Next-to-leading order QCD predictions for exclusive $J/\psi$ photoproduction in ultraperipheral Pb+Pb collisions at the LHC

TL;DR

This paper provides the first NLO QCD predictions for exclusive $J/mbda$ photoproduction in ultraperipheral Pb+Pb collisions at the LHC, highlighting the importance of quark contributions and scale uncertainties in interpreting experimental data.

Contribution

It introduces NLO collinear factorisation calculations for $J/mbda$ photoproduction in heavy-ion UPCs, emphasizing the role of quark contributions and scale dependence.

Findings

NLO predictions match LHC Pb+Pb UPC data with appropriate scale choice.

Real part of amplitude and quark PDFs are significant at NLO.

Scale dependence remains sizable but manageable at NLO.

Abstract

We present the first study of rapidity-differential cross sections of coherent exclusive $J/\psi$ photoproduction in heavy-ion ultraperipheral collisions (UPCs) at the LHC, $\text{d}\sigma(\text{Pb}+\text{Pb}\rightarrow \text{Pb}+J/\psi+\text{Pb})/\text{d}y$, to NLO in collinear factorisation. Taking the generalised parton distributions as their forward limit counterparts, the NLO contributions in the cross sections are quantified and we show that the real part of the amplitude and quark-PDF contributions must not be neglected. We further evaluate the uncertainties arising from the scale-choice at both LO and NLO, and compare our results with exclusive $J/\psi$ photoproduction data in Pb+Pb UPCs from LHC. The scale dependence in $\text{d}\sigma(\text{Pb}+\text{Pb}\rightarrow \text{Pb}+J/\psi+\text{Pb})/\text{d}y$ is sizable, but we can find a scale-choice at NLO that reproduces the Pb+Pb UPC data at both Run 1 and Run 2 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, where the interplay of both gluon and quark contributions, and in particular their relative signs, play a significant role. We end with some concluding remarks about the outlook of this observable within a tamed collinear factorisation.