When is a decay mediated by the weak force

Question

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.

Answer 1

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

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.