Electron propagator

$e^- \to e^-$

The electron self-energy $\Sigma(p)$: the sum of all 1PI corrections to the electron propagator. The full dressed propagator is $i/(\not{p} - m - \Sigma(p))$. Generates mass renormalization and contributes to the Lamb shift.

Tree-level diagrams

1 diagram.

Order $\alpha$ diagrams

2 diagrams.

Order $\alpha^{2}$ diagrams

3 diagrams.