Towards a First Precision Measurement of the Mass of the W-Boson with the ATLAS Detector at $\sqrt{s}$ = 7 TeV

The W boson, being one of the exchange bosons of the weak interaction, is one of the fundamental particles in the Standard Model of particle physics. The value of its mass cannot be derived within the framework of the Standard Model; on the contrary, it is one of the free parameters which have to measured experimentally in order for the Standard Model to make predictions. Moreover, an experimental determination of the mass of the W boson allows for a test of the internal consistency of the Standard Model, which is of high importance, since the Standard Model is expected to be only an effective theory. This means, that beyond a certain threshold of precision, inconsistencies within the Standard Model are expected to appear, pointing at a new, underlying theory of broader scope and explanatory power. A precision measurement of the mass of the W boson is challenging both in terms of modeling the processes leading to its production, as well as the understanding of the particle detector with which its decay products are measured. The ATLAS detector used for this task is one of the most advanced particle detectors currently in use. It is located at the European Laboratory for Particle Physics (CERN) close to Geneva, Switzerland. Supplied by the Large Hadron Collider with proton beams of intensities and energies never reached before in a laboratory environment, ATLAS detected more than ten million events containing leptonic decays of W bosons at a center of mass energy of 7 TeV in the year 2011. This allows for a precise measurement of the mass of the W boson. Based on a significant sub-set of these events, corresponding to an integrated luminosity of 100 pb−1, a first measurement of MW based on muonic W boson decays observed at ATLAS, is presented in this thesis. The resulting value is MW = 80.380 ± 0.045(stat.) ± 0.021(sys.) GeV. Apart from the measurement itself, studies necessary for an accurate modeling of the detector are presented.