Search for boosted Higgs boson and other resonances decaying into $b$-quark pairs using the ATLAS detector and studies of CMOS pixel sensors for the HL-LHC

The Large Hadron Collider (LHC) at CERN is the largest particle collider ever built and enables experimental study of the fundamental constituents of matter at the highest centre-of-mass energy ($\sqrt s$) ever achieved. Between 2015 and 2018, the LHC provided proton-proton collisions at a record energy of $\sqrt s$ = 13 TeV. In this thesis, two novel analyses looking for jets stemming from the hadronization of b-quarks ($b$-jets) are presented together with a detailed description of the tools and the related recent improvements for identifying the jets produced by beauty quarks ($b$-tagging). The first analysis is an inclusive search for boosted Higgs bosons decaying into a pairs of $b$-quarks using 80.5 $\textrm{fb}^{-1}$. This search has long been considered impossible due to the overwhelming QCD background. However, recent developments in the reconstruction of boosted environments made it possible to search inclusively for $H \rightarrow b \bar{b}$. In the signature used in the analysis the Higgs boson acquires high transverse momentum by recoiling against an initial jet. When the transverse momentum of the Higgs boson becomes larger than twice its mass, the subsequent decay products tend to merge and are effectively better identified by constructing a single jet with a large radius parameter of 1.0. An evidence of $W/Z$+$jets$ processes is also presented. This represents the first measurement performed by the ATLAS experiment of vector bosons in the boosted regime with heavy-quarks in the final state. In addition, exclusions limits are placed on resonant dark matter mediator models. The second analysis is a search for heavy resonances with masses above 1.1 TeV decaying to one or two $b$-quarks, producing two isolated jets in a back-to-back topology. The analysis was performed with the full data-set collected at 13 TeV which corresponds to 139 $\textrm{fb}^{-1}$. For both analyses it is crucial to identify jets initiated by heavy quarks with high accuracy. The performance of the algorithm which discriminates between jets initiated by $b$-quarks against jets initiated by $c$- or light-flavored quarks or gluons is discussed. The addition of novel $b$-tagging techniques which significantly improve the performance at high transverse momentum are also presented. Looking to the future, the physics programme at the LHC will be expanded by the High-Luminosity LHC (HL-LHC) project foreseen to start in 2024. The integrated luminosity will be increased to 3000 $\textrm{fb}^{-1}$. Such a high luminosity poses unique challenges in terms of radiation-hardness of the detectors. A novel technology of pixels detectors based on the commercially available CMOS manufacturing processes was studied during this thesis. Results of test-beam measurements show high detection efficiency under unprecedented radiation conditions.