GLOBAL QCD STUDY OF DIRECT PHOTON PRODUCTION

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 pt 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 kt of the initial state partons. The magnitude and the possible physical origin of this effect are investigated and discussed. © 1995 The American Physical Society.