Certain Uncertainties in QCD Theory Predictions for Large-P _T Photons and Other Processes

Abstract

In general there are more than one scale (large variable) determining perturbative QCD quantities: E.g. the renormalization scale μ entering the running coupling α ₈ (μ) (related with the renormalization procedure), and the factorization scale M entering the structure functions F(x,M) (determining the scale violations). Until recently, for large transverse momentum P _T ) processes, simplicity suggested the choice

$${{\mu }^{2}}={{M}^{2}}=\alpha p_{T}^{2}$$

(1)

where α = positive and of 0(1), or other choices phenomenologically equivalent; hereafter P _T will be called physical scale.