Processes

Each process is defined by its external legs—the particles entering and leaving the interaction region. Select a process to see every diagram that contributes to its amplitude.

2 external legs

e⁻ e⁻

Electron propagator

$e^- \to e^-$

The electron self-energy $\Sigma(p)$: the sum of all 1PI corrections to the electron propagator. The full dressed...

1 tree 2 1-loop 3 2-loop
γ γ

Photon propagator

$\gamma \to \gamma$

The vacuum polarization tensor $\Pi^{\mu\nu}(q^2)$: the sum of all 1PI corrections to the photon propagator. The...

1 tree 2 1-loop

3 external legs

e⁻ e⁻ γ

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...

1 tree 2 1-loop

4 external legs

e⁺ e⁻ e⁻ e⁺

Bhabha scattering

$e^+e^- \to e^+e^-$

Elastic scattering of an electron and positron. Two tree-level diagrams contribute: the s-channel (annihilation into...

2 tree 3 1-loop
e⁻ γ e⁻ γ

Compton scattering

$e^-\gamma \to e^-\gamma$

Scattering of a photon off an electron. Two tree-level diagrams (s and u channels) give the Klein-Nishina formula....

2 tree 1 1-loop
γ γ γ γ

Light-by-light scattering

$\gamma\gamma \to \gamma\gamma$

Photon-photon scattering, forbidden classically because Maxwell's equations are linear. In QED it proceeds through a...

1 1-loop
e⁻ e⁻ e⁻ e⁻

Moller scattering

$e^-e^- \to e^-e^-$

Elastic scattering of two electrons. The simplest process involving identical fermions in the initial and final...

2 tree 3 1-loop
e⁺ e⁻ γ γ

Pair annihilation

$e^+e^- \to \gamma\gamma$

Electron-positron annihilation into two photons. Related to Compton scattering by crossing symmetry. The two tree...

2 tree
γ γ e⁻ e⁺

Pair production

$\gamma\gamma \to e^+e^-$

Two photons produce an electron-positron pair. The time-reversed process of pair annihilation. Related by crossing...

2 tree