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Theses

Response of the high granularity calorimeter HGCAL and characterisation of the Higgs boson with the CMS experiment at the LHC

Abstract : This thesis presents a comprehensive characterisation of the properties of the Higgs boson in the four-lepton decay channel exploiting proton-proton collisions collected at a centre-of-mass energy of 13 TeV with the CMS experiment at the CERN LHC, corresponding to 137 fb-1 recorded during the Run-II. The four lepton decay channel is also known as the “golden channel” because of its many virtues: a large S/B ratio, a completely resolved final state, and an excellent resolution.The analysis work includes an optimization of the event categories designed to measure for the first time the Stage 1.2 STXS cross sections, for a total of nineteen cross sections in mutually exclusive phase space regions, and the development of the statistical model employed for the extraction of the results.The predictions of the Standard Model (SM) are tested measuring the H->ZZ->4l (l=e,mu) inclusive signal strength modifier, defined as the ratio of the Higgs boson production cross section to the corresponding SM prediction.The measurement gives mu=0.94±0.07(stat)+0.09(syst) at mH=125.38 GeV. The signal strength modifers of each production mechanism are also measured. The cross section of the H->4l process is also measured at generator-level, unfolding the detector effects, in a fiducial volume defined to closely match the experimental acceptance. The H->4l fiducial cross section is found to be 2.84+0.23(stat)+0.26(syst) fb. Fiducial cross sections are measured in differential bins of the transverse momentum and rapidity of the Higgs boson, the number of associated jets, and the transverse momentum of the leading associated jet. The differential cross section as a function of the transverse momentum of the Higgs boson is also used to extract limits on the trilinear self-coupling of the Higgs boson for the first time using a single-Higgs decay channel. Additional studies of CP-violation and anomalous couplings of the Higgs boson to vector bosons and fermions are also presented. All the results are found to be in agreement with the SM predictions, stressing the strength of the SM and the difficulty of unveiling physics beyond the SM (BSM). In order to probe in even further detail the predictions of the SM and to give access to rare phenomena, CERN intends to commence the operations of the High-Luminosity LHC (HL-LHC) project by the end of 2027.The HL-LHC is expected to deliver 10 times the current integrated luminosity, thus enhancing the physics reach of the LHC, but also leading to a high pile-up rate and unprecendent radiation levels. Hence, a massive upgrade of the detectors is necessary to maintain the current physics performance in such a harsh environment. The CMS Collaboration will replace the current endcap calorimeters with a High Granularity Calorimeter (HGCAL), designed to provide improved discrimination power and radiation hardness in the busy environment of the HL-LHC. The HGCAL will be the very first large-scale silicon-based imaging calorimeter ever employed in a high energy physics experiment. Hence, the validation of its design and a complete assessment of its physics performance are mandatory for the success of the project. In October 2018 the first large-scale prototype of the HGCAL, comprising of O(100) modules, was exposed to test beams with energies ranging from 20 to 300 GeV at the CERN North Area. A part of this thesis was dedicated to the assessment of the performance of the electromagnetic compartment of the prototype, leading to its complete characterisation. The high granularity of the prototype is exploited to study the longitudinal and transversal containment of electromagnetic cascades, as well as to measure the position, angular, and energy resolutions. All the results are compared with a dedicated GEANT4 Monte Carlo simulation and an excellent agreement are found for all the observables studied, thus corroborating the final design of the HGCAL and the nominal physics performance expected for the HL-LHC operations.
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Submitted on : Monday, December 20, 2021 - 4:04:12 PM
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Matteo Bonanomi. Response of the high granularity calorimeter HGCAL and characterisation of the Higgs boson with the CMS experiment at the LHC. High Energy Physics - Experiment [hep-ex]. Institut Polytechnique de Paris, 2021. English. ⟨NNT : 2021IPPAX078⟩. ⟨tel-03497358⟩

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