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Study of Heavy Flavours from Muons Measured with the ALICE Detector in Proton-Proton and Heavy-Ion Collisions at the CERN-LHC

Abstract : Ultra-relativistic heavy-ion collisions aim at investigating the properties ofstrongly-interacting matter at extreme conditions of temperature and energy density. According to quantum chromodynamics (QCD) calculations, under such conditions, the formation of a deconfined medium, the Quark-Gluon Plasma (QGP), is expected. Amongst the most important probes of the properties of the QGP, heavy quarks are of particular interest since they are expected to be produced in hard scattering processes during the early stage of the collision and subsequently interact with the hot and dense medium. Therefore, the measurement of quarkonium states and open heavy flavours should provide essential information on the properties of the system formed at the early stage of heavy-ion collisions. Indeed, open heavy flavours are expected to be sensitive to the energy density through the mechanism of in-medium energy loss of heavy quarks, while quarkonium production should be sensitive to the initial temperature of the system through their dissociation due to color screening. The measurement of the collective flow of heavy flavours provides additional insights on the possible thermalization of heavy quarks in the medium. Furthermore, one of the important medium characteristic, viscosity over entropy (eta/s), can be extracted by combining the information from measured nuclear modification factor (related to in-medium energy loss) and the magnitude of the heavy quark flow. In this regard, both quarkonium and open heavy flavour production are a field of intense experimental and theoretical researches. Despite the work devoted to these studies at SPS and RHIC, several questions are left open. With a nucleus-nucleus center of mass energy nearly 15 times larger than the one reached RHIC, the LHC which started operating in November 2009, provides a new era for studies of interacting matter at high temperature and energy density. One of the most important aspects of this new energy range is the abundant production rate of heavy quarks which are used, for the first time, as high statistics probes of the medium. The LHC delivered the first proton-proton collisions at √s = 0.9 TeV in October 2009 and reached its current maximum energy of 7 TeV in March 2010. A short proton-proton run at √s = 2.76 TeV, at the same energy than the Pb-Pb run, was performed in March 2011. The first heavy-ion run (Pb-Pb collisions at √sNN = 2.76 TeV) took place in November 2010 and the second one end of 2011. ALICE (A Large Ion Collider Experiment) is the experiment dedicated to the study of heavy-ion collisions at the LHC. ALICE also takes part in the LHC proton- proton program which is of great interest for testing perturbative QCD calculations at unprecedented low Bjorken-x values and for providing the necessary baseline for nucleus-nucleus and proton-nucleus collisions. ALICE will also collect, in the beginning of 2013, p-Pb/Pb-p collisions in order to investigate cold nuclear matter effects. ALICE measures quarkonia and open heavy flavours with (di)-electrons, (di)-muons and through the hadronic channels. This thesis work is devoted to the study of open heavy flavours in proton-proton and Pb-Pb collisions via single muons with the ALICE forward muon spectrometer. The document is organized as follows. The first chapter consists in a general introduction on heavy-ion collisions and QCD phase transitions. Chapter 2 summarizes the motivations for the study of open heavy flavours in nucleon-nucleon, nucleon-nucleus and nucleus-nucleus collisions. A particular emphasis is placed on the novelties at the LHC. Chapter 3 gives an overview of the ALICE experiment with a detailed description of the forward muon spectrometer. Chapter 4 gives a short summary of the ALICE online and offline systems. Then the analysis framework (for data and simulations) and in particular the software developed for the study of open heavy flavours is detailed. Chapter 5 summarizes the performance of the ALICE muon spectrometer for the study of the production of open heavy flavours in pp collisions via single muons and dimuons. Chapters 6 to 9 are dedicated to data analysis. Chapter 6 deals with the analysis of first pp collisions at 900 GeV. The main aim was the understanding of the response of the apparatus. These data allowed also to determine the analysis strategy for heavy flavour measurement in the single muon channel: selection of events, optimization of cuts, understanding of the background components in data. Chapter 7 presents the measurement of the production of heavy flavour decay muons in pp collisions at √s = 7 TeV. The analysis strategy is described: event and track selection, background subtraction (mainly the contribution of muons from primary pion and kaon decays), corrections, normalization and investigation of the systematic uncertainties. The experimental results are discussed and compared to perturbative QCD calculations (Fixed Order Next-to-Leading Log calculations). That concerns the transverse momentum and rapidity differential production cross sections of muons from heavy flavours decays at forward rapidity (2.5 < y < 4). Chapter 8 addresses the measurement of heavy flavour decay muon production in Pb-Pb collisions at √sNN = 2.76 TeV collected in 2010. The analysis strategy is presented. In-medium effects are investigated by means of the nuclear modification factor (RAA) of muons from heavy flavour decays. The proton-proton reference is obtained from the measurement of the differential production cross section of heavy flavour decay muons at the same center of mass energy. The nuclear modification factor is studied as a function transverse momentum (pt) and collision centrality. For comparison, results obtained with the central-to-peripheral nuclear modification factor RCP are also discussed. Chapter 9 gives an overview of the different methods investigated in ALICE for the study of the elliptic flow. In particular, the recent methods which allow to remove non-flow effects like the Q-Cumulants and Lee-Yang Zeroes are detailed. Promising results concerning the inclusive muon elliptic flow as a function of pt and centrality obtained with different flow analysis methods are compared. Finally, summary and outlooks are given.
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Submitted on : Friday, February 8, 2013 - 11:38:46 AM
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  • HAL Id : tel-00786269, version 1

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X. Zhang. Study of Heavy Flavours from Muons Measured with the ALICE Detector in Proton-Proton and Heavy-Ion Collisions at the CERN-LHC. High Energy Physics - Experiment [hep-ex]. Université Blaise Pascal - Clermont-Ferrand II, 2012. English. ⟨tel-00786269⟩

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