Coherent J/$\psi$ photoproduction at midrapidity in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV

TL;DR

This paper reports the first measurement of coherent J/ψ photoproduction cross section at midrapidity in Pb-Pb collisions at 5.02 TeV, revealing mild centrality dependence and agreement with certain theoretical models.

Contribution

It provides the first midrapidity measurement of coherent J/ψ photoproduction in Pb-Pb collisions at this energy, extending previous forward rapidity results.

Findings

Cross section compatible with previous measurements at forward rapidity.

Mild centrality dependence observed in the cross section.

Theoretical models ignoring hot medium effects describe the data well.

Abstract

The coherent J/$\psi$ photoproduction cross section is measured for the first time at midrapidity in peripheral to semicentral Pb$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV. The centrality differential cross section ${\rm d} \sigma/ {\rm d}y$ is reported for the centrality range 40-90%, together with the doubly-differential cross section ${\rm d}^2 \sigma /{\rm d}y {\rm d} p_{\rm T}$, extracted in two peripheral centrality classes. The J/$\psi$ mesons are reconstructed in the dielectron channel, in the rapidity interval $|y| < 0.9$ using the ALICE central barrel detectors. The J/$\psi$ cross section at midrapidity is statistically compatible to the earlier ALICE measurement at forward rapidity and at the same centre-of-mass energy, and shows only a mild centrality dependence over the covered range. Several sets of theoretical calculations taking into account the hadronic overlap in the collisions but ignoring possible final-state effects from a hot expanding medium are found to give a fairly good description of the current measurements within uncertainties.