Study of parton kT smearing effects in direct photon production at the Fermilab Tevatron

Previous detailed studies of direct photon production from both fixed-target and collider experiments have witnessed a pattern of deviation between the measured inclusive cross sections and the corresponding theoretical expectations in the low transverse momentum (pT) regime. Most data sets display steeper pT dependence than the next-to-leading-order (NLO) perturbative QCD (PQCD) calculations with standard choices of scales and parton distribution functions in this region. A simple implementation of higher-order soft-gluon corrections to the NLO PQCD predictions significantly improves the agreement between data and theory. This interesting feature motivated us to investigate the DOe and CDF measurements of inclusive photon cross section at √(s)=1.8 TeV from the run 1b and also at √(s)=630 GeV. We use the latest updated parton distribution function CTEQ6M in the NLO QCD calculations for direct photon cross section to describe the data. The conventional theoretical uncertainties originating from scale dependence and gluon distributions have been illustrated. We estimate the impact of additional soft-gluon radiation on the direct photon production using PYTHIA (LO PQCD), which adds transverse momentum kT to initial-state partons through a Gaussian smearing. The impact of kT effects on the discrepancy in the low-pT region is explored using a phenomenological model, wherein we merge the NLO calculations with kT correction factors. We show that this approach provides a much more acceptable description of the Fermilab Tevatron data