Nuclear modification factor in p+pb collisions at LHC and saturation

TL;DR

This paper predicts the nuclear modification factor R_{pA} for pions and photons in p+Pb collisions at LHC using saturation models fitted to HERA data, highlighting the importance of shadowing and saturation effects.

Contribution

It provides parameter-free predictions for R_{pA} at LHC energies using saturation models constrained by HERA data, emphasizing the role of gluon shadowing and saturation effects.

Findings

Moderate suppression of pions due to gluon shadowing at LHC.

Potential survival of Cronin enhancement for direct photons in some models.

Both shadowing and saturation significantly influence R_{pA} at LHC.

Abstract

We provide predictions for the nuclear modification factor R_{pA} for pions and direct photon production in p+A collisions at LHC energy at midrapidity within different saturation models fitted to HERA data. In our approach we have no free parameters to adjust and all model parameters are fitted to other reactions. Our approach gives a rather good description of PHENIX data for R_{pA} for pions. We show that, in various saturation models, the pion Cronin enhancement is replaced by a moderate suppression at LHC energy at midrapidity due to gluon shadowing effects. However, Cronin enhancement of direct photons can survive at LHC energy in models with a larger saturation scale. We show that both shadowing and saturation effects are important at LHC in p+A collisions and give rise to a rather sizable effect in the nuclear modification factor R_{pA}. Therefore, a precise measurement of p+A collisions at LHC is crucial in order to understand the underlying dynamics of heavy ion collisions.