Study of the properties of the Higgs boson in the decay channel to a b-quark pair with the ATLAS experiment at the LHC

This thesis focuses on two separate research axes: first, the search for the Standard Model (SM) Higgs boson $H$ in the associated production mode ($VH$) decaying to b-quark pairs, with data collected at $\sqrt{s}=$7 TeV and 8 TeV during the LHC Run-1, and separately at $\sqrt{s}=$13 TeV during the LHC Run-2 (until July 2016). Furthermore, the search for a high-mass CP-odd pseudoscalar $A$ boson decaying to $Zh$ pairs, with the $h$ being a SM Higgs-like boson decaying to bottom-quark pairs, with data collected at $\sqrt{s}=$13 TeV during the LHC Run-2 (until December 2015).\\ The event selection, the analysis techniques, the main backgrounds and their modeling, the signal properties, and the statistical discrimination between the signal and background in data are discussed for all analyses presented in this document, as well as the interpretation of the results.\\ In the SM $VH(b\bar{b})$ channel no clear excess over the background prediction is observed in the analyses of the LHC Run-1 and Run-2 datasets. From the Run-1 data analysis the observed (expected) significance under the hypothesis of a SM Higgs boson can be quoted at $\sigma_{obs(exp)}= 1.4(2.6)$ , with a ratio of the measured signal yield to the Standard Model expectation (signal strength) of $\mu = 0.51 \pm 0.31\text{(stat.)}\pm 0.24\text{(syst.)}$. The significance of the Run-2 data analysis is $\sigma_{obs(exp)} = 0.42(1.94)$, with a signal strength of $\mu = 0.21 ^{+0.36}_{-0.35} \text{(stat.)} \pm 0.36 \text{(syst.)}$, corresponding to a 95\% observed upper limit on the cross section times branching ratio $pp \rightarrow (W/Z)(H \rightarrow b\bar{b})$ of $1.2$ times the SM prediction, compared to the limit of $1.0^{+0.4}_{-0.3}$ expected in the absence of a SM Higgs signal.\\ In the search for a CP-odd pseudoscalar $A$ boson two upward deviations from the background-only hypothesis are observed, for masses of the $A$ boson of $m_A = 260$ GeV and $440$ GeV, with a local significance of approximately 2$\sigma$. No other significant excess over the background prediction is found, therefore 95\% upper limits are set on the production cross sections times branching fractions, $\sigma(pp\rightarrow A)\times BR(A\rightarrow Zh)\times BR(h\rightarrow b\bar{b})$, in the range of $[4.0,\:0.017]$ pb ($[6.9,\:0.026]$ pb) for $m_A = [220, 2000]$ GeV assuming gluon-fusion (b-quark-associated) exclusive production, which are furthermore interpreted in the context of 2HMDs as constraints in the $\tan(\beta)$ and $\cos(\beta-\alpha)$ parameter plane.