Light hypernuclei production in Pb-Pb collisions with ALICE at LHC

The subject of the present PhD thesis is the study of the production of light hypernuclei in ultra-relativistic Pb-Pb collisions with ALICE (A Large Ion Collider Experiment), one of the four major experiments at the LHC (Large Hadron Collider). The main physics goal of the ALICE experiment is the investigation of the properties of the strongly interacting matter at high energy density ($>$ 10 GeV/fm$^3$) and high temperature ($\approx$ 0.2 GeV) conditions. According to the lattice Quantum Chromo Dynamics (QCD) calculations, under these conditions (i.e. high temperature and large energy density) hadronic matter undergoes a phase transition to a ``plasma'' of deconfined quarks and gluons (Quark Gluon Plasma, QGP). In the first chapter of the thesis a general introduction to the heavy-ion physics will be given. Then the main quantities related to QGP formation (i.e. \textit{probes}) will be described. Finally the most important results obtained at SPS, RHIC and LHC experiments will be shown and discussed. In the second chapter a short description of the LHC and its experimental conditions will be reported and an overview of the ALICE experiment will be given. A description of the different detectors and their performances during data taking will be described; in addition a description of the computing framework will be given. The third chapter will be devoted to an introduction of the (anti)(hyper)nuclei production in heavy-ion collisions. The two main approaches which are believed to govern nuclei production (i.e. coalescence and thermal models) will be described, and an overview on the results at different energies will be shown. A comparison of the theoretical results will be also shown, with particular regards to the energies at the LHC. The fourth chapter is devoted to the description of the analysis method used to get (anti)hypertriton production yield in Pb-Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 2.76 TeV with the ALICE experiment via its mesonic decay $^{3}_{\Lambda}\mathrm H$ $ \rightarrow$ $^{3}\mathrm{He}$ + $\pi^{-}$ ( $^{3}_{\bar{\Lambda}} \overline{\mathrm H}$ $\rightarrow$ $^{3}\mathrm{\overline{He}}$+ $\pi^{+}$). In the beginning of the chapter the analysis technique used for particle identification and for the determination of secondary vertices will be described. The analysis will be divided into two distinct parts: the first one based on the data sample collected by the ALICE experiment during the first LHC heavy-ion run held at the end of 2010, while the second one based on data collected at the end of 2011. A detailed description of the study on efficiency evaluation and signal extraction will be shown for both analysis, together with a study of the systematic uncertainties. The results on the production yield of (anti)hypertriton will also be shown. The estimation of the hypertriton lifetime will be provided in the final section of the chapter. In the fifth chapter the method used to obtain the $p_{\rm T}$ spectrum of $^{3}\mathrm{He}$ will be presented. The raw spectra, the efficiency evaluation, systematic errors and feed-down from $^{3}_{\Lambda}\mathrm H$ will be presented. The final spectrum will be used to evaluate the production yield of $^{3}\mathrm{He}$($^{3}\mathrm{\overline{He}}$) in the whole ${p_{\rm T}}$ region, from 0 to $\infty$. Finally, in the last chapter, the present experimental results will be compared with published relevant results and with the most recent theoretical findings. Moreover, the measurement of the ``Strangeness Population Factor'' [S$_{3}$= $^{3}_{\Lambda}\mathrm H$/$^{3}\mathrm{He}$/($\Lambda$/p)] at the LHC energies will be provided. This quantity is a valuable tool to probe the nature of dense matter created in high-energy heavy-ion collisions and to validate theoretical models.