Measurement of isolated photon-hadron correlations in Pb–Pb collisions at 5.02 TeV with the ALICE experiment at LHC

Ultra-relativistic heavy-ion collisions, produced at the Large Hadron Collider (LHC) at CERN, allow to produce a state of strongly interacting matter, the quark-gluon plasma (QGP), in which quarks and gluons are deconfined. Direct prompt photons are produced in Compton ($\rm qg \rightarrow \gamma \rm q$) and annihilation ($\rm q \bar{q} \rightarrow \gamma \rm q$) processes. Since they are colour-neutral, they do not interact strongly with the QGP and they can tag the parton emitted back-to-back to the photon at the origin of the process, which will interact strongly with the QGP losing energy. Such tagging gives a calibrated reference of its initial momentum and direction. The study of the direct prompt photon-hadron correlations gives access to the modification of the yield of the produced hadrons, originated from the parton fragmentation, and permits to understand how the lost energy of the parton, because of the presence of the QGP, is redistributed. The work presented in this manuscript is articulated around the study of the hadron from parton fragmentation modification via the measurement of the photon-hadron correlations in Pb-Pb collisions at center of mass energy of 5.02 TeV per nucleon measured with the ALICE experiment during the LHC Run 2 (years 2015 and 2018). At first, the selection of isolated photons is discussed: photons are detected with the ALICE calorimeter EMCal and identified via calorimeter cluster shape analysis; then the purity of the candidates sample is enhanced using the isolation method. Later on, the challenges of the analysis due to the large amount of underlying event and to the presence of background photons from various sources, such as neutral meson decays and fragmentation, are illustrated. My thesis demonstrates the feasibility of the measurement in three different centrality bins in Pb-Pb collisions and the main observation made, is the strong suppression of the hadron yield in central events compared to peripheral events and pp NLO simulations, in agreement with theoretical calculations. The last part of the thesis is dedicated to the FoCal upgrade, a forward calorimeter to upgrade the ALICE detector during the Run 4 (2030-2032). My contribution to the electromagnetic calorimeter (Focal-E) is presented, illustrating the tests and the calibrations done and implemented. A few results obtained at the test beam campaigns at CERN are also presented. In conclusion, this thesis illustrates the first measurement of isolated photon-hadron correlations in Pb-Pb collisions done by the ALICE Collaboration and produces an extensive benchmarking for the future data delivered by the LHC, which will help to reduce the dominant uncertainties.