= Pion electron-to-muon decay ratio
{title2=$R_{e/\mu}=\frac{m_e^2}{m_\mu^2}\left(\frac{1-m_e^2/M_\pi^2}{1-m_\mu^2/M_\pi^2}\right)^2$}
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.
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