= Polarized massless leptonic W decay
{title2=$\frac{d\Gamma_{\epsilon_z}}{d\Omega}=\frac{g^2M_W}{128\pi^2}\sin^2\theta$}
For a resting <W boson> with real spatial <polarization vector> along $z$, the <fermion spin sum> gives $\sum|\mathcal M|^2=g^2M_W^2\sin^2\theta/2$ for massless charged <lepton> and <antineutrino> momenta. The <two-body Lorentz-invariant phase space> gives the displayed angular <decay rate>. Since $\int\sin^2\theta\,d\Omega=8\pi/3$, the integrated width is $g^2M_W/(48\pi)$, equal to the unpolarized width. The <massive-vector spin average> changes the angular distribution, not the total width. No initial <spin average> is taken for a specified pure polarization.
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