Lepton mixing angle $θ_{13} = 0$ with a horizontal symmetry $D_4$

Abstract

We discuss a model for the lepton sector based on the seesaw mechanism and on a $D_4$ family symmetry. The model predicts the mixing angle $θ_{13}$ to vanish. The solar mixing angle $θ_{12}$ is free--it will in general be large if one does not invoke finetuning. The model has an enlarged scalar sector with three Higgs doublets, together with two real scalar gauge singlets $χ_i$ ($ i = 1, 2$) which have vacuum expectation values < χ_i >_0$ at the seesaw scale. The atmospheric mixing angle $θ_{23}$ is given by $\tan θ_{23} = <χ_2>_0$ /<χ_1>_0$, and it is maximal if the Lagrangian is $D_4$-invariant; but $D_4$ may be broken softly, by a term of dimension two in the scalar potential, and then < χ_2_0$ becomes different from < χ_1_0$. Thus, the strength of the soft $D_4$ breaking controls the deviation of $θ_{23}$ from $π/ 4$. The model predicts a normal neutrino mass spectrum ($m_3 > m_2 > m_1$) and allows successful leptogenesis if $m_1 \sim 4 \times 10^{-3} \mathrm{eV}$; these properties of the model are independent of the presence and strength of the soft $D_4$ breaking.