Suppression of $ψ$(2S) production in p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV

Abstract

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 $\sqrt{s_{NN}}$ = 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.