On the hierarchy of neutrino masses

Abstract

We present a model of neutrino masses combining the seesaw mechanism and strong Dirac mass hierarchy and at the same time exhibiting a significantly reduced hierarchy at the level of active neutrino masses. The heavy Majorana masses are assumed to be degenerate. The suppression of the hierarchy is due to a symmetric and unitary operator R whose role is discussed. The model gives realistic mixing and mass spectrum. The mixing of atmospheric neutrinos is attributed to the charged lepton sector whereas the mixing of solar neutrinos is due to the neutrino sector. Small U_e3 is a consequence of the model. The masses of the active neutrinos are given by $μ_3\approx\sqrt{Δm_{@}^2}$ and $μ_1/μ_2\approx \tan^2θ_\odot$.