Studien für eine initiale Messung der Topquarkmasse mit ATLAS in dem Lepton+Jets $t\overline{t}$ Zerfallskanal und Alignment der Pixel und SCT Teildetektoren

The main topic of this thesis is a commissioning style top quark mass analysis using the ATLAS experiment at CERN. The analysis focusses on top quark pair decays in the lepton+jets decay channel. Only kinematic selection cuts and no b-tagging information is used for the event selection. This analysis is suitable for the commissioning phase of the ATLAS detector, with not yet final calibration and an incomplete understanding of the detector performance. Different methods for the reconstruction of the hadronic side of the top quark pair decays are studied and the effect of imposing the known W boson mass as constraint on the reconstruction is investigated. The analysis is modified in several ways to estimate the influence of systematic effects. The influence of the jet selection kinematics on the reconstructed top quark mass is studied, as well as the underlying jet algorithm definition and variations of the jet energy scale. The different jet algorithms under consideration are cone type and kT type algorithms with a set of different steering parameters. The cone jet algorithm with the steering parameter Rcone = 0.4 and the inclusive kT algorithm with the steering parameter R = 0.4 give the best performance. Although the top quark mass analysis is suited for the commissioning phase of the ATLAS detector, the performance of the ATLAS detector still affects the quality of the event selection. A high quality alignment of the ATLAS Inner Detector is required for an efficient lepton reconstruction and consequently for an optimal event selection. In the second part of this thesis the Local &chi² alignment approach is presented. The approach is used for the alignment of the Pixel and SCT subdetectors. The approach is first validated on a small detector setup with data that was collected in a combined testbeam run in 2004. Finally the Local &chi² alignment approach is used for the alignment of the whole Pixel and SCT subdetectors with cosmic ray data collected in fall 2008.