(Quasi-)Degeneration, Quantum Corrections and Neutrino Mixing

In the Standard Model of elementary particle physics, neutrinos play a special role among the fermions: neutrinos are exactly massless and neutrinos only appear as left-handed fermions. On the one hand, oscillation experiments suggest non-vanishing neutrino masses [1, 2] which at least have to be in the sub-eV regime. On the other hand, masses of righthanded neutrinos are unconstrained by the symmetries of the Standard Model and can in principle be much larger than the electroweak scale [3]. a. Neutrino masses and quasi-degeneration The absolute mass scale of the light neutrinos, however, is still unknown. Global fits on neutrino data give very precise measurements of the mass squared differences ∆m i j = m i −m j [4] (similar results are obtained by other groups [5, 6]):