Degenerate and Quasidegenerate Majorana Neutrinos

We study mixing and $\mathrm{CP}$ violation of three left-handed Majorana neutrinos in the limit of exactly degenerate masses, identify the weak-basis invariant relevant for $\mathrm{CP}$ violation, and show that the leptonic mixing matrix is parametrized only by two angles and one phase. After the lifting of the degeneracy, this parametrization accommodates the present data on atmospheric and solar neutrinos, as well as double $\beta$ decay. Some of the leptonic mixing Ansätze suggested in the literature correspond to special cases of this parametrization.