Search for Direct Top Squark Pair Production with the ATLAS Experiment and Studies of the Primary Vertex Reconstruction Performance

Abstract: The ATLAS detector is one of the two largest experiments installed at the Large Hadron Collider at CERN, the European Organization for Nuclear Research. During the first run, the ATLAS detector recorded data at centre of mass energies of 7 TeV and 8 TeV, enabling many precision measurements and new physics searches. One important task in ATLAS is measuring the primary vertex, the interaction point of the hardest proton-proton collision in an event. In this thesis, a study of the primary vertex reconstruction performance in data and simulated events using $t\bar{t}$ and ${Z}$ events is presented. Within the statistics available, the performance in data and simulated events is found to be compatible. Motivated by the limitations of the Standard Model of particle physics, searches for supersymmetric particles are performed with the ATLAS experiment. No signal has been observed so far, and the results are used to set exclusion limits on the masses of the supersymmetric particles. As the exclusion limits are derived from analyses which each target only a single decay mode of a supersymmetric particle, the analyses might have lower sensitivity to more complex decay scenarios. In this thesis the sensitivity of one of the ATLAS searches for direct top squark pair production to models with more complex decay modes is investigated. The study concludes that the sensitivity to models where the top squark can decay via heavier charginos and neutralinos is lower than the sensitivity to models where only decays to the lightest chargino or neutralino are present.