Cancellation of infrared divergences at finite temperature

Abstract

We consider a typical hard scattering process in a heat bath of photons and electrons at temperature, $T$, in finite temperature QED. We show that, when the hard scattering scale is much larger than the temperature, the infrared pieces of both the real and virtual parts of the cross section factorise; these can be exponentiated and cancel between each other to all orders in perturbation theory. We use the technique of Grammer and Yennie to give a prescription for the extraction of the infrared divergent parts, and for the form of the finite remainder. Symmetry arguments are used to show the finiteness of new terms arising in the $T \neq 0$ part of the computation.