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Measurement of jet properties in pp and Pb-Pb collisions at 5.02 TeV with the ALICE experiment at the LHC

Abstract : Jets, defined as collimated sprays of high-momentum particles, are experimental signatures of hard-scattered quarks and gluons produced in hadronic interactions. The jet production cross section is calculable within perturbative Quantum ChromoDynamics (pQCD), and therefore jet measurements provide stringent tests of pQCD predictions. In relativistic heavy-ion collisions, jets are well calibrated probes of the Quark-Gluon Plasma (QGP). Under extreme conditions of temperature and/or pressure, partons are deconfined and form a strongly interacting QCD medium. The initial hard scattered partons lose energy while traversing this medium due to radiative and collisional energy loss. Consequently, jet properties get modified in comparison with the vacuum case, phenomenon named jet quenching. QGP transport properties can be studied by measuring jet quenching.The charged jet production cross sections in pp collisions at √s = 2.76TeV and √s = 7TeV were measured by the ALICE experiment and compared to Leading-Order (LO) pQCD predictions. In Pb-Pb collisions, the strength of jet suppression was quantitatively assessed at √{s_NN} = 2.76TeV and √{s_NN} = 5.02TeV via the measurement of the nuclear modification factors (R_AA). The strength of charged jet suppression was quantified as a function of in-medium parton path-length based on the measured R_AA. The jet elliptic flow v₂, defined as the jet azimuthal distribution relative to the 2^{nd} order event plane, which is sensitive to the difference of the in-medium parton path-length in-plane and out-of-plane, was measured at √{s_NN} = 2.76TeV. The measured jet v₂ in mid-central Pb-Pb collisions was consistent with model predictions. The medium response has been studied through jet-track correlations at √{s_NN} = 2.76TeV as a function of centrality. The result suggested that the in-medium suppressed energy was re-distributed to large angles with respect to the jet axis. The phenomenon was described by a phenomenological calculation taking into account hydrodynamical evolution of the medium.In this thesis, two complementary aspects of jet measurements with the ALICE detector at the LHC were studied. First, the upgrade of the ALICE electromagnetic calorimeter trigger system is presented. The Di-jet Calorimeter (DCal) has been installed during LHC Long Shutdown 1 (LS1) to extend the azimuthal coverage of the existing ElectroMagnetic Calorimeter (EMCal) and PHOton Spectrometer (PHOS). The trigger system has been upgraded to account for this new detector configuration. The firmware for the Summary Trigger Unit (STU), which is the electronics of the trigger system, was upgraded to implement a brand new algorithm combining information from the calorimeters. Second, the measurement of the production cross section of charged jets reconstructed with cone resolution parameter R=0.2, 0.3, 0.4, and 0.6 in pp collisions at √s = 5.02TeV is outlined. A comparison of the production cross section to LO and Next-Leading-Order (NLO) pQCD predictions is shown. Good agreement of the production cross section with NLO pQCD calculations is found for 10 ≺ p_{T,jet}^{ch} ≺ 100GeV/c. The measurement of charged jet v₂ in mid-central (30-50%) Pb-Pb collisions at √{s_NN} = 5.02TeV is also presented. The results are compared with a toy-model Glauber simulation based on the measured path-length dependence of jet suppression. Finally, the measurement of charged jet-hadron correlations in mid-central (30-50%) Pb-Pb collisions at √{s_NN} = 5.02TeV with respect to the 2^{nd} order event plane is also presented in order to study initial collision geometry dependence of jet modification in Pb-Pb collisions.
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HOSOKAWA_2019_diffusion.pdf
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Ritsuya Hosokawa. Measurement of jet properties in pp and Pb-Pb collisions at 5.02 TeV with the ALICE experiment at the LHC. High Energy Physics - Experiment [hep-ex]. Université Grenoble Alpes; Université de Tsukuba, 2019. English. ⟨NNT : 2019GREAY025⟩. ⟨tel-02447312⟩

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