Gluon shadowing in nuclei and the role of the $c \bar c g$ state in the coherent photoproduction of $J/\psi$ in nucleus-nucleus collisions

TL;DR

This paper investigates gluon shadowing effects in nuclei by analyzing coherent $J/$ photoproduction, emphasizing the significance of including $c ar c g$ states for accurate modeling at small $x$.

Contribution

It introduces the role of $c ar c g$ Fock states in improving the description of gluon shadowing in coherent $J/$ photoproduction within the color-dipole framework.

Findings

Including $c ar c g$ states enhances the agreement with experimental data at small $x$.

Gluon shadowing ratio $R_g$ is better described when $c ar c g$ states are considered.

Rescattering of $c ar c$ states alone is insufficient for accurate modeling.

Abstract

In this brief note, we confront our results on diffractive photoproduction of $J/\psi$ mesons with the putative gluon shadowing ratio defined as $R_g= \sqrt{\sigma(\gamma A \to J/\psi A)/\sigma_{\rm IA.}}$, where $\sigma_{\rm IA}$ is the result in impulse approximation. Building on our earlier description of the process in the color-dipole approach, where we took into account the rescattering of $c \bar c$ states only, we demonstrate that the inclusion of $c \bar c g$-Fock states improves the description at small $x$ commonly associated with gluon shadowing.