Nuclear suppression of coherent $J/\psi$ photoproduction in heavy-ion UPCs and leading twist nuclear shadowing

TL;DR

This paper analyzes nuclear suppression in coherent J/psi photoproduction during heavy-ion ultraperipheral collisions, using data fitting to determine suppression factors and comparing leading twist nuclear shadowing with nuclear PDFs.

Contribution

It provides the first comprehensive determination of the nuclear suppression factor S_{Pb}(x) across a wide x-range by fitting all available experimental data and validates the leading twist approximation against these results.

Findings

S_{Pb}(x) decreases with decreasing x in 10^{-4} to 0.01 range

Leading twist approximation describes the data well

Modern nuclear PDFs reasonably reproduce the suppression factor

Abstract

We determine the nuclear suppression factor $S_{Pb}(x)$, where $x=M_{J/\psi}^2/W_{\gamma p}^2$ with $M_{J/\psi}$ the $J/\psi$ mass and $W_{\gamma p}$ the photon-nucleon energy, for the cross section of coherent $J/\psi$ photoproduction in heavy-ion ultraperipheral collisions (UPCs) at the Large Hadron Collider (LHC) and Relativistic Heavy Ion Collider (RHIC) by performing the $\chi^2$ fit to all available data on the cross section $d\sigma^{AA \to J/\psi AA}/dy$ as a function of the $J/\psi$ rapidity $y$ and the photoproduction cross section $\sigma^{\gamma A \to J/\psi A}(W_{\gamma p})$ as a function of $W_{\gamma p}$. We find that while the $d\sigma^{AA \to J/\psi AA}/dy$ data alone constrain $S_{Pb}(x)$ for $x \geq 10^{-3}$, the combined $d\sigma^{AA \to J/\psi AA}/dy$ and $\sigma^{\gamma A \to J/\psi A}(W_{\gamma p})$ data allow us to determine $S_{Pb}(x)$ in the wide interval $10^{-5} < x < 0.05$. In particular, the data favor $S_{Pb}(x)$, which decreases with a decrease of $x$ in the $10^{-4} < x < 0.01$ interval, and can be both decreasing or constant for $x< 10^{-4}$. Identifying $S_{Pb}(x)$ with the ratio of the gluon distributions in Pb and the proton $R_g(x,Q_0^2)=g_A(x,Q_0^2)/[A g_p(x,Q_0^2)]$, we demonstrate that the leading twist approximation (LTA) for nuclear shadowing provides a good description of all the data on $d\sigma^{AA \to J/\psi AA}/dy$ and $\sigma^{\gamma A \to J/\psi A}(W_{\gamma p})$ as well as on the experimental values for $S_{Pb}(x)$ derived from $\sigma^{\gamma A \to J/\psi A}(W_{\gamma p})$. We also show that modern nuclear PDFs reasonably reproduce $S_{Pb}(x)$ as well.