There are no conservation laws forbidding such decays as mu- --> e- + muon neutrino + electron anti-neutrino or neutron --> proton + electron + electron anti-neutrino. Charge, energy, and momentum are conserved as well as muon and electron lepton numbers. Why do they decay? Because they can. The decay lifetimes of the muon and tau neutrinos are determined by the strength of the weak coupling constant and by the available energy. An electron can't decay because there is no other less massive charged particle. If a neutrino has sufficient energy it can interact with a neutron and produce an electron plus a proton.
LeeH
(published on 07/24/2013)
