Alignment of the ATLAS Silicon Tracker and measurement of the top quark mass

The Large Hadron Collider (LHC) era started with its first proton-proton collisions produced in November 2009 at the CERN laboratory. In the coming decade, the high energy physics program will be dominated by the LHC and its experiments. Discoveries such as the Higgs or supersymmetric particles are some of the dreams that hopefully the LHC will bring us. This thesis is framed within the ATLAS experiment, which is one of the four large detectors located at the LHC. The work presented in this thesis is divided in two parts. The first part is dedicated to the alignment of the ATLAS silicon tracking detector using the GlobalChi2 algorithm, which is the actual baseline algorithm. It covers performance studies with Monte Carlo samples with a realistic detector description, with real cosmic rays as well as with first LHC collisions at 900 GeV. The main achievement was the production of a set of alignment constants for the real ATLAS detector. Those constants were obtained from the alignment of real cosmic ray data, and they were used to successfully reconstruct the first ever LHC collisions. The second part is devoted to perform preliminary studies of the top quark mass measurement based on Monte Carlo samples at 14 TeV, and considering 1 fb−1 of integrated luminosity. In particular, the semileptonic decay channel of the ttbar pair is used to perform a kinematic fit based on the GlobalChi2 formalism. Actually, the main goal of this part is the introduction this method, though the achieved performance is evaluated, discussing its advantages, limitations and, of course, results.