Calibration of the ATLAS B-tagger and the search for the $t\overline{t}H(H\rightarrow b\overline{b})$ process at $\sqrt{s}$ = 13 TeV with the ATLAS experiment at the LHC

Top quarks and Higgs bosons are the heaviest particles in the Standard Model of particle physics and are the subject of many analyses performed with the ATLAS experiment at the LHC at CERN. The Higgs boson was discovered in 2012 and is expected to play a major role in the way fundamental particles acquire mass, but also in potential new physics beyond the Standard Model. However, many of its properties have not been measured yet. One such property is its interaction with the top quark, represented by the top Yukawa coupling. The best way to measure this coupling is by observing the associated production of a Higgs boson with a top-antitop quark pair ($t\overline{t}H$) at the LHC. Furthermore, investigating such cases in which the Higgs boson decays into a bottom-antibottom quark pair ($t\overline{t}H(H\rightarrow b\overline{b})$) opens a window to also measuring the Yukawa coupling to the bottom quark. As the top and antitop quarks are expected to decay via the charged-current weak interaction into bottom and antibottom quarks as well, this analysis is dependent on a very efficient and precise method to identify jets originating from bottom quarks. The calibration of these identification methods employed in the ATLAS experiment is presented. It is based on 80.5 fb$^{-1}$ of data collected at a centre-of-mass energy of $\sqrt{s}=13~\text{TeV}$ in the years 2015, 2016, and 2017. This calibration produces scale factors which can be used to correct the predicted identification efficiency to the one measured in data. The relative uncertainties on these scale factors range from 8-9% for jets with a low transverse momentum ($p_{\text{T}}$) to 1% at a medium $p_{\text{T}}$ and, finally, to 3-4% at high $p_{\text{T}}$. The search for the $t\overline{t}H(H\rightarrow b\overline{b})$ process with 36.1 fb$^{-1}$ of ATLAS data collected in 2015 and 2016 is presented thereafter. The cross-section of this production mode is measured by performing a profile likelihood fit over several analysis regions involving decays of the top-antitop quark pair that produce either one or two charged leptons in the final state. The most dominant sources of uncertainty originate from the modelling of physics processes involving a top-antitop quark pair in association with a bottom-antibottom quark pair which is the main background process of this search. The ratio of the measured cross-section with respect to the one expected in the Standard Model, $\mu$, is found to be \begin{gather} \mu_{t\overline{t}H(H\rightarrow b\overline{b})} = 0.84\pm 0.29(\text{stat.})^{+0.57}_{-0.54}(\text{syst.})=0.84^{+0.64}_{-0.61}. \nonumber \end{gather} This translates into an inclusive cross-section of $\sigma_{t\overline{t}H}=426^{+326}_{-312}~\text{fb}$ when neglecting correlations between related uncertainties. The result corresponds to an observed (expected) significance of 1.4 (1.6) standard deviations and thus is not sufficient to claim an observation, as it is well compatible with both hypotheses, namely the absence as well as the presence of the $t\overline{t}H$ signal.