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Abstract

Phenomenology of sterile Neutrinos at different Scales Sterile neutrinos are one of the most simple extensions of the Standard Model (SM), but they can still have a rich variety of phenomenological implications on physical observables. As they interact with other SM particles through their mixing with the active neutrinos, they modify processes of the weak interaction. The signatures of the sterile neutrinos depend on their masses and their mixing to the active neutrinos. Two mass scales are picked up in this thesis: First, a very light sterile neutrino that oscillates with the active neutrinos is considered and it will be analyzed, whether the oscillations can be resonantly enhanced when interacting with a matter background. For a large active–sterile mixing the deficit in the atmospheric active neutrino flux after crossing the Earth could be up to 80%. In the second part indirect effects of heavy sterile neutrinos on electroweak observables are examined. When tree-level and one-loop effects are taken into account, a cancellation of the contributions of the sterile neutrinos can be present in some observables. Therefore, a larger active–sterile mixing is allowed that can then lead to sizeable influence on other processes, which can be detected in the near future.