Abstract
ALICE (A Large Ion Collider Experiment) is one of the four main experiments at the Large Hadron Collider (LHC) optimized for the study of heavy-ion collisions. The prime aim of the experiment is to study in detail the behaviour of nuclear matter at high densities and temperatures. In ALICE also
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proton-proton collisions are studied both as a reference for lead-lead collisions and in physics areas where ALICE is competitive with other LHC experiments. In order to characterise the system produced in heavy-ion collisions, it is necessary to use a wide variety of experimentally accessible observables, which can help to disentangle the different physical mechanisms that characterise the various collision stages. Among the different experimental observables, the focus of this thesis will be on anisotropic flow, one of the most important experimental observables used in heavy-ion collisions to study the properties of the QGP. For this purpose, a comprehensive study of the elliptic flow of hadrons, electrons from heavy-flavour hadron decays, and direct photons, all which are assumed to interact differently with medium, has been carried out. The v2 coefficient of low pT hadrons, mainly coming from the hadronization of soft partons within the bulk, carries direct information on the hydrodynamic properties of the partonic medium created in heavy-ion collisions. Among the different hadron species, the Φ-meson is of great interest because it is predicted to have a small hadronic cross section, and is therefore less affected by the interactions between hadrons during the hadronization phase of the collision. The v2 coefficient also allows to study the interaction strength of heavy-quarks with the expanding medium and their possible thermalisation in the medium itself. Due to their large masses, heavy quarks are produced at the initial stage of the collision, almost exclusively in hard partonic scattering processes. Therefore, they experience the full evolution of the system, propagating through the hot and dense medium and loosing energy via radiative and collisional scattering processes. Heavy-flavour hadrons and their decay products are thus effective probes to study the properties of the medium created in the heavy-ion collisions. Direct photons on the other hand, since they do not interact with the strongly-coupling medium created in these collisions, carry undistorted information about the system at their production time. The elliptic flow of direct photons strongly depends on the production mechanism. Small flow would be associated with early production while a large, hadron-like flow, would point towards late production in the medium.
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