Pion electron-to-muon decay ratio ($R_{e/\mu }=\frac{m_e^2}{m_{\mu }^2}{\left(\frac{1-m_e^2/M_{\pi }^2}{1-m_{\mu }^2/M_{\pi }^2}\right)}^2$)

At tree level with universal weak charged current coupling and massless neutrinos, the leptonic pseudoscalar decay width gives the displayed ratio. The pion decay constant, Fermi constant and common quark mixing coefficient cancel. The charged-lepton mass factor is helicity suppression: a spin-zero parent cannot emit the two massless helicities selected by the chiral weak interaction. The ratio tests this suppression and lepton universality; scalar or pseudoscalar four-fermion interactions can modify it. Radiative corrections must be included for a precision comparison.