where the masses of the electron, muon, and tau are measured respectively as m e = 0.510 998 946 (3) MeV/ c , m μ = 105.658 3745 (24) MeV/ c , and m τ = 1 776.86(12) MeV/ c ; the digits in parentheses are the uncertainties in the last digits. This gives Q = 0.666 661 (7) .