= Massless leptonic W decay width
{title2=$\Gamma_\ell=g^2M_W/(48\pi)$}
The chiral <weak charged current> vertex $-ig\gamma^\mu(1-\gamma_5)/(2\sqrt2)$ gives the spin-averaged squared <matrix element> $g^2M_W^2/3$ for an on-shell <W boson> decaying to a massless charged <lepton> and <antineutrino>. Integrating <two-body Lorentz-invariant phase space> gives $\Gamma_\ell=g^2M_W/(48\pi)$. The <massive-vector spin average> divides by three; an extra neutrino-helicity factor would incorrectly double count the <chiral projector>.
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