Type II See-Saw Mechanism, Deviations from Bimaximal Neutrino Mixing and Leptogenesis

A possible interplay of both terms in the type II see-saw formula is illustrated by presenting a novel way to generate deviations from exact bimaximal neutrino mixing. In type II see-saw mechanism with dominance of the non-canonical SU(2)_L triplet term, the conventional see-saw term can give a small contribution to the neutrino mass matrix. If the triplet term corresponds to the bimaximal mixing scheme in the normal hierarchy, the small contribution of the conventional see-saw term naturally generates non-maximal solar neutrino mixing. Atmospheric neutrino mixing is also reduced from maximal, corresponding to 1 - \sin^2 2 \theta_{23} of order 0.01. Also, small but non-vanishing U_{e3} of order 0.001 is obtained. It is also possible that the \Delta m^2 responsible for solar neutrino oscillations is induced by the small conventional see-saw term. Larger deviations from zero U_{e3} and from maximal atmospheric neutrino mixing are then expected. This scenario links the small ratio of the solar and atmospheric \Delta m^2 with the deviation from maximal solar neutrino mixing. We comment on leptogenesis in this scenario and compare the contributions to the decay asymmetry of the heavy Majorana neutrinos as induced by themselves and by the triplet.