= Polarization sum for a massive vector boson
{title2=$\sum_\lambda\epsilon_\mu(k,\lambda)\epsilon_\nu^*(k,\lambda)=-g_{\mu\nu}+k_\mu k_\nu/m^2$}
= Massive vector polarization sum
{synonym}
For $m>0$, the three physical <polarization vectors> are transverse to $k$ and have norm $-1$ in metric $(+---)$. In the rest frame their sum is $\operatorname{diag}(0,1,1,1)$; <Lorentz covariance> then gives the displayed expression for $k^2=m^2$. This includes the longitudinal mode. It cannot be specialized to $m=0$ by setting its denominator to zero.
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