Quasidegeneracy of Majorana neutrinos and the origin of large leptonic mixing

Abstract

We propose that the observed large leptonic mixing may just reflect a quasidegeneracy of three Majorana neutrinos. The limit of exact degeneracy of Majorana neutrinos is not trivial, as leptonic mixing and even $CP$ violation may occur. We conjecture that the smallness of $|{U}_{13}|$, when compared to the other elements of ${U}_{PMNS}$, may be related to the fact that, in the limit of exact mass degeneracy, the leptonic mixing matrix necessarily has a vanishing element. We show that the lifting of the mass degeneracy can lead to the measured value of $|{U}_{13}|$ while at the same time accommodating the observed solar and atmospheric mixing angles. In the scenario we consider for the breaking of the mass degeneracy, there is only one $CP$ violating phase, already present in the limit of exact degeneracy, which upon the lifting of the degeneracy generates both Majorana and Dirac-type $CP$ violation in the leptonic sector. We analyze some of the correlations among physical observables and point out that, in most of the cases considered, the implied strength of leptonic Dirac-type $CP$ violation is large enough to be detected in the next round of experiments.