Recent charmonium measurements in Pb–Pb collisions with ALICE

. Charmonia have long been recognized as a valuable probe of the nuclear matter in extreme conditions, the strongly interacting medium created in heavy-ion collisions known as quark-gluon plasma (QGP). At LHC energies, the (re-)generation process due to the abundantly produced charm quarks, was found to considerably a ff ect measured charmonium observables. Comprehensive production measurements of charmonia, including both ground and excited states, are crucial to discriminate among di ff erent (re-)generation scenarios assumed in theoretical calculations. In addition, the spin-alignment of charmonia can be a ff ected by the strong magnetic field and the large angular momentum of the medium in non-central heavy-ion collisions. Finally, the non-prompt char-monium, grants a direct insight into the nuclear modification factor of beauty hadrons, which is expected to be sensitive to the energy loss experienced by the ancestor beauty quarks inside the QGP. In this contribution, newly published results of inclusive, prompt and non-prompt J /ψ production, obtained in Pb–Pb collisions at √ s NN = 5 . 02 TeV, are presented. Recently published results of J /ψ polarization with respect to the event plane at forward rapidity in Pb–Pb collisions at √ s NN = 5 . 02 TeV are also discussed. Results are compared to available model calculations.


Introduction
Heavy quarks, specifically charm and beauty quarks, serve as excellent probes for studying the QGP, which is the strongly interacting medium formed in high-energy heavy-ion collisions.These quarks are primarily generated through initial hard partonic scatterings, allowing them to undergo the entire evolution of the QGP.Quarkonium, a bound state of a heavy quark and its corresponding anti-quark, is an essential probe to reveal information about the properties and evolution of the QGP.In the QGP, the color screening and dissociation by the surrounding medium may prevent the formation of bound states, depending on the QGP temperature and the binding energy of the state [1,2].A novel charmonium production mechanism, known as (re-)generation [3][4][5] in heavy-ion collisions, becomes particularly significant at LHC energies due to the increased density of heavy quarks in the QGP [6,7].Additionally, the measurement of non-prompt J/ψ production can offer insights into the energy loss mechanisms of beauty quarks in the QGP.The nuclear modification factor (R AA ) is one of the observables used to quantify the nuclear effects relative to a proton-proton reference.Finally, it has been proposed that the strong electromagnetic field [8], as well as the orbital angular momentum directed along the perpendicular to the reaction plane, may influence vector meson polarization in the early stages of non-central heavy-ion collisions [9].
The J/ψ are reconstructed through the e + e − and the µ + µ − decay channels at midrapidity (|y| < 0.9) and forward rapidity (2.5 < y < 4) respectively, with the ALICE experiment.The detailed analysis techniques are discussed in Refs.[6,10].The comprehensive description of ALICE detector can be found in Ref. [11].

ALI-PUB-539097
ALI-PUB-539105  [15], two microscopic transport models by Rapp et al. [4] and Zhuang et al. [16], and the energy loss model by Arléo et al. [14].The energy loss model suggests that the fragmentation of high-energy partons may become the main mechanism for J/ψ production in this kinematic region, energy loss of partons due to multiple scattering in the QGP then leads to J/ψ suppression at high p T ; both transport models describe the p Tdifferential R AA within uncertainties.The SHMc calculations coupled with a hydrodynamics inspired freeze-out parameterisation are in good agreement with the data in the low-p T region, but underestimate the measurements at high p T .The energy loss model agrees with data at high p T , even more pronounced at midrapidity than at forward rapidity.The inclusive J/ψ R AA shows an increasing trend at low-p T in central collisions, which indicates a large contribution of J/ψ production from the (re-)generation process.
The left panel of Fig. 2 shows the non-prompt J/ψ p T -differential R AA in the 0-10% centrality class in Pb-Pb collisions at √ s NN = 5.02 TeV.The results are described by models implementing collisional and radiative energy loss contributions [18][19][20][21][23][24][25].The POWLANG calculations [28], which include only collisional energy loss contributions, overestimate the R AA at intermediate and high p T .The prompt J/ψ p T -differential R AA is shown on the right panel of Fig. 2. The results are compared with the SHMc [15], the Boltzmann Transport(BT) [16,27] and the dissociation [26] models.The BT and SHMc models agree with data within uncertainties at low p T , while the dissociation model reproduces data at high p T .The polarization of inclusive J/ψ production with respect to the event plane is measured in Pb-Pb collisions at √ s NN = 5.02 TeV via the dimuon decay channel at forward rapidity [29].

Summary
In this contribution, recent measurements of chamonia production in Pb-Pb collisions at √ s NN = 5.02 TeV are reported.The inclusive J/ψ R AA shows an increasing trend at low p T , which indicates a large contribution of J/ψ production from the (re-)generation process.Non-prompt J/ψ R AA is measured in central Pb-Pb collisions at midrapidty.The strong suppression of non-prompt J/ψ at high p T is well reproduced by models implementing both collisional and radiative energy loss processes of beauty quarks inside the QGP.The first measurement of inclusive J/ψ polarization with respect to the event plane is shown as a func-tion of centrality and p T in Pb-Pb collisions at √ s NN = 5.02 TeV.A significant polarization effect, reaching 3.9σ for 2 < p T < 4 GeV/c in the 30−50% centrality interval, is presented.

Figure 3 .
Figure 3. Centrality (left) and p T dependence (right) of the λ θ polarization parameter for two centrality intervals, with respect to the event plane frame, in Pb-Pb collisions at √ s NN = 5.02 TeV at forward rapidity [29].