Etude de la densité de particules chargées et des mésons vecteurs de basses masses en collisions Pb-Pb à sqrt(s)NN = 2.76 TeV dans ALICE au LHC

The matter is composed of hadrons of which quarks and gluons are the elementary components. These do not exist in a free state in ordinary matter and are therefore permanently confined in hadrons. However, according to theoretical predictions, a few microseconds after the Big Bang, the temperature was high enough to create a deconfined state of quarks and hadrons : the Quark and Gluon Plasma (QGP). The Large Hadron Collider (LHC) at CERN (Geneva) is a particle accelerator which accelerates, among others, ions and produces collisions with energies per nucleons in the center of mass up to several TeraelectronVolts. It is thus possible to achieve temperatures to recreate the QGP phase to study its properties. The experiment ALICE (A Large Ion Collider Experiment) is dedicated to the study of such ultra-relativistic heavy-ion collisions. The lifetime of the QGP being too low, it is not possible to study it directly. It is then necessary to use indirect observables. This PhD work is directly related to the study of heavy-ion collisions at 2.76 TeV. Two observables are discussed : the density of charged particles per unit of pseudorapidity and low mass vector mesons (rho, omega and phi) in the dimuon channel. The first observable gives access to informations about the initial conditions and the underlying dynamics of particle production mechanisms. The measurement is performed in the largest pseudorapidity range reached at the LHC (10 units) thanks to the development of an original analysis method called " displaced vertex technique ". The technique employed and the results obtained are described in Chapter 3. The study of low mass vector mesons allows to probe the production of strangeness via the phi meson and chiral symmetry through the ! spectral function modification. The analysis was conducted using the ALICE muon spectrometer and the results obtained from the production rate of the phi with respect to rho and omega are shown in Chapter 4. In this chapter, a study on the sensitivity of the detector to the effects related to the chiral symmetry restoration is also conducted.