J/ψ suppression at forward rapidity in Pb–Pb collisions at √s_NN=5.02 TeV

Abstract The inclusive J/ψ production has been studied in Pb–Pb and pp collisions at the centre-of-mass energy per nucleon pair s NN = 5.02  TeV , using the ALICE detector at the CERN LHC. The J/ψ meson is reconstructed, in the centre-of-mass rapidity interval 2.5 < y < 4 and in the transverse-momentum range p T < 12  GeV / c , via its decay to a muon pair. In this Letter, we present results on the inclusive J/ψ cross section in pp collisions at s = 5.02  TeV and on the nuclear modification factor R AA . The latter is presented as a function of the centrality of the collision and, for central collisions, as a function of the transverse momentum p T of the J/ψ. The measured R AA values indicate a suppression of the J/ψ in nuclear collisions and are then compared to our previous results obtained in Pb–Pb collisions at s NN = 2.76  TeV . The ratio of the R AA values at the two energies is also computed and compared to calculations of statistical and dynamical models. The numerical value of the ratio for central events (0–10% centrality) is 1.17 ± 0.04 ( stat ) ± 0.20 ( syst ) . In central events, as a function of p T , a slight increase of R AA with collision energy is visible in the region 2 < p T < 6  GeV / c . Theoretical calculations qualitatively describe the measurements, within uncertainties.