Global QCD study of direct photon production

Abstract

A global QCD analysis of the direct photon production process from both fixed target and collider experiments is presented. These data sets now completely cover the parton x range from 0.01 to 0.6, thereby providing a stringent test of perturbative QCD and parton distributions. Previous detailed studies of direct photons emphasized fixed target data. We find most data sets have a steeper ${\mathit{p}}_{\mathit{t}}$ distribution than the QCD prediction. Neither global fits with new parton distributions nor improved photon fragmentation functions can resolve this problem since the deviation occurs at different x values for experiments at different energies. A more likely explanation is the need for additional broadening of the ${\mathit{k}}_{\mathit{t}}$ of the initial state partons. The magnitude and the possible physical origin of this effect are investigated and discussed.