= Scalar-QED photon-tadpole symmetry factor
{title2=$S_{\mathrm{graph}}=1/2$}
The two identical <photon> fields at a <scalar electrodynamics> <Seagull vertex>[seagull] give a <vertex> multiplicity two. Joining them into one closed <photon> line gives <Feynman-diagram symmetry factor> $1/2$. Directly expanding $-e^2A^2\phi^\dagger\phi$ gives only one <photon> <Wick contraction>, independently checking the compensation. In a transverse gauge the remaining <tensor> <trace> is $d-1$; a massless <Tadpole diagram>[photon tadpole] is a <scaleless integral> in <dimensional regularization>.
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