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 Higgs boson decaying to a pair of b-quarks (bb) in the vector boson associated production mode (VH) with the data collected with the ATLAS experiment at 13 TeV corresponding to an integrated luminosity of 79.8 fb$^{-1}$. An 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 1.16 $\pm$ 0.26. The analysis is combined with the results obtained previously and the analysis performed with other decay modes from one hand and other production modes on the other hand. The observation of the production and decay modes can be claimed with an observed (expected) significance of 5.3 (4.8) standard deviations for the first one and 5.4 (5.5) standard deviations for the second. Several improvements are 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. Due to the new conditions, the ATLAS Inner-Tracker 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 sensors aiming at reducing their thickness and increasing the geometrical acceptance are presented. The electrical properties and detection efficiencies have been measured on beams and are presented in this document.