Seesaw mechanism and quark-lepton symmetry

A previous short analysis of the seesaw mechanism, based on quark-lepton sy mmetry, experimental data and hierarchical neutrino spectrum, is enlarged t o include small but not zero U-e3 and inverted mass hierarchy. The structur e of the heavy neutrino mass matrix obtained in several cases is discussed. We find two leading forms for this matrix. One is diagonal and stands at t he unification scale or above. The other is off-diagonal and stands at the intermediate scale. (C) 2000 Published by Elsevier Science B.V. All rights reserved.