Measurement of an excess in the yield of J/$\psi$ at very low-$p_{\rm T}$ in Pb--Pb collisions with the ALICE detector

TL;DR

This paper reports an unexpected excess of low-$p_{T}$ J/$\\psi$ mesons in Pb--Pb collisions at the LHC, suggesting coherent photoproduction as a novel mechanism in such nuclear interactions.

Contribution

It provides the first evidence of J/$\psi$ coherent photoproduction at impact parameters smaller than twice the nuclear radius in heavy-ion collisions.

Findings

Observed excess of low-$p_{T}$ J/$\psi$ yield in Pb--Pb collisions.

Estimated the coherent photoproduction cross section in peripheral events.

Proposed coherent photoproduction as the origin of the excess.

Abstract

We report on the measurement of J/$\psi$ production at very low transverse momentum ($p_{\rm T} < $ 300 MeV/$c$) in Pb--Pb collisions performed with the ALICE detector at the LHC. We find an excess in the yield of J/$\psi$ with respect to expectations from hadronic production. Coherent photo-production of J/$\psi$ is proposed as a plausible origin of this excess. We show the nuclear modification factor of very low-$p_{\rm T}$ J/$\psi$ as a function of centrality. Then we measure the J/$\psi$ coherent photoproduction cross section in peripheral events assuming that it is the mechanism at the origin of the measured excess. It's worth noting that the observation of J/$\psi$ coherent photoproduction in Pb--Pb collisions at impact parameters smaller than twice the nuclear radius has never been observed so far and would open new theoretical challenges.