Gauge invariance and gluon poles for direct photon production

Abstract

We discuss the hadron tensor of the direct photon production. We study the effects which lead to the soft breaking of factorization by inspection of the corresponding QCD gauge invariance. We emphasize that the special role is played by the contour gauge for gluon fields. We demonstrate that the different prescriptions in the gluonic pole contributions are dictated by the presence of initial or final state interactions in diagrams. Moreover, the different prescriptions that correspond to the initial of final state interactions are needed to ensure the QCD gauge invariance.