Revisiting pseudo-Dirac neutrinos

We study the pseudo-Dirac mixing of left- and right-handed neutrinos in the case where the Majorana masses ML and MR are small when compared with the Dirac mass, MD. The light Majorana masses could be generated by a non-renormalizable operator reflecting effects of new physics at some high energy scale. In this context, we obtain a simple model independent closed bound for MD. A phenomenologically consistent scenario is achieved with ML,MR≃10−7 eV and MD≃10−5–10−4 eV. This precludes the possibility of positive mass searches in the planned future experiments like GENIUS or in tritium decay experiments. If on the other hand, GENIUS does observe a positive signal for a Majorana mass ⩾10−3 eV, then with very little fine tuning of neutrino parameters, the scale of new physics could be in the TeV range, but pseudo-Dirac scenario in that case is excluded. We briefly discuss the constraints from cosmology when a fraction of the dark matter is composed of nearly degenerate neutrinos.