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

TL;DR

This paper reports the first measurement of the $a8$(2S)-to-J/$a8$ double ratio and nuclear modification factor for $a8$(2S) in Pb--Pb collisions at 5.02 TeV, providing insights into charmonium regeneration in the QGP.

Contribution

First extraction of $a8$(2S)-to-J/$a8$ double ratio and nuclear modification factor at LHC energies with improved pp reference data.

Findings

Significant $a8$(2S) signal observed at low $p_T$ in central collisions.

Results favor models with charmonium regeneration mechanisms.

Comparison with theoretical models shows consistency within uncertainties.

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 at the LHC. $\psi$(2S) production relative to J/$\psi$ is one possible discriminator between the two different regeneration scenarios. At RHIC and at the LHC, there is so far no significant observation of the $\psi$(2S) in nucleus--nucleus collisions in central events at low transverse momentum, where regeneration is the dominating process. The combined Run 2 data set of ALICE allows 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 improved precision. Results are compared with model calculations.