Neutrino masses and mixing with seesaw mechanism and universal breaking ofextended democracy

In the framework of a minimal extension of the SM, where the only additiona l fields are three right-handed neutrinos, we suggest that the charged lept on, the Dirac neutrino and the right-handed Majorana neutrino mass matrices are all, to leading approximation, proportional to the democratic matrix. With the further assumption that the breaking of this extended democracy is universal for all leptonic mass matrices, a large mixing in the 2-3 sector can be obtained and is linked to the seesaw mechanism, together with the e xistence of a strong hierarchy in the masses of right-handed neutrinos. The structure of the resulting effective mass matrix of light neutrinos is sta ble against the RGE evolution, and a good fit to all solar and atmospheric neutrino data is obtained. (C) 2001 Elsevier Science B.V. All rights reserv ed.