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

Abstract

We report on the measurement of J/$ψ$ 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/$ψ$ with respect to expectations from hadronic production. Coherent photo-production of J/$ψ$ is proposed as a plausible origin of this excess. We show the nuclear modification factor of very low-$p_{\rm T}$ J/$ψ$ as a function of centrality. Then we measure the J/$ψ$ 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/$ψ$ 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.