Studio della produzione di bottomonio in collisioni Pb-Pb con lo spettrometro per muoni dell'esperimento ALICE a LHC

It has been more than four decades since the basic idea of the possible existence of a new state of matter in which quarks and gluons are no longer confined within hadrons was proposed. Contrary to atoms and molecules which can be ionized to reveal their constituents, quarks and gluons must clump together to form colour-neutral objects and cannot be observed as free particles; however, under particular conditions of extremely high temperature and energy density a deconfinement can occur, thus leading to the formation of the so-called Quark-Gluon Plasma (QGP). The QGP is supposed to be established in the early stage of the Universe, just for a few millionths of a second shortly after the Big Bang; a detailed study of its properties by means of ultrarelativistic heavy-ion collisions has been expected to shed light on the fundamental theory of the strong interaction and provide more information on the cosmological QGP epoch, which is effectively shielded from astronomical observations by the subsequent evolution of the Universe. The steady technological progress over the past twenty years has allowed to vastly expand the opportunities of investigation of QGP properties, reaching energies of √sNN = 5.02 TeV in collisions of Pb nuclei at the CERN LHC. Among CERN experiments, ALICE is the one explicitly conceived to the study of such collisions and in particular to the suppression of bottomonium states (bound states of bottom and anti-bottom quarks), whose production is expected to be significantly suppressed due to the high-density colour charge screening mechanism that characterizes the strongly interacting deconfined medium originated in the heavy-ion collisions. In particular, the ALICE experiment has collected in November 2018 its largest Pb-Pb data sample (integrated luminosity = 530 μb-1 at √sNN = 5.02 TeV, which provides a statistics more than twice higher with respect to the previous one collected in November 2015. The present work makes use of these data and is dedicated to the analysis of suppression of bottomonium states, evaluated quantitatively through the nuclear modification factor (RAA), computed as the ratio between the resonances yields in Pb-Pb and the corresponding production cross section in pp collisions, normalized to the number of nucleon-nucleon collisions in Pb-Pb configuration. In the following, the reduction and sorting of data sample will be presented; thereafter, the evolution of bottomonium yields through the study of the invariant mass spectrum will be discussed. Finally, a comparison between the results achieved with the present data sample and those achieved by means of the statistics collected in the previous data taking will be given.