Study of the Higgs self coupling in ATLAS detector at LHC. Study of performances of the timing detector for the high luminosity phase of LHC

A bosonic particle with a mass equal to 125 GeV was observed in 2012, by ATLAS and CMS collaborations at the Large Hadron Collider (LHC). This particle was associated with the Higgs Boson or BEH boson, predicted fifty years before its discovery by Francois Englert, Robert Brout and Peter Higgs. This particle validates the BEH mechanism, explaining the origin of the mass of known particles and the electroweak symmetry breaking. Since its discovery, it has become crucial to probe the various properties of the Higgs boson such as the Higgs self-coupling. The success in probing Higgs self-coupling will bring another probe of the standard model and will provide a direct measurement of the Higgs field potential in the vacuum. This measure is performed through a global analysis of the di-Higgs (HH) production at LHC, decaying into various channels. This thesis focuses on the study of the decay of the Higgs boson pair into two light leptons with the same charge (referred to as 2lSS) within the context of the Run2 at LHC, providing an integrated luminosity of 139 fb^-1 and a centre of mass energy of √(s) = 13 TeV. The study based on Monte-Carlo simulations aims to develop a machine learning-based strategy to discriminate the signal (2lSS originating from the decay of the Higgs boson pair ) from the background (all other processes producing 2lSS events). An analysis of background noise estimation and systematic uncertainty estimation is also presented in this work. Finally, although the measurement of the self-coupling can be constrained by Run2, its direct measurement is expected in the High-Luminosity phase of the LHC (HL-LHC). This phase involves a five-fold increase in instantaneous luminosity, requiring an upgrade of the ATLAS detector to ensure performance comparable to Run2 performances, despite the increase of radiation and pile-up effects. As a result, a new high-granularity timing detector (HGTD) will be added. A study on the readout electronics of this future detector is presented in this thesis, determining its performance in the test bench and during irradiation tests. (author)