Study of the effect of the initial geometry on elliptic flow and charged hadron production in Pb-Pb collisions with ALICE

Abstract

The aim of the ALICE experiment (A Large Ion Collider Experiment) at the LHC is the study of the nuclear matter under conditions of extreme energy density and temperature. Under these conditions the formation of a deconfined phase called Quark Gluon Plasma (QGP) is predicted by lattice QCD. In this phase, quarks and gluons are no longer confined to individual nucleons. A transition from the QGP state into a hadronic state should have occurred during the early stages of Universe, due to its expansion and to the consequent decrease of the temperature. Collisions of heavy nucleons at relativistic energy create in the laboratory the conditions for the hot and dense environment required for the phase transition. The ALICE experiment is dedicated to the study of the deconfined state of strongly interacting matter. Heavy-ions are extended object and the system created in central nucleus- nucleus collisions is different from the one created in peripheral collisions. In par- ticular, for non-central collisions, in the plane perpendicular to the beam direction, the geometrical overlap region is highly anisotropic. This initial spatial asymmetry is converted via interactions into an anisotropy in the momentum space. Measure- ments of this modulation, known as anisotropic transverse flow, provide insight into the collective evolution and the early stages of a relativistic heavy-ion collision. The QGP exhibits strong collectivity, behaving as a nearly perfect liquid as observed at RHIC. The collective properties of the system can be stud- ied through the transverse momentum (pT ) distributions and the measurement of the anisotropy of the particle distributions. The pT distributions allow to extract information about the collective transverse expansion (radial flow) and the tem- perature at the moment when the hadrons decouple from the system. On the other hand the magnitude of the anisotropic flow can be characterized by the coefficients in the Fourier expansion of the azimuthal distribution of particles. The dominant coefficient for non central collision is the second harmonic, v2, which is called elliptic flow. It has been observed and extensively studied in nuclear collisions from sub-relativistic energies on up to RHIC and LHC energies. For the collisions of two smooth spheres, one would expect all odd harmonics to vanish due to symmetry reasons. However, due to event-by-event fluctuations in the positions of the participating nucleons inside the nuclei, the shape of the initial energy density of the heavy-ion collision is, in general, not symmetric with respect to the reaction plane, defined by the beam direction and the impact parameter. This gives rise to non-zero odd harmonic coefficients. In recent years the understanding of the initial geometry fluctuations and their role in the formation of final state anisotropic flow has significantly improved. Con- trolling the initial conditions in heavy-ion collisions would provide the possibility of detailed studies of the properties of the high density hot QCD matter. The Event Shape Engineering (ESE) technique allows the selection of different event shapes for a definite centrality and colliding system. The event selection is based on the azimuthal distribution of produced particles, using the so-called flow vector. Recent Monte-Carlo studies show a strong correlation between the (final state) event shape selection and the (initial state) eccentricity of the collision. This opens many new possibilities to study the properties of the system created in high energy nucleus-nucleus collisions, allowing to characterize events according to the initial geometry. In particular the event shape selection allows investigating the non trivial cor- relation between elliptic and radial flow. In this thesis we present a measurement of the pT distributions of primary particles in strongly or weakly anisotropic envi- ronments, at fixed impact parameter. In the first chapter a brief introduction to the physics of the QGP is given, focusing on the global characteristics of heavy ion collisions and the time evolution of the created system. A brief overview of the most recent results at RHIC and LHC experiments is also presented. In the second chapter we present a basic introduction to hydrodynamic models, which provide the theoretical framework, for the understanding the connection be- tween initial condition dynamic and the hydrodynamic response of the system cre- ated in nucleus-nucleus collisions. Furthermore, recent measurements of a large set of flow observables associated with event-shape fluctuations and collective expan- sion in heavy ion collisions are discussed. The experimental results are presented and compared to theoretical calculations. New types of fluctuation measurements, that can further improve our understanding of the event-shape fluctuations and collective expansion dynamics, are discussed. The third chapter is devoted to the description of the ALICE experiment. After a general overview of the apparatus, the the ALICE particle identification capabilities are described. In chapter 5 an approach to select the eccentricity of the event with the Event Shape Engineering is presented. The effect of this selection on the elliptic flow coefficient and identified particle spectra in Pb–Pb collisions at \sqrt{s_{NN}} = 2.76 TeV center-of-mass energy are discussed in chapter 6 and 7, respectively. The final results on the elliptic flow and the pT distributions of charged hadrons in event shape selected events are discussed in chapter 8