Vertex function

In quantum electrodynamics, the vertex function is the one particle irreducible correlation function involving ψ, ${\bar {\psi }}$ and the vector potential A.

$\Gamma ^{\mu }\equiv -{1 \over e}{\delta ^{3}\Gamma _{eff} \over \delta {\bar {\psi }}\delta \psi \delta A_{\mu }}$

It's unfortunate that the effective action Γ_eff and the vertex function Γ^μ happen to be described by the same letter.

The dominant (and classical) contribution to Γ^μ is the gamma matrix γ^μ, which explains the choice of the letter.

The one-loop correction to the vertex function. This is the dominant contribution to the anomalous magnetic moment of the electron.