2015/10/28 by Massacrier, Laure
#FOS: Physical sciences #High Energy Physics - Experiment (hep-ex) #Nuclear Experiment (nucl-ex)
paper · doi:10.48550/arxiv.1510.08315
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.