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Isolated photon production in p-Pb collisions at √(s_NN ) = 5.02 TeV with the ALICE experiment at the LHC

Abstract : Quantum Chromo Dynamics is the theory associated to the strong interaction in the standard model. It predicts that partons (quarks and gluons) are confined into hadrons at standard thermodynamic conditions. A state of deconfined hadronic matter, the Quark-Gluon Plasma (QGP), is predicted for a high energy density and would have existed in the early state of the Universe. ALICE (A Large Ion Collider Experiment) at CERN-LHC (Large Hadron Collider) is dedicated to hadronic matter study in p-p, p-Pb (Pb-p) and in Pb-Pb collisions where QGP formation is expected. In this thesis, we will present the study of photons coming directly from parton-parton hard scatterings, the prompt photons. Their production cross-section is calculable in perturbative Quantum Chromo Dynamics theory. Such a measurement in p-Pb collisions enables the study of cold nuclear matter effects whose knowledge is crucial to distinguish key signatures of the QGP from nuclear effects in Pb-Pb collisions. Experimentally, the prompt photon can be discriminated using the isolation analysis method. In this thesis, the measurement of the isolated photon cross-section in p-Pb collisions at √(s_NN )= 5.02 TeV with the EMCal calorimeter of the ALICE experiment, is presented. The results are consistent, within the uncertainties, with theoretical calculations. The comparison with the measured cross-section in p-p analysis is performed via the determination of the nuclear modification factor.The last one is compatible with the unity and does not clearly indicate modification of the prompt photon production due to nuclear effects in p-Pb collisions.
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Lucile Ronflette. Isolated photon production in p-Pb collisions at √(s_NN ) = 5.02 TeV with the ALICE experiment at the LHC. Nuclear Experiment [nucl-ex]. Université de Nantes, 2016. English. ⟨tel-02506136⟩

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