Probing fully coherent radiation and parton densities using (virtual) photons at the LHC

TL;DR

This paper investigates how fully coherent radiation effects influence prompt photon and virtual photon production in pPb collisions at the LHC, highlighting the potential of Drell-Yan processes to probe nuclear parton distribution functions.

Contribution

It introduces the impact of fully coherent radiation effects on prompt photon production and demonstrates the use of Drell-Yan processes to effectively probe nuclear PDFs.

Findings

Fully coherent energy loss affects prompt photon production at backward rapidity.

Fully coherent energy gain influences prompt photon production at forward rapidity.

Drell-Yan process is effective for constraining nuclear PDFs using pseudo-data.

Abstract

Prompt photon production in pA collisions has long been suggested as a sensitive probe of the nuclear parton distribution functions (nPDFs). In this study, we present recent results on another cold nuclear matter effect, namely fully coherent radiation induced by parton multiple scattering, which may influence the nuclear dependence of prompt photon production. Medium-induced radiation effects, implemented in leading-order direct and fragmentation photon processes, are computed for pPb collisions at the LHC. At backward rapidity, photons are sensitive to fully coherent energy loss (FCEL), while at forward rapidity, fully coherent energy gain (FCEG) plays a crucial role due to the dominance of the $qg \to q\gamma$ scattering channel. In contrast, for virtual photon production, the impact of fully coherent radiation is marginal, making Drell-Yan (DY) one of the best ways to probe nuclear PDFs. The power of the DY process is demonstrated by reweighting nPDF sets at next-to-leading order using realistic pseudo-data for LHC Run 3.