Search for the production of a Higgs boson in association with top quarks and decaying into a b-quark pair and b-jet identification with the ATLAS experiment at LHC

In July 2012, the ATLAS and CMS experiments announced the discovery of a new particle, with a mass about $125$ GeV, compatible with the Standard Model Higgs boson. In order to assess if the observed particle is the one predicted by the Standard Model, the couplings if this Higgs boson to fermions have to be measured. In particular, the top quark has the strongest Yukawa coupling to the Higgs boson. The associated production of a Higgs boson with a pair of top quarks ($t\bar{t}H$) gives a direct access to this coupling. The $t\bar{t}H$ process is accessible for the first time in the Run 2 of the LHC thanks to an upgrade of the detector, especially the IBL which improves $b$-tagging, and the increase of the center of mass energy to $13$ TeV and of the integrated luminosity to $36.1$ fb$^{-1}$ in 2016. This thesis presents the search for $t\bar{t}H$ events with the Higgs boson decaying to a pair of $b$-quarks using data collected by the ATLAS detector in 2015 and 2016. The description of the background and the extraction of the $t\bar{t}H(H\rightarrow bb)$ signal in data are obtained by a statistical matching on predictions to data. In particular the $t\bar{t}+$jets background is the main limitation to signal sensitivity and is scrutinized. The identification of jets originating from $b$-quarks, called $b$-tagging, is a vital input to the search of $t\bar{t}H(H\rightarrow bb)$ events because of the four $b$-quarks in the final state. For Run 2 the definition of b-flavoured-jets in Monte Carlo simulations is revisited to improve the understanding of $b$-tagging algorithms and their performance. Standard $b$-tagging algorithms do not separate jets originating from a single $b$-quark from those originating from two $b$-quarks. Thus a specific method has been developed and is reviewed in this thesis.