Measurement of the Higgs boson coupling to $b$-quarks in the associated production with a vector boson with the ATLAS detector

This thesis presents the measurement of the Higgs boson coupling to a pair of $b$-quarks. The $H→b\bar{b}$ decay is predicted by the Standard Model to have a branching ratio of 58%, the largest among all decay modes. Therefore, accessing this decay is crucial to constrain the overall Higgs boson decay width. The measurement is performed in the production process with a $Z$ or $W$ boson, whose leptonic decays ensure high rejection of the multi-jet background and efficient triggering of the signal signature. The analysis uses proton-proton collision data collected by the ATLAS experiment during the years 2015-2018, as produced by the Large Hadron Collider at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV, corresponding to an integrated luminosity of 139 fb$^{-1}$. The production of the Higgs boson in association with a $Z$ or $W$ boson is measured with an observed (expected) significance of 5.3 (5.1) and 4.0 (4.1) standard deviations, respectively, constituting the observation of the $ZH$ process and the evidence of the $WH$ one. The measured signal strength of $\mu(VH→b\bar{b})$ = 1.02$^{+0.12}_{-0.11}$(stat.)$^{+0.14}_{-0.13}$(syst.) is highly compatible with the Standard Model expectation. Differential measurements of the $VH, H→b\bar{b}$ cross section are performed as a function of the vector boson transverse momentum, yielding good agreement with theoretical predictions. This thesis also presents performance studies of the impact of $b$- and $c$-hadron interactions with the ATLAS pixel detector material on the tagging efficiency of jets originating from $b$-quarks, known as $b$-jets. In addition, a new technique to identify $b$-jets without using tracking information is presented and it is expected to significantly improve the tagging efficiency at very high transverse momenta.