary rotations cancel. The charged W boson couples to one up-type quark and one down-type quark, the combination of the two unitary rotations of up and down quarks results in the CKM matrix.

Now I know that the process Bs --> muon + anti-muon, recently discovered at the LHC, comes mostly from a Z penguin with a W in the loop. This is a rare flavour changing process. Does this mean that at the one-loop level that the Z boson should also have flavour-changing decays to a strange + anti-bottom?
 
jkp said:
Does this mean that at the one-loop level that the Z boson should also have flavour-changing decays to a strange + anti-bottom?
I would expect those, but according to the http://pdglive.lbl.gov/Rsummary.brl?nodein=S044&inscript=Y&fsizein=1&wholedec0=Y , no such decay was found yet.
 
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