Vertex function

$e^-e^-\gamma$

The dressed vertex $\Gamma^\mu(p,p')$: the sum of all 1PI corrections to the electron-photon vertex. Decomposes into the Dirac form factor $F_1(q^2)$ (charge) and the Pauli form factor $F_2(q^2)$ (magnetic moment). The one-loop correction gives the Schwinger result $F_2(0) = \alpha/2\pi$.

Tree-level diagrams

1 diagram.

Order $\alpha$ diagrams

2 diagrams.