Probing Gluon Shadowing in Heavy Nuclei through Bayesian Reweighting of J/$\psi$ Photoproduction in Ultra-Peripheral Collisions

TL;DR

This paper uses Bayesian reweighting of nuclear parton distribution functions with J/psi photoproduction data from heavy-ion collisions to better constrain gluon shadowing effects in nuclei, especially at very small Bjorken-x.

Contribution

It introduces a novel Bayesian reweighting approach to incorporate experimental data into nPDFs, significantly reducing uncertainties in the gluon distribution at small x.

Findings

Pronounced gluon shadowing in Pb nuclei at x=10^{-4}

Reduced uncertainties in gluon density across 10^{-5} < x < 10^{-3}

Improved agreement of nPDFs with experimental data

Abstract

The gluon distribution in nuclei plays a pivotal role in understanding quantum chromodynamics (QCD) under extreme nuclear environments, yet remains poorly constrained compared to quark distributions. Coherent \jpsi photoproduction in ultra-peripheral heavy-ion collisions ($\gamma + A \rightarrow \mathrm{J}/\psi + A$) provides a unique solution to this challenge, serving as a sensitive probe of nuclear gluon densities. In this study, we perform Bayesian reweighting on the EPPS21 and nCTEQ15 sets of nuclear parton distribution functions (nPDF) by incorporating coherent \jpsi photoproduction measurements from both RHIC and LHC. The Bayesian-reweighted gluon modification factors $\mathrm{R_g^{A}}(x, Q^2 = 2.4\ \mathrm{GeV}^2)$ reveal pronounced nuclear shadowing in the Pb nuclei, with $\mathrm{R_g^{\mathrm{Pb}}} \approx 0.60$ at $x = 10^{-4}$, while simultaneously achieving a great reduction of the uncertainties in the density of the gluon across the critical Bjorken-$x$ range $10^{-5} < x < 10^{-3}$ compared to initial predictions of the nPDF. This work establishes coherent \jpsi photoproduction as a precision tool for gluon nPDF extraction, overcoming traditional deep-inelastic scattering limitations through perturbative QCD-calibrated probes. The constrained nPDFs demonstrate improved consistency with the experimental data across collider energies, particularly in the shadowing-dominated regime.