Mesures Des Corrélations γ-hadron Dans Les Collisions P-p Avec L'expérience ALICE Au LHC

With the advent of the Large Hadron Collider (LHC) end of 2009, the new accelerator at CERN collides protons and heavy-ions at unprecedented high energies. ALICE, one of the major exper-iment installed at LHC, is dedicated to the study of nuclear matter under extreme conditions of energy density with the opportunity of creating a partonic medium called the Quark-Gluon-Plasma (QGP). This new experimental facility opens new avenues for the understanding of fundamental properties of the strong interaction and its vacuum.The reach the objectives of this scientific program, it is required to select a set of appropriate probes carrying relevant information on the properties of the medium created in ultra-relativistic heavy-ion collisions. Based on the information delivered by all the observables and guided by modelization of the fundamental principles in action, a coherent picture will emerge to interpret the observed phenomena. In the first part of the present document I describe the context of the scientific program, the general concepts involved in heavy-ion collisions at ultra relativistic energies, and the main results obtained so far in the field.Among the observables of interest, the production of hadrons jets is particularly attractive. Jets are the result of the hadronisation process of high transverse momentum partons and are observed in the detectors as a beam of collimated hadrons. High transverse momentum partons are created by hard scattering of partons (2→2 type of processes) constituting the colliding projectiles. The jet structure measured, for example as the distribution of the factional jet energy carried by the individual hadrons inside the jet, is the observable of choice. In heavy ion collisions, high transverse momentum partons are created concurrently with the hot and dense medium of interest and their kinematical properties are modified as they traverse the medium. This modification, imprinted in the jet structure, is observed by comparing the jet structure measured in heavy-ion collisions (in medium jet structure) with the jet structure measured in proton-proton collisions (vacuum jet structure). Such a measurement raises however a technical difficulty:whereas jets can be easily identified and measured in proton-proton collisions, in heavy-ion collisions the large hadronic background from the underlying event (the underlying event is everything except the two hard scattered jets and is generated by the beams particle break up and by initial and final state radiation) makes the jet identification measurement quite challenging. In addition the initial momentum of the hard scattered parton is unknown since only the final jet momentum can be measured i.e. the momentum of the parton as it emerges from the medium. This complicates the interpretation of the measurement.To overcome these difficulties, I have selected a particular 2→2 process which creates a direct photon (direct photon at variance with decay photon) in the final state together with a high transverse momentum parton. The momentum of the photon ((?)), since it does not interact strongly with the medium, calibrates the momentum of the parton((?)=-(?)).Therefore the photon momentum is a measure of the parton momentum when created and the jet momentum the momentum of the parton after it has traversed the medium. In addition since the photon momentum (energy and direction) defines also the jet momentum, jet reconstruction algorithms are not required anymore. Instead of studying photon-jet correlations (where the jet is fully reconstructed), it is sufficient to study photon-hadron correlations from all the hadrons in the event. However, the relatively weak cross section for the production of these particular hard scattering processes makes the measurement quite challenging. An introduction to 2 particle correlations is given in the second part of this document followed by a description of the ALICE detection systems used for this measurement.The strategy I have followed for this study starts with a validation of the measurement. It consists first in studying with the help of Monte Carlo simulations the accuracy of the selected ob-servable in revealing and quantifying the phenomenon under study. Second, it consists in verifying the ability to measure the observable and its robustness with the detectors setup of the ALICE experiment. The validation procedure and results are discussed in the third part of this document. I have particularly studied the possibility to extract two quantities from the 2 particle azimuthal correlation measured in proton-proton collisions:(ⅰ) the average total transverse momentum () generated at the partonic level by the Fermi motion and initial and final state radiation, and (ⅱ) the per trigger yield of jet hadrons as a function of the fractional jet energy(xE) they carry each (correlation function). The same quantities have been studied from simulated heavy-ion collisions with the objective to analyze the effects due to the presence of highly dense color medium. The distribution of kT values becomes broader in a way that can be directly related to the transport properties of the medium and the correlation function is modified so that the number of high xE hadrons are suppressed (jet quenching) and the number of low xE hadrons is increased (radiative gluon production) with an amplitude proportional to the transport coefficient and to distance tra-versed inside the medium. To finish this part of the document dedicated to Monte Carlo studies (on which I have spent most of my time as a PhD student) with another detailed study n the possibility to exploit the photon-jet observable as a tomographic tool (following a suggestion by X.N. Wang). The idea is to localize the hard scattering well inside the medium (by selecting hadrons with low xE values) or on the surface of the medium (by selecting hadrons with large xE values). One would therefore choose the distance in the medium through which the hard scattered parton travels and probe the medium from its densest part (center) to its less dense part (surface). I found that such a measurement will be quite challenging.In the forth part of the present document, I address the analysis of the data collected in 2010 for proton-proton collisions at (?)= 7 TcV. During this first long data taking period at LHC, the ALICE detection system was not yet complete. In particular, the incomplete coverage of the electromagnetic calorimeters and the absence of a selective photon trigger was a severe handicap for the photon-jet measurement. The resulting event statistics available for the measurement of this observable was limited to the photon energy range-below 10 GeV. This low energy domain is not well suited for the identification of direct photons because of their scarcity in the overwhelming background generated by decay photons. On top of that, the time between the availability of the data and the scheduled time for my defense was too short to perform an in-depth analysis. Most of the results presented from this analysis in the present document must therefore to be considered as very preliminary, but the key features are there. The results concern the 2 jet and mono jet structure observed in the photon-jet azimuthal correlation, the measured value of (kT) and the xE distributions.This very preliminary analysis of the first data collected at LHC and presented in this document is the first only toward a comprehensive study of the photon-jet observable. Since the writing of the document, the analysis has progressed and provided a few results which were considered ripe by the collaboration to be presented at the Quark Matter conference in May 2011. The data which will be collected in 2011 in proton-proton and Pb-Pb collisions will be much richer in photon-jet events thanks to the complete coverage of the ALICE electromagnetic calorimeter and thansk to a very high energy photon trigger provided by the calorimeter as well.For completeness, I finish the present document with the description of my contribution, as the main person in charge, to the quality assurance and monitoring tasks for the two ALICE electromagnetic calorimeters during data taking.