= Photon polarization sum
The sum over the two physical transverse photon <polarization vectors> is not the full tensor $-\eta_{\mu\nu}$. It differs by gauge-dependent terms proportional to the photon <four-momentum>. When the amplitude satisfies the <Ward identity>, those terms vanish on contraction and one may use $-\eta_{\mu\nu}$ as a replacement inside the squared amplitude. This replacement must not be mistaken for a free-standing equality of physical polarization tensors.
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