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

Abstract

We determine the nuclear suppression factor $S_{Pb}(x)$, where $x=M_{J/ψ}^2/W_{γp}^2$ with $M_{J/ψ}$ the $J/ψ$ mass and $W_{γp}$ the photon-nucleon energy, for the cross section of coherent $J/ψ$ photoproduction in heavy-ion ultraperipheral collisions (UPCs) at the Large Hadron Collider (LHC) and Relativistic Heavy Ion Collider (RHIC) by performing the $χ^2$ fit to all available data on the cross section $dσ^{AA \to J/ψAA}/dy$ as a function of the $J/ψ$ rapidity $y$ and the photoproduction cross section $σ^{γA \to J/ψA}(W_{γp})$ as a function of $W_{γp}$. We find that while the $dσ^{AA \to J/ψAA}/dy$ data alone constrain $S_{Pb}(x)$ for $x \geq 10^{-3}$, the combined $dσ^{AA \to J/ψAA}/dy$ and $σ^{γA \to J/ψA}(W_{γp})$ 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 $dσ^{AA \to J/ψAA}/dy$ and $σ^{γA \to J/ψA}(W_{γp})$ as well as on the experimental values for $S_{Pb}(x)$ derived from $σ^{γA \to J/ψA}(W_{γp})$. We also show that modern nuclear PDFs reasonably reproduce $S_{Pb}(x)$ as well.