= W boson propagator
{c}
{title2=$D_{\mu\nu}(p)=\frac{-i(g_{\mu\nu}-p_\mu p_\nu/M_W^2)}{p^2-M_W^2+i0}$}
In unitary gauge, the free massive <W boson> has the displayed momentum-space propagator. At external momentum small compared with $M_W$, its leading term is $ig_{\mu\nu}/M_W^2$. Inserting it between two <weak charged current> vertices produces the local <Fermi interaction>, with $G_F/\sqrt2=g^2/(8M_W^2)$ when each vertex is written with $1-\gamma_5$. Higher momentum terms give higher-dimensional operators; other gauges distribute the longitudinal contribution differently while giving the same physical amplitudes.
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