Exclusive J/ψ photoproduction in ultraperipheral Pb+Pb collisions at the CERN Large Hadron Collider calculated at next-to-leading order perturbative QCD

Abstract

We present the first next-to-leading-order (NLO) perturbative QCD (pQCD) study of rapidity-differential cross sections of coherent exclusive photoproduction of J/ψ mesons in heavy-ion ultraperipheral collisions (UPCs) at the CERN Large Hadron Collider (LHC), dσ/dy(Pb+Pb→Pb+J/ψ+Pb). For this, we account for the photon-nucleon NLO cross sections at the forward limit, the t dependence using a standard nuclear form factor, and the photon fluxes of the colliding nuclei. Approximating the generalized parton distributions with their forward-limit parton distribution functions (PDFs), we quantify the NLO contributions in the cross sections, show that the real part of the amplitude and quark-PDF contributions must not be neglected, quantify the uncertainties arising from the scale choice and PDFs, and compare our results with ALICE, CMS, and LHCb J/ψ photoproduction data in Pb+Pb UPCs, exclusive J/ψ photoproduction data from HERA, and LHCb data in p+p. The scale dependence in dσ/dy(Pb+Pb→Pb+J/ψ+Pb) is significant, but we can find a scale choice that reproduces the Pb+Pb UPC data at both 2.76 and 5.02 TeV collision energies. This process has traditionally been suggested to be a direct probe of nuclear gluon distributions. We show that the situation changes rather dramatically from LO to NLO: the NLO cross sections reflect the nuclear effects of both gluons and quarks in a complicated manner, where the relative signs of the LO and NLO terms in the amplitude play a significant role.