Parton κT and large- pT production of direct photons and π0 mesons

Abstract

Predictions of perturbative QCD (pQCD) for the inclusive production of direct photons and mesons at high p T have been tested using new high-statistics measurements from Fermilab experiment E706. Inclusive π 0 and direct-γ cross sections in the kinematic range 3.5 < p T < 12 GeV/ c with central rapidities have been measured for 530 and 800 GeV/ c proton beams and a 515 GeV/ c π − beam incident on a Be target. Current Next-to-Leading Order (NLO) pQCD calculations fail to describe the data for usual choices of scales. Other aspects of the data indicate the presence of a substantial initial state parton transverse momentum ( κ T ) in the hard scattering. Incorporating a kinematic model of κ T effects improves the agreement between the calculations and the observed cross sections. Gluon distributions are extracted from Deep Inelastic Scattering (DIS), Drell-Yan (DY), and our direct photon data using fitting procedures similar to those used by the CTEQ collaboration; calculations using these new parton distribution fits yield jet cross sections consistent with CDF and DØ measurements.