# Momentum flow in QED vertex

1. Aug 18, 2008

### kilokhan

say I have a QED vertex with an anti-fermion (p) going in and a fermion (q) and photon(k) coming out. Since the momentum flow is supposed to be along the direction of particle number flow, wouldn't this mean that the delta function at that vertex would be $$\delta(-p-q-k)$$ ? that doesn't seem to make sense. What is the correct delta function, and how do I get it?

2. Aug 19, 2008

### Avodyne

You can't have a fermion coming in and an antifermion coming out; that violates charge conservation ...