Diagrams / Pair production (u-channel)

Pair production (u-channel)

Perturbative order	$\alpha$ (tree level)
Topology	u-channel
Symmetry factorCounts overcounting in the perturbation series. If a diagram has internal symmetries—ways to relabel internal lines and get the same diagram back—the symmetry factor divides that out. Most QED diagrams have S=1; the vacuum polarization bubble has S=1/2 because swapping the two internal fermion lines gives the same diagram.	1.0
Divergence	finite

Pair production

Crossed version: the photon absorption order is swapped.

The computational chain

From diagram to number—every step of the amplitude calculation.

Crossed pair production diagram

The u-channel version of pair production: the photon absorption order is reversed. Must be summed with the t-channel for the complete amplitude.