Exploiting the ATLAS detector in a search for stop squark in a compressed mass spectrum using the Higgs boson

In past decades, experimental and theoretical efforts offered several reasons to believe that the Standard Model of particles (SM) is nothing more than an effective low-energy approximation of a more fundamental theory. The theory of Supersymmetry (SUSY) seems to be a natural candidate to extend the SM. Of particular interest among SUSY particles is the stop squark, due to the fact that it gives the dominant contribution to the Higgs boson mass radiative corrections. In this thesis, I present a search for direct stop squark pair production in data collected by the ATLAS detector at the CERN Large Hadron Collider (LHC) during 2015-16 in proton-proton collisions with a centre-of-mass energy ps = 13 TeV. This search takes advantage of the full capabilities of ATLAS further increased by the improvements made to the ATLAS pixel detector during 2014 - 2015. Both the installation of a new pixel detector layer (Insertable B - Layer) and the upgrade of the data-acquisition system of the pre-existing layers are presented in detail in this thesis, along with their impact to the b-jets reconstruction efficiency. This search for stop squark relies on events with a pair of b-jets compatible with the decay of an Higgs boson to select the signal candidate events. Results of this selection are interpreted in models with long decay chains involving a heavy neutralino X02 or the heavier stop squark et2. No excess is observed in the data with respect to the SM predictions for any of the models and decay chains, thus excluding at 95% confidence level et2 and et1 masses up to about 800 GeV and extending the exclusion region of supersymmetric parameter space covered by previous LHC searches.