A GLOBAL QCD STUDY OF DIRECT PHOTON PRODUCTION
J. Huston, E. Kovacs, S. Kuhlmann, H. L. Lai, J. F. Owens, W. K. Tung
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
p
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
k
t
of the initial state partons. The magnitude and the possible physical origin of this effect are investigated and discussed.