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

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.

An inclusive search is performed for supersymmetry in final states containing jets and an apparent imbalance in transverse momentum, ptvecmiss, due to the production of unobserved weakly interacting particles in pp collisions at a centre-of-mass energy of 8 TeV. The data, recorded with the CMS detector at the CERN LHC, correspond to an integrated luminosity of 18.5 inverse femtobarns. The dimensionless kinematic variable alphaT is used to discriminate between events with genuine ptvecmiss associated with unobserved particles and spurious values of ptvecmiss arising from jet energy mismeasurements. No excess of event yields above the expected standard model backgrounds is observed. The results are interpreted in terms of constraints on the parameter space of several simplified models of supersymmetry that assume the pair production of top squarks. The search provides sensitivity to a broad range of top squark decay modes, including the two-body decay top squark to c chi0, where c is a charm quark and chi0 is the lightest neutralino, as well as the four-body decay top squark to b f bar-f' chi0, where b is a bottom quark and f and bar-f' are fermions produced in the decay of an intermediate off-shell W boson. These modes dominate in scenarios in which the top squark and lightest neutralino are nearly degenerate in mass. For these modes, top squarks with masses as large as 260 and 230 GeV are excluded, respectively, for the two- and four-body decays.