Inclusive dijet photoproduction in ultraperipheral heavy ion collisions at the CERN Large Hadron Collider in next-to-leading order QCD

V Guzey,M Klasen

doi:10.1103/physrevc.99.065202

Abstract

We compute the cross section of inclusive dijet photoproduction in ultraperipheral Pb-Pb collisions at the LHC using next-to-leading order perturbative QCD. We demonstrate that our theoretical calculations provide a good description of various kinematic distributions measured by the ATLAS collaboration. We find that the calculated dijet photoproduction cross section is sensitive to nuclear modifications of parton distribution functions (PDFs) at the level of 10 to 20%. Hence, this process can be used to reduce uncertainties in the determination of these nuclear PDFs, whose current magnitude is comparable to the size of the calculated nuclear modifications of the dijet photoproduction cross section.

Highlights

Ultraperipheral collisions (UPCs) of relativistic ions correspond to large impact parameters between the nuclei exceeding the sum of their radii, so that short-range strong interactions between the ions are suppressed and reactions proceed rather via the emission of quasireal photons by the colliding ions
UPCs have become an active field of research, driven by experimental results obtained at the BNL Relativistic Heavy Ion Collider (RHIC) and the CERN Large Hadron Collider (LHC); for a recent experimental review see, e.g., [2]
To assess the impact of measurements of the inclusive dijet photoproduction cross section on nuclear parton distribution functions (nPDFs), we focus on the xA distribution

Summary

INTRODUCTION

Ultraperipheral collisions (UPCs) of relativistic ions correspond to large impact parameters between the nuclei exceeding the sum of their radii, so that short-range strong interactions between the ions are suppressed and reactions proceed rather via the emission of quasireal photons by the colliding ions. In the framework of the leading logarithmic approximation of perturbative QCD [31], it can be interpreted as evidence of large nuclear gluon shadowing, Rg = fg/A(x, μ2 )/[A fg/N (x, μ2 )] ≈ 0.6 at x = 10−3 and μ2 = 3 GeV2 ( fg/A and fg/N are gluon densities in Pb and the proton, respectively) This value of Rg agrees with predictions of the leading twist nuclear shadowing model [32], which are characterized by small theoretical uncertainties in this kinematic region. The calculations using EPPS16 nPDFs and predictions of the leading twist nuclear shadowing model give similar results This suggests that inclusive dijet photoproduction on nuclei can be used to reduce uncertainties in the determination of nPDFs, which are currently significant and comparable in size to the magnitude of the calculated nuclear modifications of the dijet photoproduction cross section.

PHOTOPRODUCTION OF DIJETS IN UPCS IN NLO PERTURBATIVE QCD

PREDICTIONS FOR DIJET PHOTOPRODUCTION IN Pb-Pb COLLISIONS AT THE LHC

CONCLUSIONS