Neutrino masses and the Cabibbo expansion

We adopt a bottom-up approach to reconstruct the possible neutrino mass matrix patterns, in the hypothesis that $\lambda = \sin \theta_C$, where $\theta_C$ is the Cabibbo angle, is a useful expansion parameter in the neutrino sector as it is in the quark sector. A numerical analysis of these Cabibbo structures shows that an inverted hierarchy spectrum is statistically favored by the neutrino oscillation data. We then discuss how apparently conflicting mass and mixing data for quarks, charged leptons and neutrinos can fit in a unified perspective. We show that large mixing angles can appear through the seesaw mechanism, even if the neutrino Dirac mass matrix has hierarchical eigenvalues and small mixing angles. Finally, it is shown that the form of the heavy Majorana mass matrix may point towards theories with extra-dimensions.