{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.


 Polarization sum for a massive vector boson

ID: polarization-sum-for-a-massive-vector-boson