A4-based seesaw model for realistic neutrino masses and mixing

We present an A4-based model where neutrino masses arise from a combination of seesaw mechanisms. The model is motivated by several small mixing and mass parameters indicated by the data. These are theta(13), the solar mass splitting, and the small deviation of theta(23) from maximal mixing (= pi/4). We take the above as indications that at some level the small quantities are well approximated by zero. In particular, the mixing angles to zeroth order should be either 0 or pi/4. Accordingly, in this model the type-II seesaw dominates and generates the larger atmospheric mass splitting and sets theta(23) = pi/4. The other mixing angles are vanishing as is the solar splitting. We show how the A4 assignment for the lepton doublets leads to this form. We also specify the A4 properties of the right-handed neutrinos which result in a smaller type-I seesaw contribution that acts as a perturbation and shifts the angles theta(12) and theta(13) into the correct range and the desired value of Delta m(solar)(2) is produced. The A4 symmetry results in relationships between these quantities as well as with a small deviation of theta(23) from pi/4. If the right-handed neutrino mass matrix M-R is chosen real then there is no leptonic CP violation and only normal ordering is admissible. If M-R is complex then inverted ordering is also allowed with the proviso that the CP phase delta is large, i.e., similar to pi/2 or -pi/2. The preliminary results from NO nu A favoring normal ordering and d near -pi/2 imply quasidegenerate neutrino masses in this model.