Background studies on the H→ZZ*→4l channel at LHC Run 1. Prospects of the bbH(→γγ) mode and studies for an improved pixel detector system for the ATLAS upgrade towards HL-LHC

The discovery of a scalar boson, known as the Higgs boson, marked the first LHC data period (2010 - 2012). Using mainly di-photon and di-Z decays, with the latest leading to a four lepton final state, the mass of the boson was measured with a precision of < 0.2 %. Relevant couplings were estimated by combining several final states, while corresponding uncertainties would largely benefit from the increased statistics expected during coming LHC data periods (Run 2, Phase II). The H→ZZ*→4l channel, in spite of its suppressed brunching ratio, benefits from a weak background, making it a prime choice for the investigation of the new boson’s properties. In this thesis, the analysis aimed to the observation of this mode with the ALTAS detector is presented, with a focus on the measurement and control of the reducible electron background. In the context of preparation for future high luminosity data periods, foreseen from 2025 onward, two distinct studies are conducted: 1. The first concerns the observability potential of the Higgs associated production mode in conjunction with two b-quarks. A multivariate analysis based on simulated data confirms a very weak expected signal in the H→di-photon channel. 2. The second revolves around the conception and development of an inner silicon detector capable of operating in the hostile environment of high radiation and in-creased occupancy, expected during LHC Phase II. Main studies were concentrated on improving radiation hardness, geometrical and detection efficiency. Through fabrication process simulation and SIMS measurements, doping profiles and electrical characteristics, expected for innovative technologies, are explored. Prototypes were designed and evaluated in test beams and irradiation experiments in order to assess their performances and that of associated read-out electronics.