Lepton universality ($g_e=g_{\mu }=g_{\tau }$)

The Standard Model assigns the same gauge coupling to each lepton generation in the weak charged current. Different decay rates still arise from masses and phase space. Ratios of leptonic pion decay rates cancel the common hadronic decay constant and quark mixing coefficient, isolating universality and helicity suppression. Electromagnetic radiative corrections must be included for precision comparisons to inclusive measured decay rates.