Probing the small-$x$ nuclear gluon distributions with isolated photons at forward rapidities in p+Pb collisions at the LHC

TL;DR

This paper investigates how isolated photon production in p+Pb collisions at the LHC can effectively probe small-$x$ nuclear gluon distributions, highlighting the impact of isolation cuts and rapidity asymmetries.

Contribution

It demonstrates that isolated photon measurements are highly sensitive to small-$x$ nuclear gluon PDFs and identifies optimal conditions for their experimental study.

Findings

Photons provide greater small-$x$ sensitivity than pions.

Isolation cuts minimally affect $x$ sensitivity.

Forward-backward yield asymmetry probes nPDFs independently.

Abstract

Inclusive direct photon production in p+Pb collisions at the LHC is studied within the NLO perturbative QCD. Our aim is to quantify the dominant $x$ regions probed at different rapidities and to identify the best conditions for testing the nuclear gluon parton distribution functions (nPDFs) at small $x$. A comparison to the inclusive pion production reveals that from these two processes the photons carry more sensitivity to the small-$x$ partons and that this sensitivity can be further increased by imposing an isolation cut for the photon events. The details of the isolation criteria, however, seem to make only a small difference to the studied $x$ sensitivity and have practically no effect on the expected nuclear modifications. We consider also the yield asymmetry between forward and backward rapidities which can be used to probe the nPDFs irrespectively of whether an accurate p+p baseline is available.