Abstract :
The mass matrices of charged fermions have a simple structure if expressed in powers of the small parameter σ=(mc/mt)1/2. It is suggested that the mass matrix of the three heavy neutrinos occurring in grand unified theories can be expressed in terms of the same parameter. The requirement that these heavy neutrinos carry different U(1) generation quantum numbers gives rise to an almost unique form for this matrix. By applying the see-saw mechanism, the mass splitting of the two lightest neutrinos comes out to be tiny, favoring the vacuum oscillation solution for solar neutrinos. The mixing matrix is of the bimaximal type but contains also CP violating phases.
