Gauge-boson propagator
= Gauge-boson propagator
For the Euclidean <covariant gauge> term $(\partial_\mu A^\mu)^2/(2\xi)$, the free <gauge-boson propagator> is
$$
D_{\mu\nu}(k)=\frac1{k^2}\left(\delta_{\mu\nu}+(\xi-1)\frac{k_\mu k_\nu}{k^2}\right).
$$