$\psi$(2S) production in Pb-Pb collisions measured by ALICE at the LHC

TL;DR

This paper reports the first significant measurement of $ ext{ψ(2S)}$ production in Pb-Pb collisions at the LHC, providing insights into charmonium regeneration mechanisms in the quark-gluon plasma.

Contribution

It presents the first significant $ ext{ψ(2S)}$ signal at low $p_T$ in Pb-Pb collisions and compares the $ ext{ψ(2S)}$-to-J/ψ ratio and nuclear modification factor with models and previous experiments.

Findings

Significant $ ext{ψ(2S)}$ signal observed at low $p_T$ and forward rapidity.

The $ ext{ψ(2S)}$-to-J/ψ ratio varies with centrality and $p_T$.

Results favor regeneration scenarios in charmonium production.

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 J/$\psi$ nuclear modification factor at the LHC. The $\psi(2S)$ production relative to J/$\psi$ is a possible discriminator between the two regeneration scenarios. Studies of $\psi(2S)$ production in central nucleus-nucleus collisions at low transverse momentum ($p_{\rm T}$) are crucial, particularly at the LHC, where regeneration appears to be dominant. A significant $\psi$(2S) signal is extracted at low $p_{\rm T}$ and forward rapidity in the dimuon decay channel for the first time. This measurement relies on the $\psi(2S)$ cross section measured recently in pp collisions at $\sqrt{s_{\rm }}=5.02$ TeV with an unprecedented precision compared to previous ALICE results. In this contribution, we present newly published results on the $\psi(2S)$-to-J/$\psi$ ratio and the $\psi(2S)$ nuclear modification factor in Pb-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV. Results are reported as a function of centrality and $p_{\rm T}$ for $p_{\rm T}<12$ GeV/c and are compared to available NA50 and CMS measurements. Comparisons to transport and statistical hadronization model predictions are also provided to shed light on the charmonium states recombination mechanism