Prompt photon production and photon-jet correlations at the LHC

TL;DR

This paper compares advanced theoretical predictions with LHC data on photon and photon-jet production, finding good agreement and providing predictions to help understand nuclear effects like shadowing in proton-lead collisions.

Contribution

It introduces next-to-leading order predictions matched to parton showers for photon production and photon-jet correlations, with new predictions for the ratio R_gamma and parton momentum fractions in lead ions.

Findings

Good agreement between theory and ATLAS/CMS data

Reduced scale uncertainties in predictions

Predictions for R_gamma and x_Pb^obs relevant for nuclear effect studies

Abstract

Next-to-leading order predictions matched to parton showers are compared with recent ATLAS data on inclusive photon production and CMS data on associated photon and jet production in pp and pPb collisions at different centre-of-mass energies of the LHC. We find good agreement and, as expected, considerably reduced scale uncertainties compared to previous theoretical calculations. Predictions are made for the ratio of inclusive photons over decay photons $R_\gamma$, an important quantity to evaluate the significance of additional photon sources, e.g. thermal radiation from a Quark-Gluon-Plasma, and for distributions in the parton momentum fraction in lead ions $x_{\rm Pb}^{\rm obs}$, that could be determined by ALICE, ATLAS, CMS and LHCb in ongoing analyses of photon+jet production in pPb collisions at $\sqrt{s_{NN}}=5.02$ TeV. These data should have an important impact on the determination of nuclear effects such as shadowing at low $x$.