Study Of Jet Fragmentation With ALICE At LHC Through γ-hadron Correlation Measurement

With the advent in 2008 of the Large Hadron Collider (LHC), the new accelerator at CERN will collide heavy-ions at unprecedented high energies. Using this experimental facility, the ALICE collaboration will study matter under extreme conditions of energy density opening new avenues for the understanding of fundamental properties of the strong interaction and its vacuum. In particular, it will explore the properties of primordial matter which degrees of freedom are quarks and gluons: the quark-gluon plasma.In the present thesis, I present a contribution to the collaboration wide effort in preparing the challenging task of data analysis and in selecting the most appropriate probes carrying relevant information on the properties of the medium created in ultrarelativistic heavy-ion collisions. I have selected high momentum jets as a tomographic probe of the medium: the observable is the correlation of high-momentum direct photon with hadrons emitted in the opposite direction. In this measurement, often referred to as the golden probe, the direct photon is a measure of the initial energy of the parton, created by the same hard process as the photon in the initial stage of the collision, and the hadrons, being the final state particles of the fragmenting parton, measure the parton after it has traversed the medium.I will report on the first part of my task, namely the validation of the direct-photon - hadron correlation observable in proton-proton collisions. This measurement is mandatory as it will serve as a base line for the same measurement in heavy-ion collisions. Medium effects modifying the jet fragmentation properties will be revealed by comparing the same measurement with the two collision systems. For this purpose, the detection and identification performances of direct photons have been studied in great details. The systematic errors have been evaluated, in particular those due to the large photon background generated by the electromagnetic decay of neutral mesons. The statistical errors have been estimated based on the running condition in a standard operation year of the LHC. All the studies have been performed from Monte-Carlo generated data.I conclude my work by validating, first, the method in providing an accurate measurement of the fragmentation function and, second, by defining under which kinematical conditions the measurement is feasible with ALICE.For completeness, I have discussed the object of my personal work in the general context of our current knowledge on particle physics and in the more particular context of heavy-ion physics.This work was started two years ago with the study of the response of the ALICE photon spectrometer, PHOS, and of its performance with respect to photon identification. During a 4 months stay at CERN as a summer student and great efforts in these two years, I have accomplished most of the work on the correlation studies. The later gave rise to an article which has been submitted to the European Journal of Physics C.