Etude de la fragmentation des partons par la mesure de corrélations photon-hadron auprès de l'expérience ALICE au LHC

The strong interaction theory, Quantum Chromodynamic (QCD), predicts a new phase of nuclear matter at very high temperature and/or very high density. This state, which should have been the state of the Univers some microseconds after the Big Bang, is composed of deconfined quarks and gluons known as the quark-gluon plasma (QGP). The measurement of its composition and properties is a challenge for the nuclear physics of the 21st century and should lead to a better understanding of the fundamental symetries and mechanisms related to the quarks confinement inside hadrons and the strong interaction generally. The Large Hadron Collider (LHC) accelerator at CERN (European Organization for Nuclear Research) allows to reach the thermodynamic conditions required to create the quark-gluon plasma using ultra-relativistic heavy ion collisions (Pb). The ALICE experiment (A Large Ion Collider Experiment) allows to access several probes to characterize the QGP through particles reconstruction and. Among these probes, high energy parton interaction is used to performe a medium tomography to access characteristics such as density or temperature. Parton energy loss inside hot and deconfined medium is estimated from the modification of the energy distribution of hadrons produced by fragmentation. The parton initial state, especially its energy, is obtained by measuring correlations between a photon (almost unsensitive to the medium) and back-to-back hadrons produced by the parton fragmentation. This thesis is dedicated to the photon-hadron correlations analysis in order to study the modification of the parton fragmentation due to the quark-gluon plasma. The context of the LHC start and first data taking in 2010 have required an important experimental work to understand precisely the different detectors used for charged particles and photons reconstruction. First part of this thesis is devoted to the characterization of the electromagnetic calorimeter (EMCal), the central detector for energy measurement and photon identification. The second part is dedicated to the photon-hadron correlation measurement, for the 7 TeV proton-proton collisions and 2.76 TeV Lead-Lead collisions. An important work has been done to improve the prompt photon identification, one of the key point of this analysis.