Recent Neutrino Data and Type III Seesaw with Discrete Symmetry

TL;DR

This paper explores how a Type III seesaw model with discrete $A_4 imes Z_2$ symmetry can naturally produce neutrino mixing angles consistent with recent experimental data, including a sizable $ heta_{13}$ and maximal CP violation.

Contribution

It introduces effective dimension-5 operators in the Type III seesaw model with $A_4 imes Z_2$ symmetry, explaining deviations from tri-bimaximal mixing and accommodating recent neutrino data.

Findings

Deviations from TBM mixing explain large $ heta_{13}$.

All neutrino data, including $ heta_{13} eq 0$, can be fitted.

Leptonic CP violation is predicted to be maximal.

Abstract

In light of the recent neutrino experiment results from Daya Bay and RENO Collaborations, we study phenomenology of neutrino mixing angles in the Type III seesaw model with an discrete $A_4 \times Z_2$ symmetry, whose spontaneously breaking scale is much higher than the electroweak scale. At tree level, the tri-bimaximal (TBM) form of the lepton mixing matrix can be obtained from leptonic Yukawa interactions in a natural way. We introduce all possible effective dimension-5 operators, invariant under the Standard Model gauge group and $A_4 \times Z_2$, and explicitly show that they induce a deviation of the lepton mixing from the TBM mixing matrix, which can explain a large mixing angle $\theta_{13}$ together with small deviations of the solar and atmospheric mixing angles from the TBM. Two possible scenarios are investigated, by taking into account either negligible or sizable contributions from the light charged lepton sector to the lepton mixing matrix. Especially it is found in the latter scenario that all the neutrino experimental data, including the recent best-fit value of $\theta_{13} = 8.68^{\circ}$, can be accommodated. The leptonic CP violation characterized by the Jarlskog invariant $J_{CP}$ has a non-vanishing value, indicating a signal of maximal CP violation.