Seesaw neutrino masses and mixing with extended democracy ∗

In the context of a minimal extension of the Standard Model with three extra heavy right-handed neutrinos, we propose a model for neutrino masses and mixing based on the hipothesis of a complete alignment of the lepton mass matrices in flavour space. Considering a uniform quasi-democratic structure for these matrices, we show that, in the presence of a highly hierarchical right-handed neutrino mass spectrum, the effective neutrino mass matrix, obtained through the seesaw mechanism, can reproduce all the solutions of the solar neutrino problem. All the enthusiasm around the Super-Kamiokande evidence for atmospheric neutrino oscillations [1] has lead to a great activity in the search for models [2] which can reproduce the solar and atmospheric neu-trino data [3]. In Ref. [4] we have proposed a simple model for lepton masses and mixing which fulfils these requirements. As a starting point, we consider the existence of three heavy right-handed neutrinos which, together with the seesaw mechanism, will generate small neutrino masses. Furthermore, we assume that the charged lepton, Dirac and right-handed neutrino mass matrices (M l , M D and M R respectively) are aligned in flavour space in a such a way that they are all " democratic " , i.e., they are proportional to a matrix whose elements are, in leading approximation , all equal to one. We will refer to this assumption as " extended democracy ". In the context of the Standard Model (SM) with three additional heavy right-handed neutrinos, the lepton mass Lagrangian has the form: