The minimal extension of the SM and the neutrino oscillation data

Abstract

We study the simplest Standard Model extension with only one extra right-handed neutrino. In this case there are two massless m1,2 and two massive m3,4 neutrinos, and in principle both solar and atmospheric anomalies can be described in two different scenarios, m3 ≪ m4 (scheme I) and m3 ≃ m4 (scheme II). However, neither bi-maximal mixing nor the dark matter problem are explained in this minimal extension. Only scheme II can accommodate simultaneously maximal mixing for atmospheric neutrinos and the small mixing angle MSW solution for the solar anomaly. This scenario can be tested in the BOREXINO experiment.