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.

This book reports a search for theoretically natural supersymmetry (SUSY) at the Large Hadron Collider (LHC). The data collected with the ATLAS detector in 2012 corresponding to 20 /fb of an integrated luminosity have been analyzed for stop pair production in proton–proton collisions at a center-of-mass energy of 8 TeV at the Large Hadron Collider (LHC) in the scenario of the higgsino-like neutralino. The author focuses on stop decaying into a bottom quark and chargino. In the scenario of the higgsino-like neutralino, the mass difference between charginos and neutralinos (Δm) is expected to be small, and observable final-state particles are likely to have low-momentum (soft). The author develops a dedicated analysis with a soft lepton as a probe of particles from chargino decay, which suppresses the large amount of backgrounds. As a result of the analysis, no significant SUSY signal is observed. The 95% confidence-level exclusion limits are set to masses of stop and neutralino assuming Δm = 20 GeV. The region with ΔM (the mass difference between stop and neutralino) 70 GeV is excluded for the first time at stop mass of less than 210 GeV. The author also excludes the signals with ΔM 120 GeV up to 600 GeV of stop mass with neutralino mass of less than 280 GeV. The author clearly shows very few remaining parameter spaces for light stop (e.g., topology of stop decay is extremely similar to the SM top quark) by combining his results and previous ATLAS analyses. His results provide a strong constraint to searches for new physics in the future.

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

Astrophysical observations implying the existence of Dark Matter and Dark Energy, which are not described by the Standard Model (SM) of particle physics, have led to extensions of the SM predicting new particles that could be directly produced at the Large Hadron Collider (LHC) at CERN. Based on 2015 and 2016 ATLAS proton-proton collision data, this thesis presents searches for the supersymmetric partner of the top quark, for Dark Matter, and for DarkEnergy, in signatures with jets and missing transverse energy. Muon detection is key to some of the most important LHC physics results, including the discovery of the Higgs boson and the measurement of its properties. The efficiency with which muons can be detected with the ATLAS detector is measured using Z boson decays. The performance of high-precision Monitored Drift Tube muon chambers under background rates similar to the ones expected for the High Luminosity-LHC is studied.