= Photon polarization vector
{title2=$\epsilon^\mu(k)$}
= Photon polarisation vector
{synonym}
For a real <on shell> <photon> of null momentum $k$, a physical polarization satisfies $k\cdot\epsilon=0$ and is defined modulo $\epsilon\mapsto\epsilon+\alpha k$. The two-dimensional quotient carries the two physical <photon> helicities. In a transverse gauge choose $\epsilon^0=0$ and $\epsilon^*\cdot\epsilon=-1$. A <Ward identity> ensures that replacing an external polarization by its momentum gives zero in the summed physical amplitude.
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