Nuclear suppression from coherent $J/\psi$ photoproduction at the Large Hadron Collider

Understand

Using the data on coherent $J/\psi$ photoproduction in Pb-Pb ultraperipheral collisions (UPCs) obtained in Runs 1 and 2 at the Large Hadron Collider (LHC), we determined with a good accuracy the nuclear suppression factor of $S_{Pb}(x)$ in a wide range of the momentum fraction $x$, $10^{-5} \leq x \leq 0.04$.

• In the small-$x$ region $x < 10^{-3}$, our $\chi^2$ fit favors a flat form of $S_{Pb}(x) \approx 0.6$ with approximately a 5% accuracy for $x=6 \times 10^{-4} - 10^{-3} $ and a 25% error at $x=10^{-4}$.
• At the same time, uncertainties of the fit do not exclude a slow decrease of $S_{Pb}(x)$ in the small-$x$ limit.
• At large $x$, $S_{Pb}(x)$ is constrained to better than 10% precision up to $x=0.04$ and is also consistent with the value of $S_{Pb}(x)$ at $\langle x \rangle =0.042$, which we extract from the Fermilab data on the $A$ dependence of the cross section of coherent $J/\psi$ photoproduction on fixed nuclear targets.