Observation of the Higgs boson coupling to b-quarks with the ATLAS detector

This thesis presents the results of the search for the Standard Model Higgs boson decay to a pair of bottom quarks, carried with data at $\sqrt{s} = 13$ TeV collected with the ATLAS detector at the LHC between 2015 and 2017. \\ The Standard Model Higgs boson - with a mass of $m_H = 125$ GeV - is expected to decay to a pair of bottom quarks, also referred to as $H \to b\overline{b}$, more than half of the times (58\%). An experimental confirmation of this process is crucial to probe the mechanism of quark mass generation of down type quarks and heavily constrain the possibility of new physics in the Higgs sector. \\ The search has been carried selecting the Higgs boson produced in association with a vector boson (VH), which ensures high background rejection thanks to the additional leptonic signatures in the final state. \\ The core of this thesis work is the analysis performed in 2018 with $80 ~\ifb$ of ATLAS data. The analysis strategy, the event selection and the treatment of the systematic uncertainties affecting the measurement are presented in details in the document. The measured signal strength ($\sigma \cdot BR$/ $\sigma_{SM} \cdot BR_{SM}$) is $\mu = 1.16^{+0.16}_{-0.16}(\text{stat.})^{+0.21}_{-0.19}(\text{syst.})$, while the significance of the measurement is 4.9 standard deviations ($\sigma$) compared to an expected significance of 4.3 standard deviations. The sensitivity of the analysis has been enhanced combining this result with the Run-1 analysis at 7 and 8 TeV, for a total of $105~ \ifb$. A total observed (expected) significance of 4.9 (5.1) standard deviations has been obtained, with a signal strength of $\mu^{bb}_{VH} = 0.98^{+0.22}_{-0.21} = 0.98 \pm 0.14(\text{stat.})^{+0.17}_{ -0.16}(\text{syst.})$. The sensitivity to the VH production mode has been increased combining the $VH, H \to b \overline{b}$ Run-2 result with other Run-2 analyses sensitive to the VH production mode. Fixing the branching ratios of each final state to its SM value, the overall signal strength of the VH process is $\mu_{VH} = 1.13 ^{+0.24}_{-0.23} = 1.13 \pm 0.15(\text{stat.})^{+0.18}_{ -0.17}(\text{syst.})$, with an observed significance of 5.3 standard deviations, and an expected significance of 4.8 standard deviations. Similarly, the sensitivity to the $H \to b \overline{b}$ decay has been enhanced by a combination with other Run-1 and Run-2 analyses sensitive to this decay channel. The signal strength of the $H \to b \overline{b}$ decay has been obtained fixing the production modes to their SM predictions. The combined value is $\mu_{H \to bb} = 1.01 \pm 0.2 = 1.01 \pm 0.12(\text{stat.})^{+0.16}_{ -0.15}(\text{syst.})$, with an expected (observed) significance of 5.4 (5.5) standard deviations. Both combinations have a significance above the $5\sigma$ threshold, meaning that for the first time the VH production mode and the $H \to b \overline{b}$ decay have been measured with enough precision to claim their observation. The thesis summarizes the main aspects of my contribution to the 2018 round of this analysis and the next one, expected to be public for the end of 2019. Furthermore, it also focuses on another aspect of my contribution in ATLAS during the three years of PhD, which is the energy calibration of low energy electrons, used for example in the $H \to 4\ell$ analysis, with $J/\psi \to e^-e^+$ events.