B-tagging Calibration and Observation of Higgs Boson Decays to a pair of bottom quarks with the ATLAS Detector

This thesis firstly describes a measurement of the $b$-jet tagging efficiency with a tag-and-probe method using data recorded by the ATLAS detector in 2015 and 2016. The efficiency measurement method, the object selection, the event and probe jet selection, the evaluation of the statistical and systematic uncertainties, and the final results are discussed. The $b$-tagging efficiencies have been measured as a function of the jet transverse momentum, pseudorapidity and of the average number of pile-up collisions. The efficiencies measured in data have been compared to those predicted from simulation, and simulation-to-data efficiency scale factors have been determined. The efficiency scale factors are close to unity, with total uncertainty ranging between 2% and 12%. The second part of this thesis focuses on the search for the decays of the Standard Model (SM) Higgs boson to bottom quarks in the associated production mode with vector bosons. Two measurements have been performed and are described here: the first one is based on 36.1 $\mathrm{fb}^{-1}$ of 13 TeV $pp$ collisions recorded by ATLAS during the LHC Run-2 until 2016; the second one includes the additional 44 $\mathrm{fb}^{-1}$ of 13 TeV $pp$ collisions recorded in 2017. For the study based on 36 $\mathrm{fb}^{-1}$ of data, the object reconstruction, event selection and classification, the techniques to discriminate the signal from the background, the main background and their modelling, the signal properties and the interpretation of results are discussed. For the study based on 80 $\mathrm{fb}^{-1}$ data, the main difference from the previous study are highlighted. Both results provide a strong evidence of the $VH$, $H\to b\bar{b}$ process, with statistical significances of 3.5 and 4.9 standard deviations, respectively. The signal strength, defined as the ratio between the measured cross section times branching ratio and the SM prediction, is measured with the 2015-2017 dataset to be $\mu^{bb}_{VH} = 1.16^{+0.27}_{-0.25} = 1.16\pm 0.16~\mathrm{(stat.)}^{+0.21}_{-0.19}~\mathrm{(syst.)}$, indicating good agreement with the SM. The combination of the $VH$, $H\to b\bar{b}$ results based on 80 $\mathrm{fb}^{-1}$ of 13 TeV collisions with the results of the same analysis performed on data collected at lower (7 and 8 TeV) centre-of-mass energies in 2011 and 2012, and witht those of the searches of $H\to b\bar{b}$ decays in other Higgs boson production modes (vector-boson fusion, associated production with a top-quark pair) have led to the observation of the $H\to b\bar{b}$ decay with a significance of 5.4~standard deviations, and to a measurement of the signal strength $\mu^{bb}= 1.01\pm 0.20 = 1.01\pm 0.12~\mathrm{(stat.)}^{+0.16}_{-0.15}~\mathrm{(syst.)}$. A combination of the results of the $VH$, $H\to b\bar{b}$ results based on 80 $\mathrm{fb}^{-1}$ of 13 TeV collisions with the results of searches for the same production mode on the same amount of data in the diphoton and $ZZ^*\to 4\ell$ Higgs boson final states has led to the observation of $VH$ production with a significance of 5.3~standard deviations, and to a measurement of the signal strength $\mu^{VH} = 1.13^{+0.24}_{-0.23} = 1.13 \pm 0.15~\mathrm{(stat.)}^{+0.18}_{-0.17}~\mathrm{(syst.)}$, in good agreement with the SM.