Massive-vector spin average
= Massive-vector spin average
{title2=$\overline{|\mathcal M|^2}=\tfrac13\sum_{\mathrm{spins}}|\mathcal M|^2$}
An unpolarized decay of a massive <spin>-one particle averages the squared amplitude over three parent <spin> states. Its polarization sum is $-\eta_{\mu\nu}+p_\mu p_\nu/m^2$. For a conserved external current the longitudinal $p_\mu p_\nu$ term drops out, but this does not replace the <spin> average by $1/2$: the physical massive parent still has three states.