Type II seesaw mechanism, deviations from bimaximal neutrino mixing, and leptogenesis

A possible interplay of both terms in the type II seesaw formula is illustrated by presenting a novel way to generate deviations from exact bimaximal neutrino mixing. In the type II seesaw mechanism with dominance of the noncanonical SU(2)L triplet term, the conventional seesaw term can naturally give a small contribution to the neutrino mass matrix. Nevertheless, if the triplet term corresponds to the bimaximal mixing scheme in the normal hierarchy, the small contribution of the conventional seesaw term naturally generates nonmaximal solar neutrino mixing. Atmospheric neutrino mixing is also reduced from maximal, corresponding to 1-sin22θ23 of order 0.01. Also, small but nonvanishing Ue3 of order 0.001 is obtained. It is also possible that the Δm2 responsible for solar neutrino oscillations is induced by the small conventional seesaw term. Larger deviations from zero Ue3 and from maximal atmospheric neutrino mixing are then expected. This scenario links the small ratio of the solar and atmospheric Δm2 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