Nonzero θ13 in SO (3) →A4 lepton models

The simplest neutrino mass models based on A4 symmetry predict θ13=0 at tree level, a value that contradicts recent data. We study models that arise from the spontaneous breaking of an SO(3) symmetry to its A4 subgroup, and find that such models can naturally accommodate a nonzero θ13 at tree level. Standard Model charged leptons mix with additional heavy ones to generate a θ13 that scales with the ratio of the A4-breaking to SO(3)-breaking scales. A suitable choice of energy scales hence allows one to reproduce the correct lepton mixing angles. We also consider an alternative approach where we modify the alignment of flavons associated with the charged lepton masses, and find that the effects on θ13 are enhanced by a factor that scales as mτ/mμ.