Centrality dependence of ψ (2S) suppression in p-Pb collisions at s N N = 5.02 TeV

The inclusive production of the ψ (2S) charmonium state was studied as a function of centrality in p-Pb collisions at the nucleon-nucleon center of mass energy s N N = 5.02 TeV at the CERN LHC. The measurement was performed with the ALICE detector in the center of mass rapidity ranges −4 . 46 < y cms < −2 . 96 and 2 . 03 < y cms < 3 . 53, down to zero transverse momentum, by reconstructing the ψ (2S) decay to a muon pair. The ψ (2S) production cross section σ ψ (2S) is presented as a function of the collision centrality, which is estimated through the energy deposited in forward rapidity calorimeters. The relative strength of nuclear effects on the ψ (2S) and on the corresponding 1S charmonium state J/ ψ is then studied by means of the double ratio of cross sections [σ ψ (2S) / σ J/ ψ ] pPb / [σ ψ (2S) / σ J/ ψ ] pp between p-Pb and pp collisions, and by the values of the nuclear modification factors for the two charmonium states. The results show a large suppression of ψ (2S) production relative to the J/ ψ at backward (negative) rapidity, corresponding to the flight direction of the Pb-nucleus, while at forward (positive) rapidity the suppressions of the two states are comparable. Finally, comparisons to results from lower energy experiments and to available theoretical models are presented.