Suppression of ψ (2S) production in p-Pb collisions at s N N = 5 . 02 TeV

The ALICE Collaboration has studied the inclusive production of the charmonium state ψ (2S) in proton-lead (p-Pb) collisions at the nucleon-nucleon centre of mass energy s N N = 5.02 TeV at the CERN LHC. The measurement was performed at forward (2 . 03 < y cms < 3 . 53) and backward (−4 . 46 < y cms < −2 . 96) centre of mass rapidities, studying the decays into muon pairs. In this paper, we present the inclusive production cross sections σ ψ (2S) , both integrated and as a function of the transverse momentum p T , for the two y cms domains. The results are compared to those obtained for the 1S vector state (J/ ψ ), by showing the ratios between the production cross sections, as well as the double ratios [ σ ψ (2S) /σ J /ψ ] pPb / [ σ ψ (2S) /σ J/ ψ ] pp between p-Pb and proton-proton collisions. Finally, the nuclear modification factor for inclusive ψ (2S) is evaluated and compared to the measurement of the same quantity for J/ ψ and to theoretical models including parton shadowing and coherent energy loss mechanisms. The results show a significantly larger suppression of the ψ (2S) compared to that measured for J/ ψ and to models. These observations represent a clear indication for sizeable final state effects on ψ (2S) production.