Measurements of quarkonium production and polarization in Pb–Pb collisions with ALICE

. Quarkonia are excellent probes of deconfinement in heavy-ion collisions. For J /ψ , a bound state of c ¯ c quarks, the (re-)generation is found to be the dominant production mechanism at the LHC energies. Production measurements of non-prompt J /ψ , originating from beauty-hadron decays, allow one to access the interaction of beauty-quarks with the quark-gluon plasma (QGP). Polarization and spin alignment measurements can be used to investigate the characteristics of the formed medium. Moreover, it has been hypothesized that quarkonium states can be polarized by the strong magnetic field generated in the initial state of the collision and by the large angular momentum of the medium in non-central heavy-ion collisions. In these proceedings, the measurements of the inclusive, prompt, and non-prompt J /ψ nuclear modification factor R AA in Pb–Pb collisions at √ s NN = 5 . 02 TeV are shown. The measured non-prompt J /ψ fraction extends down to very low p T with a significantly improved precision compared to previous publications. The results from the first publication on the J /ψ polarization with respect to the event-plane in Pb–Pb collisions at √ s NN = 5 . 02 TeV at forward rapidity are presented as well. The results are compared with available calculations.


Introduction
Heavy quarks (charm and beauty) are an excellent probe to study the quark-gluon plasma (QGP), the strongly interacting medium created in high-energy heavy-ion collisions.They are mainly produced via initial hard partonic scatterings and thus experience the entire QGP evolution.Quarkonia are bound states composed of a heavy quark and its corresponding antiquark.In the QGP, the color screening of the surrounding medium may prevent the heavy quark and anti-quark from forming their bound states depending on the QGP temperature and binding energy of the state [1].A new charmonium production mechanism, known as (re-)generation [2][3][4], is found to be significant at the LHC energies due to the increase of heavy quark densities in the QGP [5,6].Moreover, the measurement of non-prompt J/ψ production can provide insights into the beauty quark energy loss mechanisms in the QGP.The nuclear modification factor (R AA ) is an interesting observable to study the medium influence on particle production yields and it is defined as the ratio of the yield in heavy-ion collisions to that in pp collisions at the same center-of-mass energy, scaled by the number of binary nucleonnucleon collisions.Finally, it has been argued that the strong electromagnetic field created at the very early stage of the heavy-ion collision [7], as well as the angular momentum of the medium in non-central collisions [8], may affect vector meson polarization.
The left panel of Fig. 1 shows the p T -integrated inclusive J/ψ R AA measured at midrapidity as a function of the average number of participant nucleons (⟨N part ⟩) in Pb-Pb collisions at √ s NN = 5.02 TeV.In order to exclude J/ψ photoproduction processes [11], a selection of the dielectron with p T > 0.15 GeV/c is applied.The R AA exhibits a sizeable increase in moving from semi-central to central collisions, described by models including (re-)generation.The right panel of Fig. 1 compares the p T -differential inclusive J/ψ R AA in 0-10% and 0-20% centrality intervals at midrapidity and forward rapidity, respectively.A larger R AA is observed at midrapidity and low p T as a consequence of the higher cc density, which leads to larger J/ψ yields being produced via (re-)generation.The statistical hadronization model [12] describes data at low p T and underestimates them at high p T .The transport model agrees with data in the whole measured p T region [3].The left panel of Fig. 2 shows the non-prompt J/ψ fraction as a function of p T , measured in Pb-Pb collisions at √ s NN = 5.02 TeV, in three different centrality classes.The results are compatible with CMS in the common p T interval, which refers to the centrality class 0-100% [14].The right panel of Fig. 2 shows the p T differential non-prompt J/ψ R AA in the 0-10% centrality class in Pb-Pb collisions at √ s NN = 5.02 TeV, compared to similar results from other experiments as well as with model calculations.The new ALICE data extends the non-prompt J/ψ R AA measurement down to very low p T and are consistent with CMS [14] and ATLAS [15] data in the overlapping p T region.The non-prompt J/ψ R AA is comparable to the non-prompt D 0 R AA measurement performed by the ALICE collaboration [16] (small differences could arise from the decay kinematics).The suppression of the nonprompt J/ψ R AA at high p T can be described by model calculations from Refs.[17,18].Both calculations include beauty quark energy loss in the QGP via both radiative [19] and collisional [20] processes.

Figure 1 .
Figure 1.Left panel: Inclusive J/ψ R AA at midrapidity, integrated over p T , as a function of ⟨N part ⟩ in Pb-Pb collisions at √ s NN = 5.02 TeV.Right panel: p T dependence of the inclusive J/ψ R AA at midrapidity and forward rapidity in the 0-10% and 0-20% centrality intervals, respectively.The results are compared to model calculations from Refs.[3, 4, 12, 13].

Figure 2 .
Figure 2. Left panel: p T dependence of the non-prompt J/ψ fraction in Pb-Pb collisions at √ s NN = 5.02 TeV and midrapidity.The results are compared to the corresponding CMS measurements in the centrality range of 0-100%[14].Right panel: Non-prompt J/ψ R AA as a function of p T compared with similar measurements from CMS[14] and ATLAS[15].Non-prompt D 0 R AA measurements from ALICE [16] are shown as well.Results are compared with model calculations from Refs.[17,18].

Figure 3 .
Figure 3. Centrality (left panel) and p T dependence (right panel) of the λ θ polarization parameter.The vertical bars and open boxes represent the statistical and systematic uncertainties, respectively [21].