Study of partonkTsmearing effects in direct photon production at the Fermilab Tevatron

Abstract

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 ${(p}_{T})$ regime. Most data sets display steeper ${p}_{T}$ 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 D\O{} and CDF measurements of inclusive photon cross section at $\sqrt{s}=1.8\mathrm{TeV}$ from the run $1b$ and also at $\sqrt{s}=630\mathrm{GeV}.$ 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 ${k}_{T}$ to initial-state partons through a Gaussian smearing. The impact of ${k}_{T}$ effects on the discrepancy in the low-${p}_{T}$ region is explored using a phenomenological model, wherein we merge the NLO calculations with ${k}_{T}$ correction factors. We show that this approach provides a much more acceptable description of the Fermilab Tevatron data.