Kinoshita-Lee-Nauenberg theorem, magnetic mass, and thermal photon production

We study the infrared singularities associated with ultrasoft transverse gluons in the calculation of photon production by a quark-gluon plasma. Despite the fact that the Kinoshita-Lee-Nauenberg theorem works in this context and provides cancellations of infrared singularities, it does not prevent the production rate of low invariant mass dileptons to be sensitive to the magnetic mass of gluons and therefore the rate to be nonperturbative. (c) 2000 The American Physical Society.