Dealing with ambiguities in higher order QCD predictions for large pT processes involving real photons

We discuss the ambiguities which affect predictions for large pT processes in the framework of perturbative QCD: indeed, the choice of both the renormalization scheme (RS) where the coupling constant is defined and of the factorization scheme (FS) connected with parametrization of long distance physics, is arbitrary. We specialize to the case of large pT processes involving real photons for which next to leading order calculations have been performed. The variation of the predictions for the inclusive spectrum with respect to RS and FS parameters is illustrated by drawing contour lines. We discuss various algorithms to get rid of these ambiguities and give arguments to understand the shape of the contour plots and to show that all proposed algorithms yield, for reasonably large values of pT, very close answers. Also included is a discussion of the choice of scale for the Drell-Yan pair mass-distribution. We conclude by emphasizing that this treatment of ambiguities gives a sounder basis for comparing with experiment and extracting precise information about the value of Λ and gluon properties. (orig.)