•Soft photons

To find the contribution of emission of one soft photon to a given process, we must multiply the tree-order amplitude with the factor from Weinberg (13.1.1)-(13.1.2) and integrate over q from zero to the cutoff.

The factor reads:

where a summation over p (p1,p2,...) for all external electrons is understood, and q is the momentum of the emitted photon.

Converted by Mathematica  (July 10, 2003)