Searches for Supersymmetric Partners of Top and Bottom Quarks in Proton-Proton Collisions at $\sqrt{s}=7$ TeV

Supersymmetry (SUSY) is one of the most compelling theories to describe physics beyond the Standard Model (SM). In the framework of R-parity conserving minimal extension of the SM, supersymmetric particles are produced in pairs and the lightest supersymmetric particle (LSP) is stable. Due to the mixing of the right-handed and left-handed supersymmetric quarks, and the strong Yukawa coupling, the stops and the sbottoms (supersymmetric partners of top and bottom quarks) are lighter than the other supersymmetric quarks. Moreover, considering the"naturalness" of the Higgs mass, the stop mass is expected to be order of several hundreds GeV even if other colored supersymmetric particles are much heavier. If kinematically allowed, they could be produced via direct pair production or from the decay of gluinos (supersymmetric partner of gluons). This result in complex final states consisting of the missing transverse momentum (from the LSP) and bottom-jets (from the decay of stops or sbottoms). In this thesis, searches for the stop and the sbottom will be presented focusing on the most probable models using the proton-proton collisions at $\sqrt{s}=7$~TeV recorded by the ATLAS detector. The data with the integrated luminosity of about 2 fb$^{−1}$ are used. Searches in four kinds of topologies have been performed; no-lepton multijet selection, one-lepton multi-jet selection, no-lepton di-jet selection, and two-lepton multi-jet selection optimized for each targeted SUSY model. None of them shows the significant excess from the SM predictions. These results are used to set limits on the several SUSY models. They are the most stringent limits obtained by the collider experiments and lead to the strong limits on the naturalness of the Higgs mass in the supersymmetric models.