Direct photon production and PDF fits reloaded

TL;DR

This paper revisits the role of direct photon production in global PDF fits, incorporating recent NNLO calculations and electroweak corrections, and assesses its impact on the gluon distribution using LHC data.

Contribution

It provides a comprehensive analysis of NNLO QCD and electroweak effects on direct photon production and its influence on gluon PDF constraints in global fits.

Findings

NNLO corrections improve agreement with ATLAS data.

Including 8 TeV data constrains the medium-x gluon.

Updated predictions for 13 TeV direct photon production.

Abstract

Direct photon production in hadronic collisions provides a handle on the gluon PDF by means of the QCD Compton scattering process. In this work we revisit the impact of direct photon production on a global PDF analysis, motivated by the recent availability of the next-to-next-to-leading (NNLO) calculation for this process. We demonstrate that the inclusion of NNLO QCD and leading-logarithmic electroweak corrections leads to a good quantitative agreement with the ATLAS measurements at 8 TeV and 13 TeV, except for the most forward rapidity region in the former case. By including the ATLAS 8 TeV direct photon production data in the NNPDF3.1 NNLO global analysis, we assess its impact on the medium-x gluon. We also study the constraining power of the direct photon production measurements on PDF fits based on different datasets, in particular on the NNPDF3.1 no-LHC and collider-only fits. We also present updated NNLO theoretical predictions for direct photon production at 13 TeV that include the constraints from the 8 TeV measurements.