Neutrino mass model with S3 symmetry and seesaw interplay

Abstract

We develop a see-saw model for neutrino masses and mixing with an S3\times Z3 symmetry. It involves an interplay of Type-I and Type-II see-saw contributions of which the former is subdominant. The S3 \times Z3 quantum numbers of the fermion and scalar fields are chosen such that the Type-II see-saw generates a mass matrix which incorporates the atmospheric mass splitting and sets \theta_{23} = \pi/4. The solar splitting and \theta_{13} are absent, while the third mixing angle can achieve any value, \theta_{12}^0. Specific choices of \theta_{12}^0 are of interest, e.g., 35.3^\circ (tribimaximal), 45.0^\circ (bimaximal), 31.7^\circ (golden ratio), and 0^\circ (no solar mixing). The role of the Type-I see-saw is to nudge all the above into the range indicated by the data. The model results in novel interrelationships between these quantities due to their common origin, making it readily falsifiable. For example, normal (inverted) ordering is associated with \theta_{23} in the first (second) octant. CP-violation is controlled by phases in the right-handed neutrino Majorana mass matrix, M_{\nu R}. In their absence, only normal ordering is admissible. When M_{\nu R} is complex the Dirac CP-phase, \delta, can be large, i.e., \sim \pm \pi/2, and inverted ordering is also allowed. The preliminary results from T2K and NOVA which favour normal ordering and \delta \sim -\pi/2 are indicative, in this model, of a lightest neutrino mass of 0.05 eV or more.