$\psi$(2S) production and nuclear modification factor in nucleus-nucleus collisions with ALICE

Abstract

Charmonium production is a probe sensitive to deconfinement in nucleus--nucleus collisions. The production of J/$\psi$ via regeneration within the QGP or at the phase boundary has been identified as an important ingredient for the description of the observed centrality and $p_{\rm T}$ dependence of the J/$\psi$ nuclear modification factor at the LHC. $\psi$(2S) production relative to J/$\psi$ is one possible discriminator between the two different regeneration scenarios. At RHIC and LHC, there is so far no significant observation of the $\psi$(2S) in central nucleus-nucleus collisions at low transverse momentum, where regeneration is expected to be the dominant production mechanism. The combined Run 2 data set of ALICE allows one to extract a significant $\psi$(2S) signal in such a kinematic region at forward rapidity in the dimuon decay channel. In this contribution, we present for the first time results on the $\psi$(2S)-to-J/$\psi$ (double) ratio and the $\psi$(2S) nuclear modification factor in Pb--Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV, calculated with respect to a new pp reference with an improved precision compared to an earlier publication. Results are compared with model calculations.