Abstract: The success of particle physics experiments, like those at the Large Hardon Collider (LHC) at CERN, relies on a worldwide interdisciplinary collaboration in a variety of different fields. This thesis contributes to two vital aspects in this area of research:in the first part of a search for heavy trilepton resonances decaying to a Z boson and an electron or muon is presented,
while the second part focusses on research and development of radiation tolerant silicon tracking detectors designed for the upgrade of the ATLAS detector for the future luminosity upgrade of the LHC.
The search for trilepton resonances is based on pp collision data taken at a centre-of-mass energy of 8 TeV by the ATLAS experiment at the LHC corresponding to an integrated luminosity of 20.3 /fb. To reconstruct the narrow resonance, events with at least three leptons (electrons or muons) with a high-transverse momentum are selected. Two of these leptons are required to be consistent with originating from a Z boson decay. Since no significant excess above Standard Model background predictions is observed, 95% confidence level upper limits on the production cross section of trilepton resonances beyond the Standard Model are derived. The results of this analysis are interpreted in the context of vector-like lepton and type-III seesaw models. For the vector-like lepton model, most heavy lepton mass values in the range 113–176 GeV are excluded. For the type-III seesaw model, most mass values in the range 100–474 GeV are excluded.

The second part of this thesis focusses on the development of radiation-tolerant silicon strip detectors for the luminosity upgrade of the ATLAS detector envisaged to commence in the year 2016. This thesis includes the results of several studies which contribute to multiple key aspects required for a successful upgrade of the silicon strip detector of the ATLAS Inner Tracker. Among these are the results of a beam test providing the first comparative results between planar n-in-p and 3D silicon strip sensors, where the electrodes are etched into the silicon substrate. The six sensors in this beam test were irradiated with fluences of up to 3x10e15 neq/cm^2. Detailed investigations on the collected charge showed that both sensor geometries collect enough charge to equip the silicon microstrip layers at the upgraded Inner Tracker. The beam test measurements also provided the first observation of charge multiplication in 3D detectors, an effect attributed to an avalanche of charges in detector regions with high electric fields. Finally a summary on the installation of a centralised test facility for large scale structures and irradiated modules is presented including a direct comparison of competing design approaches for the Inner Tracker, called stave and supermodule approach