Muon decays are almost always written as $$\mu^- \rightarrow e^-+ \bar\nu_e +\nu_\mu.$$
The reason given on wikipedia is that one of the product neutrinos of muon decay must be a muon-type neutrino and the other an electron-type antineutrino due to conservation of leptonic family numbers.
Does this mean that muons can't decay into quarks plus a muon-type neutrino, which can be lighter particles than the muons itself and with electric charge added up to -1? If they can't, why?
Best Answer
No, muons can't decay into quarks because quarks are confined; the final product cannot be quarks, but rather composite particles made of quarks, such as mesons and baryons. The lightest mesons are the pions, which are already heavier than the muon, so any such decay is forbidden by energy conservation.
On the other hand, the extremely heavy tau can and does decay to light mesons quite often, as you can see in the PDG entry.