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

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.

Isolated photon production in proton-nucleus collisions at forward rapidity

“…A similar conclusion was made in the NLO pQCD calculation presented in Ref. [14]. Experimentally the isolation cut is defined by imposing a limit on the transverse energy in the cone, which is not possible to implement in our leading order calculation.…”

“…[14] by performing a NLO petrurbative quantum chromodynamics (pQCD) calculation with EPS09 nuclear parton distribution functions [13]. Further, the suppression is expected to get stronger at more forward rapidities.…”

“…in Ref. [14]. At high energy (or at very small x), the partonic densities become very large, of the order of 1=α s , and a convenient framework to describe QCD in this region is given by the color glass condensate (CGC) effective theory [15].…”

An update on nuclear PDFs (LHeC)

“…Although these measurements should definitely have an impact, for the larger systematic uncertainties at small p T , the best constraints appear to be limited to p T 10 GeV. In addition, the probed x region in photon pro-duction tends to be shifted to significantly higher x than what a naive leading-order proxy x ∼ (2p T / √ s)e −η would indicate [11]. Thus, although the impact of forward-photon measurements has not yet been estimated in detail, the obtainable constraints may be limited at small-x and small Q 2 .…”

Nuclear PDF constraints from p+Pb collisions at the LHC

“…The EPS09-based predictions for this observable are presented in figure 5 for three rapidity intervals, 2 < |η| < 3, 3 < |η| < 4 and 4 < |η| < 5, see Ref. [12] for details. Also the isospin effect arising from the different charge densities in proton and nuclei is shown.…”