Abstract: Supersymmetry poses one of the best motivated theories for particle physics beyond the Standard Model. It can provide a dark matter candidate and enable grand unification. The supersymmetric partner of the top quark, the top squark, can provide a cancellation of diverging radiative corrections to the Higgs-boson mass and thereby avoid the hierarchy problem of the Standard Model. In this thesis, a search for top squarks in final states with jets and missing transverse momentum is presented. Proton-proton collisions at a centre-of-mass energy of √s = 13 TeV, produced by the Large Hadron Collider, were recorded by the ATLAS detector. The analysed dataset corresponds to an integrated luminosity of 36.1 fb-1. No significant excess above the Standard Model background was observed. The analysis results are interpreted as exclusion limits on the top-squark mass and the neutralino mass in various scenarios that address specific aspects of the supersymmetry

A search for the pair production of top squarks, each with R -parity-violating decays into two Standard Model quarks, is performed using 17.4 fb -1 of √s=8 TeV proton-proton collision data recorded by the ATLAS experiment at the LHC. Each top squark is assumed to decay to a b- and an s-quark, leading to four quarks in the final state. Background discrimination is achieved with the use of b-tagging and selections on the mass and substructure of large-radius jets, providing sensitivity to top squark masses as low as 100 GeV. Finally, no evidence of an excess beyond the Standard Model background prediction is observed and top squarks decaying to $\overline{b}$$\overline{s}$ are excluded for top squark masses in the range 100 ≤ m $\overline{t}$ ≤ 315 GeV at 95% confidence level.

The sensitivity of this search to models with a top squark decaying to jets and a neutralino through a on-shell top quark extends to 775 GeV in top squark mass and to 650 GeV in models with a top squark decaying through an intermediate chargino.

Zusammenfassung: The thesis is focused on supersymmetry searches for the top-squark pair production with one lepton, jets and missing transverse momenta for compressed scenarios. Compressed scenario refers to signal models, where mass difference between some supersymmetric particles is small.All considered decay channels are assumed to be with branching ration of 100%. The results are based on data corresponding to an integrated luminosity of 20/fb recorded at center-of-mass energy of 8 TeV with ATLAS detector.The first analysis is targeting the decay channel where a top squark decays to a bottom quark and a chargino, with subsequent decay of the chargino to a W-boson and a neutralino. Targeted compressed scenarios are signal models with the mass difference between the top squark and the chargino is below 20 GeV. The analysis allows to exclude the top squark masses from 125 GeV to 350 GeV at 95% CL.The second analysis is targeting the decay channel where a top squark decays to a top and a neutralino via on- or off-shell decay. Targeted compressed scenarios are signal models with small difference between the mass of the top squark and the sum of masses for the top and the neutralino. Excluded the region for top squark masses from 200 GeV to 300 GeV at 95% CL

This thesis focuses on searches for squarks with the ATLAS detector in "compressed" scenarios where the scalar top is very close in mass to the lightest supersymmetric particle. These models are theoretically appealing because the presence of a quasi-degenerate scalar top enhances the self-annihilation cross-section of the lightest supersymmetric particle, acting therefore as a regulator of the dark matter relic density. Two main analyses are presented: the first is a search for scalar tops decaying to charm quarks. The identification of jets originating from the charm quark is very challenging due to its short lifetime. The calibration of tools for charm-tagging has paved the way to measuring the decay of the Higgs boson to pairs of charm quarks. The second analysis presented is the development of a novel technique for reconstructing low momentum b-hadrons. This tool has enabled the ATLAS collaboration to explore topologies that were previously inaccessible.