Photoproduction of low-$p_{\rm T}$ J/$\psi$ from peripheral to central Pb$-$Pb collisions at 5.02 TeV

TL;DR

This paper reports the observation of excess low-$p_{\rm T}$ J/$\psi$ production in Pb$-$Pb collisions at 5.02 TeV, indicating coherent photoproduction occurs even in hadronic interactions with nuclear overlap.

Contribution

It provides the first measurement of coherent photoproduction of J/$\psi$ in central Pb$-$Pb collisions at 5.02 TeV, extending previous studies and confirming the phenomenon across a wider centrality range.

Findings

Excess of J/$\psi$ yield at very low $p_{\rm T}$ in peripheral to semi-central collisions.

Measurement of nuclear modification factor and cross section as a function of centrality.

Confirmation of coherent photoproduction in more central collisions with >5$\sigma$ significance.

Abstract

An excess of J/$\psi$ yield at very low transverse momentum ($p_{\rm T} < 0.3$ GeV/$c$), originating from coherent photoproduction, is observed in peripheral and semicentral hadronic Pb$-$Pb collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_{\rm NN}} = 5.02$ TeV. The measurement is performed with the ALICE detector via the dimuon decay channel at forward rapidity ($2.5<y<4$). The nuclear modification factor at very low $p_{\rm T}$ and the coherent photoproduction cross section are measured as a function of centrality down to the 10% most central collisions. These results extend the previous study at $\sqrt{s_{\rm NN}} = 2.76$ TeV, confirming the clear excess over hadronic production in the $p_{\rm T}$ range 0$-$0.3 GeV/$c$ and the centrality range 70$-$90%, and establishing an excess with a significance greater than 5$\sigma$ also in the 50$-$70% and 30$-$50% centrality ranges. The results are compared with earlier measurements at $\sqrt{s_{\rm NN}} = 2.76$ TeV and with different theoretical predictions aiming at describing how coherent photoproduction occurs in hadronic interactions with nuclear overlap.