Search for the scalar top quark in the final state with jets and missing transverse momentum with the ATLAS detector at the LHC

Supersymmetry theory (SUSY) can explain unsolved problems in the Standard Model of particle physics, such as, dark matter observed in the universe, the unification of coupling constants of the electromagnetic, weak, and strong interactions and hierarchy problem of the mass of the Higgs boson. The 125 GeV mass of the Higgs boson, which was observed at the LHC, is too light for the naturalness in the Standard Model unless new mechanism to stabilize the Higgs mass is introduced. The superpartners of the top quarks, which are called scalar top quarks, can provide a key solution for the observed Higgs mass according to the prediction of natural SUSY, but they have not been observed so far. The search for the scalar top quarks has been performed at the ATLAS. The scalar top quark decays into a top quark and a neutralino if it is kinematically allowed. In the hadronic final states of top quarks, the method of the top reconstruction is one of the principal features in the analysis for the production of boosted tops due to the large mass difference of a scalar top quark and a neutralino. The top reconstruction is performed in two steps: to reconstruct jets of top-quark decay products and to identify top-quarks, called top-quark tagging, using the reconstructed jets. In this thesis, a new method, DNN top tagger, is introduced for both the jet reconstruction and top-quark tagging, optimized for the high-mass scalar top quarks. The new method shows high top-tagging efficiencies and good background rejections, which help significantly improve 27% of the significance for signal models where the final states top-quarks are highly boosted. The search uses at 139 fb$^{-1}$ of $\sqrt{s}$ = 13 TeV data of proton-proton collisions at Large Hadron Collider recorded by the ATLAS. No significant excess over the Standard Model predictions is observed. The exclusion of the mass of the scalar top quark is extended up to 1.4 TeV for the mass of the neutralino below 200 GeV at 95% confidence level.