Diagrams / Bare vertex

Bare vertex

Perturbative order	$\alpha$ (tree level)
Topology	tree
Symmetry factorCounts overcounting in the perturbation series. If a diagram has internal symmetries—ways to relabel internal lines and get the same diagram back—the symmetry factor divides that out. In QED, most diagrams have S=1 because fermion arrows break the symmetry between internal lines.	1.0
Divergence	finite

Contributes to

Vertex function

The bare QED vertex. The only interaction vertex in the theory.

The computational chain

From diagram to number—every step of the amplitude calculation.

Feynman rule

$$-ie\gamma^\mu$$

The sole interaction vertex of QED. Every diagram in the theory is built from this vertex, the electron propagator, and the photon propagator. The $\gamma^\mu$ matrix encodes the vector coupling of the photon to the electron current $\bar{\psi}\gamma^\mu\psi$.