In Muon decay, the decay width is: [itex]\Gamma[/itex][itex]\mu[/itex] = hbar/[itex]\tau[/itex][itex]\mu[/itex] = GF² / (192π³(hbar.c)6) . (m[itex]\mu[/itex]c²)5(1+[itex]\epsilon[/itex]) It's this (m[itex]\mu[/itex]c²)5 that bothers me.. How do you decide the powers in other reactions? I know it has something to do with the degrees of freedom, but I don't know how. Also: what would be the answer to following question? p+ + p- -> top particles. The decay width will go according to GF.(Mt)x What is the value of x?