Study of the decay properties of the Higgs boson into two b quarks and upgrade of the ATLAS inner tracker

This thesis is focussing on the search for the Standard Model (SM) Higgs boson decaying to a pair of b-quarks in the vector boson associated production mode (VH) with the data collected with the ATLAS experiment at √s = 13 TeV corresponding to an integrated luminosity of L= 79.8 fb$^{−1}$. The selection of the events, reconstruction of the objects,the modelling of the backgrounds and signal, the discrimination of the events, and the resulting statistical analysis of the properties of the signal are discussed for this analysis. A clear excess of events compatible with the SM Higgs boson is detected with an observed (expected) significance of 4.9 (4.3) standard deviations and a signal strength of µ$_{vh}^{BB}$ =1.16±0.16(stat.)$_{-0.19}^{+0.21}$(syst.). The analysis is combined with the results obtained in Run-1 and the analysis performed with other decay modes (di-photon and $ZZ$∗ → 4 leptons, in the VH production mode) and other production modes (VBF and ttH, in the $B$$\bar{b}$ decay mode). The observation of the decay to b-quarks is claimed with an observed (expected) significance of 5.4 (5.5) standard deviations with a signal strength of µH→bb =1.01±0.12(stat.)$_{-0.15}^{+0.16}$(syst.) as well as the VH production mode with an observed (expected) significance of 5.3 (4.8) standard deviations with a signal strength of µV H =1.13±0.15(stat.)$_{-0.17}^{0.18}$(syst.). Several improvements are also discussed concerning the QCD background rejection and background modelling using multivariate techniques. From 2026 the LHC will enter in a new phase, providing collisions at an unprecedented rate. To understand the potential gains from that period, an extrapolation study is presented. Two scenarios of extrapolation are proposed, and the results are interpreted in terms of cross-section and signal strength. The uncertainties on the signal strength are going from 10 (15) % for the WH mode to 43 (50) % for the ggZh one for scenario 2 (1), with a systematical limitation for the WH and qqZh processes and a predominance of the statistical uncertainties for ggZh . Due to the new conditions, the ATLAS Inner-Tracker (ITk) will have to be upgraded to benefit from the new technologies developed to be more radiation hard and improve its resolution. Three productions of planar sensor aiming at reducing their thickness and increasing the geometrical acceptance are presented. The electrical properties and detection efficiencies have been measured on beams after irradiation campaigns.