Feynman Rules - Covariant Gauges

• Loops: each loop gives an integral over the unconstrained loop momentum, k,

$\int \frac{d^{D}k}{(2\pi {)}^D}$ where $D=4-2ϵ$ in Dimensional Regularisation.

• Closed fermion/ghost loops: introduce an extra minus sign for each closed fermion and ghost loops.
• External fermion lines: introduce a relative negative sign between diagrams with (anti-commuting) fermions interchanged.
• Symmetry factors: divide by the number of symmetries of the graph.
• Dirac algebra: go against the spin flow when writing an amplitude.