Constraints on the Proton's Gluon Distribution from Prompt Photon Production

TL;DR

The paper investigates how prompt photon production in hadronic collisions can constrain the proton's gluon distribution within a consistent NLO QCD framework that includes both direct and fragmentation photon production and a realistic isolation treatment. It performs combined fits to DIS structure-function data and a comprehensive set of prompt-photon measurements, quantifying theoretical uncertainties with particular emphasis on scale dependence and fragmentation functions. The results show that scale uncertainties dominate, especially at large x, and yield gluon densities that differ from some global fits, with enhancements near x ~ 0.01 and suppressions near x ~ 0.15; importantly, the data can be described without invoking intrinsic k_T smearing. Overall, the work clarifies the potential and limits of prompt photons as gluon probes and informs future PDF analyses and heavy-flavor predictions.

Abstract

We analyze the capability of prompt photon production in pp and pp(bar) collisions to constrain the gluon distribution of the proton, considering data from fixed-target experiments as well as collider measurements. Combined fits are performed to these large-p_T direct gamma cross sections and lepton-proton deep-inelastic scattering data in the framework of next-to-leading order perturbative QCD. Special attention is paid to theoretical uncertainties originating from the scale dependence of the results and from the fragmentation contribution to the prompt photon cross section.