Electron Identification with the TRT and the W Boson Mass Measurement at ATLAS

This thesis covers two physics analyses for the ATLAS experiment at CERN. The first one, describing the mapping and optimisation of the electron identification in the TRT subdetector system, introduces new methods utilising the, so far unused, dE/dx information, carried in the time-over-threshold variable, as well as possible ways of combining this with the current algorithms. Based only on real data, its results will also be of use as input for a tuning of the Monte Carlo simulation of the system. Introducing a probability based approach for particle identification the overall pion rejection factor reaches value from 25 to 70. Especially at low energies an improvement, compared to the current method, of up to 300% is reached. In the second analysis the systematic error in the W boson mass measurement, due to the theoretical description of real photons emitted by the final states in leptonic decays, is characterised. By observing the effect on parameters used for the mass measurement for successive theory improvements (orders in QED), the overall systematic error has been estimated to be about 8 MeV (5 MeV) for electrons (muons).