Muon decay and neutrino mass

The calculation of the $\mu$ decay is performed in V-A theory taking into account that, in agreement with the observation of the oscillations, the neutrinos must be massive. Provided that the masses of the electron and muon neutrinos are not higher than the known upper values, the result is a possible modification of the Fermi constant, no effect at all on the polarization of the outgoing electron and possibly an endpoint effect on the electron energy and angle distributions.