HEAVY NEUTRINO THRESHOLD EFFECTS IN LOW-ENERGY PHENOMENOLOGY

Right-handed neutrinos with mass of O(10(12) - 10(13)) GeV are required to implement the seesaw mechanism and generate neutrino masses capable of playing a role in structure formation. Moreover models of fermion masses often relate the Yukawa couplings involving these neutrinos to the up-quark Yukawa couplings. Here we study the effects of such couplings on the radiative corrections to quark masses. We find that b - tau equality at M(GUT) may still give the correct m(b)/m(tau)-ratio at low energies, but only if there is large mu - tau mixing in the charged leptonic sector. We propose specific mass matrix ''textures'' dictated by a U(1) family symmetry whose structure preserves m(b) = m(tau) at M(GUT). In these schemes, due to the large nu(mu) - nu(tau) mixing, it is possible to give a simultaneous solution to the solar neutrino deficit and the atmospheric neutrino problem.