Hadrons and direct photon in pp and pA collisions at LHC and saturation effects

TL;DR

This paper studies hadron and direct photon production in proton-proton and proton-nucleus collisions at LHC energies, highlighting the sensitivity of forward rapidity measurements to saturation effects and predicting nuclear modification factors.

Contribution

It provides a detailed analysis of saturation effects on particle production at LHC, including predictions for direct photon and pion yields and nuclear modification factors using various saturation models.

Findings

Maximum sensitivity to saturation at forward rapidities in pp collisions.

Direct photon to pion ratio can reach 20-10 at high rapidities.

Saturation effects cause a peak in the rapidity distribution of direct photons.

Abstract

We investigate hadrons and direct photon production in pp and pA collisions at the energies of RHIC and LHC within the color-dipole approach employing various saturation models. We show that greatest sensitivity to saturation effects is reached at very forward rapidities for pp collisions at LHC (\sqrt{s}=14 TeV). The ratio of direct-photon to pion production can be about 20-10 (at \eta=7-8). Therefore, direct photon production at forward rapidities should provide a rather clean probe. We calculate the rapidity dependence of the invariant cross-section and find some peculiar enhancement at forward rapidities which is more pronounced for direct photon production. We show that this peak is further enhanced by saturation effects. We provide predictions for the nuclear modification factor R_{pA} for pions and direct photon production in pA collisions at LHC energy at midrapidity. We show within various saturation models that the pion Cronin enhancement at RHIC is replaced by a moderate suppression at LHC energy at midrapidity due to gluon shadowing effects. Cronin enhancement of direct photons can survive at LHC energy within models with a larger saturation scale.