Multiple Parton Emission Effects in Next-to-Leading Order Direct Photon   Production
  A recent global analysis of direct photon production at hadron collider and fixed target experiments has noted a disturbing trend of disagreement between next-to-leading-order (NLO) calculations and data. The conjecture has been made that the discrepancy is due to explicit multiple parton emission effects which are not accounted for in the theoretical calculations. We investigate this problem by merging a NLO calculation of direct photon production with extra multiple parton emissions via the parton shower (PS) algorithm. Our calculation maintains the integrity of the underlying NLO calculation while avoiding ambiguities due to double counting of multiple parton emissions. We find that the NLO+PS calculation can account for much of the theory/CDF data discrepancy at $\sqrt{s}=1.8$ TeV. It can also account for much of the theory/UA2 discrepancy if a very large virtuality is assumed to initiate the initial state parton shower. For lower energy data sets ({\it e.g.} $\sqrt{s}< 63$ GeV), NLO+PS calculations alone cannot account for the data/theory discrepancy, so that some additional non-perturbative $k_T$ smearing is needed. 
Howard Baer and Mary Hall Reno