First measurement of the branching ratio fraction BR(W $\to \tau \nu \to \mu \nu \nu$)/BR(W $\to \mu \nu$) in $\sqrt{s}$ = 7 TeV proton-proton collisions with the ATLAS detector and realization of a production facility for large scale micromegas drift boards

The CERN research complex at Geneva is the worldwide largest high energy particle lab hosting with the Large Hadron Collider (LHC) the most powerful accelerator. A major step in our understanding of the Standard Model, as the base of particle physics, was achieved by the LHC experiments ATLAS and CMS with the discovery of the Higgs boson in 2012. The search for new particles and in general new physics is the driving motivation for high energy particle physics experiments. New particles can be either detected at the so called high energy frontier, by their direct production in particle collisions, or via hints in precision measurement. As new particles or couplings occur as contributions to known Standard Model processes, for example the branching ratio of a decay may be altered. This thesis presents the first branching ratio fraction measurement of the W boson decay BR(W→τν→μνν)/BR(W→μν) at the LHC. This process is of interest, as hints for a violation of the lepton universality were found in decays to the third generation. The measurement was implemented as a template fit of the Monte Carlo simulated detector signals for the relevant processes to the measured data and the expected sensitivity has been estimated. This analysis was based on the well understood data and simulation used for the measurement of the W mass at $\sqrt{s}$ = 7 TeV. Within the kinematic range of that measurement, the branching ratio BR(W→τν→μνν) = 0.228${}_{-0.023\left(0.010\right)}^{+0.024\left(0.010\right)}$ with a sensitivity of13 % was found,with a perspective to a 4 % sensitivity for relaxed kinematic cuts, which becomes compatible for the inclusion in the world average. As the LHC is under constant development and improvement and the perspective for the high luminosity LHC with larger interaction rates was set, the detectors need to improved to keep up with the event reconstruction at higher occupancies to allow for future precision measurements. In the perspective of the ATLAS New Small Wheel upgrade for a improved event triggering during the data taking at high multiplicities, a production facility for high planarity large scale gas detector components was setup in the scope of this thesis. Besides the mechanical infrastructure for the component production, the quality control methods have been developed and characterized, the starting production was lead and the whole series production was accompanied. The quality control results show, that not only the production could be finished by far as the first production site in the collaboration, but also 90 % of the components fulfill the quality requirement of the ATLAS collaboration and the remaining parts could be accepted for the assembly after a small reworking. In this work the full chain of the detector construction, from the tooling development over the production principle to the quality control methods, documentation and results is presented.