Small $x$ gluon shadowing from LHC data on coherent $\mathrm{J/}\psi$ photoproduction

TL;DR

This paper introduces a new method to extract the coherent photo-nuclear cross section for J/ψ production from LHC Pb-Pb collision data, enabling insights into small-x gluon shadowing effects.

Contribution

A novel disentanglement technique for coherent J/ψ photoproduction contributions in heavy-ion collisions is developed and demonstrated with LHC data.

Findings

Extracted photo-nuclear cross sections up to 470 GeV photon-lead energy.

Computed nuclear suppression factors consistent with gluon shadowing models.

Provided new constraints on small-x gluon distributions in nuclei.

Abstract

The cross section for coherent $\mathrm{J/}\psi$ photoproduction in Pb-Pb collisions is the sum of two contributions, one from low-, the other from high-energy photon-nucleus interactions. A novel method to disentangle both contributions allowing one to extract the coherent photo-nuclear cross section for coherent $\mathrm{J/}\psi$ production, $\sigma_{\gamma \rm Pb}$, is presented. The utility of the method is demonstrated using measurements from peripheral and ultra-peripheral Pb-Pb collisions at the LHC. Applying the proposed method to the available data it is possible to obtain $\sigma_{\gamma \rm Pb}$ up to a photon-lead centre-of-mass energy of 470 GeV, which corresponds to $x$ of 4.4x10$^{-5}$. To illustrate a possible use of the extracted photo-nuclear cross sections, the corresponding nuclear suppression factors are computed and they are compared to predictions of gluon shadowing calculated in the leading-twist approximation approach.