If a $\pi ^+ \to \nu_\mu+\mu^+$, here quark flavour is lost entirely. I am aware the weak force mediates decays where quark flavour is not conserved. But If I consider a $p+\bar p$ annihilating, here quark flavour is lost as well but the decay is not mediated by the weak force, but rather by photons (or the strong force?).

When do we use the w and z bosons in a decay?


$u \bar d$ are the quarks we start with. We end with up with 0 quarks, losing all flavour, is this still a process mediated by the weak forces W bosons? I was initially under the impression that the products still had to contain quarks for a weak force decay.

  • $\begingroup$ Why do you think flavor is not conserved in $p\bar{p}$ annihilation? $\endgroup$ – knzhou May 22 at 19:29
  • $\begingroup$ Ah, the flavour is 0 before and after, but what about the first decay process? A hadron decays into a lepton, I can't see how flavour is conserved. $\endgroup$ – Vishal Jain May 22 at 19:32
  • $\begingroup$ What is "the first decay process"? $\endgroup$ – knzhou May 22 at 19:36
  • $\begingroup$ In general, if you want to get your questions answered, you need to spell out all the reasoning you used to arrive at your conclusions. $\endgroup$ – knzhou May 22 at 19:37
  • $\begingroup$ @knzhou I have made an edit, sorry I was vague about that, by first decay I mean the pi+ decaying into leptons. $\endgroup$ – Vishal Jain May 22 at 19:44

One works with feynman diagrams to keep count of quantum numbers:

p+ mu netrino

Pions decay because they consist of a quark and an antiquark and the $W^+$meson changes the up into a down through the weak interaction, allowing down-antidown annihilation into a virtual $W^+$, and then the $W^+$ decays.


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