Division of systems engineering

At the LHC energies, ultrarelativistic heavy-ion collisions lead to the emergence of an extreme state of matter, the quark-gluon plasma or QGP. The transition from hadronic matter to a phase of deconfined partons is described by quantum chromodynamics, theory of the strong interaction between quarks and gluons. The QGP properties can be studied via the measurement of so-called hard probes such as heavy-flavour quarks. Formed at the early stage of the collision, quarkonia, resonances of two quarks of the same heavy flavour, interact with the medium throughout its evolution. Depending on the intensity of the interaction mechanisms at play, the pairs are dissociated and the final-state production rate is thus suppressed. The measurement of this suppression offers a privileged access to QGP features. In particular, the production of quarkonia in heavy-ion collisions is commonly advertised as a key observable of the modification of the strong interaction.

This thesis is devoted to the study of the ϒ production in Pb–Pb collisions at √s_(NN) = 5.02 TeV at the LHC. The different ϒ(nS) states are reconstructed via their decay into muon pairs detected by the muon spectrometer of the ALICE experiment. The yields are measured as a function of the collision centrality, rapidity and transverse momentum. In order to quantify the suppression, a similar analysis is performed with data collected in proton–proton collisions, at the same centre-of-mass energy. The results indicate a strong suppression of the ϒ(1S) production, increasing from peripheral towards the most central collisions. For the first time, the ϒ(2S) excited state can be measured at forward rapidity in nucleus–nucleus collisions. Its production is more than two times more suppressed than the ground state. The comparison with predictions by phenomenological models allows to interpret the measurements and to constrain the suppression mechanisms of quarkonia.

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