Studies of charmonia production ratios as a function of the centrality in PbPb collisions at LHCb

This PhD thesis focuses on the study of the Quark-Gluon Plasma (QGP), a deconfined state of matter formed in high-energy heavy-ion collisions. In particular, I investigate the production of quarkonium states, with emphasis on the charmonium mesons J/ψ and ψ(2S), reconstructed through their dimuon decays in the LHCb detector at CERN. Since different quarkonium states have different binding energies, comparing the production of J/ψ and ψ(2S) in PbPb collisions provides a sensitive probe of QGP suppression mechanisms and offers valuable tests for theoretical models describing quarkonium behavior in a hot and dense medium. The central goal of this work is the measurement of the ψ(2S)/J/ψ production ratio as a function of collision centrality, using PbPb data collected by the LHCb detector during Run 2 (2018) and Run 3 (2024) of the LHC at √s_NN ≈ 5 TeV. I developed a full analysis chain including event simulation, signal selection, and evaluation of reconstruction, selection, trigger, and muon identification efficiencies. The study based on Run 2 data led to a publication in JHEP07(2025)235 in 2025, while the analysis of Run 3 data is currently ongoing, with the upgraded detector and extended centrality coverage promising improved precision and broader physics reach. In addition to this main focus, part of my research was devoted to the study of spin-dependent phenomena in fixed-target collisions with the SMOG system. I performed the first measurement of Λ hyperon transverse polarization in proton–neon interactions at √s_NN = 68.4 GeV, probing spin–orbit correlations in a novel kinematic regime relevant for transverse-momentum-dependent parton distributions. This work was published in JHEP09(2024)082 in 2024 and is included in the thesis as an appendix. Finally, I contributed to the development and testing of an alarm panel for the upgraded vertex detector (VELO), installed in 2024, which is currently used to support data-taking operations. This technical contribution enhances the overall reliability and performance of the detector, thereby supporting its operation in heavy-ion environments.