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Abstract

In this thesis, the search for direct pair production of the supersymmetric top quark partner with the HEPTopTagger algorithm is presented. An integrated luminosity of 20.3 fb^-1 of proton-proton collision data at sqrt s = 8 TeV collected by the ATLAS detector at the Large Hadron Collider is used. The top squark, ~t1, is assumed to decay into a top quark and a neutralino, ~chi01, with BR(~t1 -> t ~chi01 ) = 100%. The analysis targets the fully hadronic final state: the HEPTopTagger is employed to reconstruct the hadronically decaying top quarks with pT > 200 GeV; the MT2 variable together with the missing transverse momentum is used to measure the momentum imbalance of the undetected neutralinos. In order to reliably apply the HEPTopTagger to search for the top squark, its performance is studied in a sample enriched in top quark pairs in the lepton+jets channel. The effciency of identifying moderate to high pT top quarks is measured to vary from 10% to 45%. A novel method is developed to estimate in-situ the uncertainty on the energy scale of the subjets, representing the top quark decay products. The simulation has been found to be a reliable tool to predict the Standard Model background and the HEPTopTagger can be safely employed in the search for top partners. In the search for direct top squark pair production, no significant excess over the Standard Model background expectation is found, and exclusion limits are set as a function of the top squark and neutralino masses. Top squark masses between 250 - 720 GeV are excluded for neutralino masses of a few tens of GeV.