Inclusive ψ(2S) production in p–Pb collisions with ALICE

Abstract

The ALICE Collaboration has studied the inclusive $\psi(\rm 2S)$ production in p-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV at the CERN LHC. Measurements are performed, in the $\mu^{+}\mu^{-}$ decay channel, in a forward (2.03$<y_{\rm cms}<$3.53) and in a backward (-4.46$<y_{\rm cms}<$-2.96) centre of mass rapidity ranges, as a function of transverse momentum or event activity. The $\psi(\rm 2S)$ production is compared to the $\rm J/\psi$ one through the double ratio $[\sigma_{\psi(\rm 2S)}/\sigma_{\rm J/\psi}]_{\rm pPb}/[\sigma_{\psi(\rm 2S)}/\sigma_{\rm J/\psi}]_{\rm pp}$ between the cross sections evaluated in p-Pb and pp collisions and by calculating the $\rm J/\psi$ and $\psi(\rm 2S)$ nuclear modification factors. Results indicate that the $\psi(\rm 2S)$ production is suppressed with respect to pp and, in particular in the backward rapidity region, the suppression is stronger than the $\rm J/\psi$ one. This unexpected difference between the $\rm J/\psi$ and $\psi(\rm 2S)$ behaviour, not accounted for by theoretical models based on nuclear parton shadowing and coherent energy loss mechanisms, indicates the presence of sizeable final state effects on the $\psi(\rm 2S)$ production.