Non unitarity and radiative corrections in type I see-saw

Since the discovery of neutrino oscillations it is beyond any doubt that the standard model has to be extended to accommodate neutrino mass terms. One popular option is to introduce singlet neutral fermions, which can have Majorana mass and allow furthermore a Dirac mass term with the active neutrinos. We study a model in which the neutral fermions are of the order of TeV scale and can considerably mix to the active flavors. This has on the one hand an impact on the active neutrino-mixing-matrix, which is now non-unitary. Furthermore, the heavy states can propagate in loops and thus contribute to gauge boson self-energies, what has influence on the electroweak precision observables. We find a regime in which the tree level effects of non-unitarity are in an interplay with the radiative effects which leads to interesting phenomenological results.