Traina, Michelangelo (2019) Measurement of the D0 meson production in Pb-Pb collisions with the ALICE experiment at the LHC. [Magistrali biennali]
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The D0 is the lightest particle with a charm quark and is particularly well suited for the study of charm production and interaction with the Quark-Gluon Plasma (QGP). This phase of matter sees quarks in the asymptotic freedom regime and can form only when very high temperatures are reached. The ultra-relativistic lead-lead collisions studied at the LHC are one of such scenarios, allowing to investigate in-medium energy loss mechanisms for heavy quarks. The data collected by ALICE experiment during the latest 2018 run will allow the most precise measurement to date of such effects. In this thesis work, the D0 production in 2018 Pb-Pb collisions at Sqrt(sNN) = 5.02 TeV is assessed via the reconstruction of its decay into a charged pion and kaon. The aim is setting important boundaries for the understanding of the interaction of charm quarks with the high-temperature QCD medium, especially at transverse momenta in the 1-to-3 GeV/c range, where its relative yield is to be interpreted as stemming from the entanglement of several phenomena. The reconstruction is accomplished by studying the decay topology, in order to best exploit the excellent spatial resolution and particle-identification capabilities of the ALICE detector. In particular, a selection process discarding the so-called combinatorial background, distinctively abundant in the high-multiplicity environment of Pb-Pb collisions, is carried out. Each pair of tracks candidate to be considered as stemming from a D0 decay must fulfil a set of conditions, which are optimised through the maximisation of the statistical significance of the signal. In order to circumvent gross misestimations due to possible background fluctuations, the signal samples are produced through HIJING and PYTHIA Monte Carlo simulations. Subsequently, a study of the cut efficiency and of the systematic uncertainties associated to procedures such as yields evaluation and significance maximisation (usage of Monte Carlo simulations for the signal, choice of fit function for the background etc.) is performed. The overall goal is the extraction of the nuclear modification factor, denoted by RAA. This observable is the ratio of the transverse-momentum D0 production spectra obtained in the Pb-Pb and in the p-p colliding systems, where the latter is rescaled in the hypothesis that the heavy-ion collision be a superposition of independent nucleon-nucleon collisions. Whenever this hypothesis is correct, then RAA = 1 within uncertainties – this has been observed to be the case, for instance, in p-Pb collisions, where an extended volume of QGP matter is not expected to form. However, if so-called final-state interactions between the charm and the QGP take place, then a suppression of the nuclear modification factor for a specific transverse momentum interval is expected. The described tasks are carried out with state-of-the-art computational tools such as dedicated data analysis framework AliRoot and C++ programming language. Furthermore, Machine Learning techniques are to be employed. These are gaining increasing importance in science as they provide effective methods to deal with problems characterised by several degrees of freedom.
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