A frequentist analysis of the n=3 type-I seesaw model

The extension of the Standard Model by right-handed neutrinos can not only explain the active neutrino masses via the seesaw mechanism, but is also able to answer a number of open questions in cosmology. In particular, masses below the TeV scale are of interest as they can lead to a plethora of signatures in experimental searches. In the thesis, the first full frequentist analysis of this extension of the Standard Model by three right-handed neutrinos, with masses between 60 MeV and 500 GeV, using the Global and Modular BSM Inference Tool (GAMBIT) is presented. The analysis is based on the Casas-Ibarra parameterization and includes a large range of experimental constraints: active neutrino mixing, indirect constraints from, e.g., electroweak precision observables and lepton universality, and numerous direct searches for right-handed neutrinos. To study their overall effect, combined profile likelihood results for the most relevant (phenomenologically) parameter projections are derived. Furthermore, the role of (marginally) statistically preferred regions in the parameter space is discussed. Finally, the flavour mixing pattern of the three right-handed neutrinos for different values of the lightest neutrino mass are explored. The results comprise the most comprehensive assessment of the model with three right-handed neutrinos below the TeV scale so far, and provide a robust ground for exploring the impact of future constraints or detections.