Heavy neutrino threshold effects in low energy phenomenology

Right handed neutrinos with mass of ${\cal O} \; (10^{12}-10^{13})$ GeV are required to implement the see-saw 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.Comment: 22 pages, latex file, including 8 latex figures. Revised version (typos corrected, references added), to be published in Nucl. Phys.

Neutrino Masses from Gauge Symmetries

A very simple extension of the standard model to include an Abelian family symmetry is able to describe the hierarchy of quark and lepton masses and their mixing angles together with the unification of gauge couplings. We consider the implications of this model for neutrino masses and mixing angles and show that they are determined up to a discrete ambiguity corresponding to the representation content of the Higgs sector responsible for the Majorana mass matrix.Comment: (CERN-TH.7412/94, Latex file 16 p, no figures