# When is a decay mediated by the weak force

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?

Edit:

$$u \bar d$$ 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.

• Why do you think flavor is not conserved in $p\bar{p}$ annihilation? – knzhou May 22 at 19:29
• 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. – Vishal Jain May 22 at 19:32
• What is "the first decay process"? – knzhou May 22 at 19:36
• 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. – knzhou May 22 at 19:37
• @knzhou I have made an edit, sorry I was vague about that, by first decay I mean the pi+ decaying into leptons. – Vishal Jain May 22 at 19:44

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.