Background suppression for a top quark mass measurement in the lepton+jets ttbar decay channel and Alignment of the ATLAS silicon detectors with cosmic rays

The first part of this thesis deals with the suppression of background sources contributing to the event sample used for the determination of the top quark mass. Jet merging scales of a kT jet algorithm are used as event shapes and a multivariate technique is applied to discriminate signal from W+jets physics background. Several kinematic variables are reviewed upon their capability to suppress wrong jet associations. The second part deals with track-based alignment of the silicon part of the ATLAS detector using cosmic rays. Additions to the LocalChi2 alignment algorithm are presented, which are supposed to improve alignment with cosmic ray data. The achieved alignment precision is evaluated and compared to previous results.