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

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.