Quarkonium polarization in Pb--Pb collisions with the ALICE experiment at the LHC

The study of the formation and the properties of the strongly interacting medium produced in ultrarelativistic heavy-ion collisions, named Quark-Gluon Plasma (QGP), is the main research goal of the ALICE experiment at the CERN Large Hadron Collider (LHC). Quarkonia, bound states of two heavy quarks (charm or bottom), represent a valuable tool in the investigation of the color deconfinement, since their binding can be strongly affected by the presence of the QGP, which may lead to their dissociation and subsequent re-generation, depending on the initial density of the heavy quarks that constitute them. Among the various observables typically studied in hadronic collisions, quarkonium polarization, which measures the spin alignment of a particle with respect to a chosen axis, plays an important role. On one side, the relatively small value measured at the LHC in proton-proton collisions has represented a serious challenge for the commonly-used theoretical models. On the other, in heavy-ion collisions quarkonium polarization could also be sensitive to the formation of the QGP. The work discussed in this thesis represents the first measurement of J/$\psi$ and $\varUpsilon(\rm{1S})$ polarization in heavy-ion collisions at the LHC. The present analysis, performed in the dimuon decay channel at forward rapidity ($\rm{2.5}<\it{y}<\rm{4}$), exploits the full Run 2 data sample collected by the ALICE experiment in Pb--Pb collisions at $\sqrt{s_{\rm{NN}}}=5.02$ TeV. In the first part of the thesis, an overview of the main theoretical and experimental aspects related to quarkonium production and polarization is presented, together with a description of the ALICE experiment apparatus. In the second part the measurement of the J/$\psi$ polarization is discussed. This analysis is performed as a function of the transverse momentum, in three $p_{\rm{T}}$ ranges from 2 to 10 GeV/$\textit{c}$, and in four centrality classes. The results will be compared with the existing measurements in pp collisions, in order to evaluate the potential modification of the polarization moving from small to large collision systems. Moreover, the possibility to use polarization to investigate the formation of an intense magnetic field in heavy-ion collisions will be also discussed, presenting a feasibility study of J/$\psi$ polarization with respect to the event plane. Finally the measurement of $\varUpsilon(\rm{1S})$ polarization is shown. This result is obtained for $p_{\rm{T}}<15\;\mathrm{GeV}/c$, integrating over centrality from 0 to 90%. Beyond the quarkonium polarization analysis, which represents the main topic of this thesis, a part of the research activity carried out during my PhD was dedicated to the study of $\psi(2\rm{S})$ production in p--Pb collisions at $\sqrt{s_{\rm{NN}}}=8.16$ TeV. The results of this analysis show that the strongly interacting system created in p--Pb collisions at LHC energy can dissociate this state, contrary to what happens for the more strongly bound J/$\psi$. This analysis is performed as a function of transverse momentum, rapidity and collision centrality and is discussed in a dedicated appendix.