Quantitative constraints on the gluon distribution function in the proton from collider isolated-photon data

TL;DR

This paper uses Bayesian reweighting of collider isolated-photon data across multiple energies to refine the gluon distribution in the proton, significantly reducing uncertainties and impacting predictions like Higgs production.

Contribution

It introduces a Bayesian reweighting method applied to isolated-photon data to improve the gluon PDF with quantifiable uncertainty reduction.

Findings

Isolated-photon data constrains gluon density at x~0.02

NLO predictions fit most datasets from 200 GeV to 7 TeV

Gluon PDF uncertainty reduces by up to 20%, affecting Higgs cross section predictions

Abstract

The impact of isolated-photon data from proton-(anti)proton collisions at RHIC, SppbarS, Tevatron and LHC energies, on the parton distribution functions of the proton is studied using a recently developed Bayesian reweighting method. The impact on the gluon density of the 35 existing isolated-gamma measurements is quantified using next-to-leading order (NLO) perturbative QCD calculations complemented with the NNPDF2.1 parton densities. The NLO predictions are found to describe well most of the datasets from 200 GeV up to 7 TeV centre-of-mass energies. The isolated-photon spectra recently measured at the LHC are precise enough to constrain the gluon distribution and lead to a moderate reduction (up to 20%) of its uncertainties around fractional momenta x~0.02. As a particular case, we show that the improved gluon density reduces the PDF uncertainty for the Higgs boson production cross section in the gluon-fusion channel by more than 20% at the LHC. We conclude that present and future isolated-photon measurements constitute an interesting addition to coming global PDF analyses.