One-loop Feynman diagram
= One-loop Feynman diagram
A <Feynman diagram> with one independent closed cycle and hence one independent <loop momentum>. An $n$-vertex polygon of <Dirac propagators> in four dimensions has <superficial degree of divergence> $4-n$ before symmetry cancellations. For a <fermion loop>, the <closed fermion loop sign> is separate from any <symmetry factor> or pairwise <charge conjugation> cancellation.