In July 2012, CERN announced the discovery of the Higgs boson, the last missing piece of the Standard Model (SM). The Higgs boson was observed in different channels. Precise measurement of its properties is now very important to investigate for possible deviations from the SM. This thesis presents a search for the Higgs boson produced in association with top quarks and decaying to a b quark pair, the ttH(H→bb) channel, using proton-proton collisions at √s = 13 TeV, collected with the ATLAS detector in 2015 and 2016. This document details in particular the full reconstruction of the ttH(H→bb) system and the discrimination between signal and the main background, tt+jets. The ttbb is a subset of the tt+jets backgrounds recent studies show that there is a large fraction of ttbb events with jets containing two b-hadrons. A new algorithm has been developed to discriminate such jets from single b-hadrons jets. The description of this tool is presented in this thesis.

The discovery in 2012 of the Higgs boson at the Large Hadron Collider (LHC) represents a milestone for the Standard Model (SM) of particle physics. Most of the SM Higgs production and decay rates have been measured at the LHC with increased precision. However, despite its experimental success, the SM is known to be only an effective manifestation of a more fundamental description of nature. The scientific research at the LHC is strongly focused on extending the SM by searching, directly or indirectly, for indications of New Physics. The extensive physics program requires increasingly advanced computational and algorithmic techniques. In the last decades, Machine Learning (ML) methods have made a prominent appearance in the field of particle physics, and promise to address many challenges faced by the LHC. This thesis presents the analysis that led to the observation of the SM Higgs boson decay into pairs of bottom quarks. The analysis exploits the production of a Higgs boson associated with a vector boson whose signatures enable efficient triggering and powerful background reduction. The main strategy to maximise the signal sensitivity is based on a multivariate approach. The analysis is performed on a dataset corresponding to a luminosity of 79.8/fb collected by the ATLAS experiment during Run-2 at a centre-of-mass energy of 13 TeV. An excess of events over the expected background is found with an observed (expected) significance of 4.9 (4.3) standard deviation. A combination with results from other \Hbb searches provides an observed (expected) significance of 5.4 (5.5). The corresponding ratio between the signal yield and the SM expectation is 1.01 +- 0.12 (stat.)+ 0.16-0.15(syst.). The 'observation' analysis was further extended to provide a finer interpretation of the V H(H → bb) signal measurement. The cross sections for the VH production times the H → bb branching ratio have been measured in exclusive regions of phase space. These measurements are used to search for possible deviations from the SM with an effective field theory approach, based on anomalous couplings of the Higgs boson. The results of the cross-section measurements, as well as the constraining of the operators that affect the couplings of the Higgs boson to the vector boson and the bottom quarks, have been documented and discussed in this thesis. This thesis also describes a novel technique for the fast simulation of the forward calorimeter response, based on similarity search methods. Such techniques constitute a branch of ML and include clustering and indexing methods that enable quick and efficient searches for vectors similar to each other. The new simulation approach provides optimal results in terms of detector resolution response and reduces the computational requirements of a standard particles simulation.

Since the discovery of the Higgs boson in 2012, the diphoton ([gamma][gamma]) decay channel of the Higgs boson has been one of the most potent channels due to the contrast between the smoothly falling background and sharply peaked signal in the diphoton invariant mass. In 2018, with up to 80 fb-1 of data at [square root]s = 13 TeV, ATLAS observed Higgs boson production in association with a pair of top quarks (ttH) with a significance of 5.8 [sigma] by combining measurements in the [gamma][gamma], bb, ZZ, and multi-lepton Higgs decay channels. The ttH production cross-section was measured to be 670 [plus or minus] 90 (stat) +110 -100 (syst) fb. The diphoton channel was one of the main contributors to this result, alone providing a significance of 4.1 [sigma]. With 140 fb-1, a search for non-resonant Higgs pair production in the bb[gamma][gamma] final state was performed. No significant signal was observed and upper limits at 95% confidence level were set. The observed limit on the SM cross-section was 130 fb, or 4.2 times the predicted value. The observed Higgs trilinear coupling modifier was constrained to be between [-1.5, 6.7]. Both the ttH (H [rightwards arrow] [gamma][gamma]) and HH bb[gamma][gamma] analyses will benefit tremendously from the increased statistics expected from the High-Luminosity LHC (HL-LHC). To ensure the continued efficiency of the detector in the harsh HL-LHC environment, ATLAS will install a new Inner Tracker (ITk) consisting of silicon pixel sensors in its innermost layer. At SLAC National Accelerator Laboratory, a variety of electrical tests are performed for the construction of a prototype integrated pixel system, in order to provide early feedback and validation of the ITk design.