Neutrino masses and mixing in $A_5$ with flavour antisymmetry

Abstract

We discuss consequences of assuming that the (Majorana) neutrino mass matrix $M_\nu$ and the charged lepton mass matrix $M_l$ satisfy, $S_\nu^T M_\nu S_\nu=-M_\nu~,T_l^\dagger M_lM_l^\dagger T_l=M_lM_l^\dagger$ with respect to some discrete groups $S_\nu,T_l$ contained in $A_5$. These assumptions lead to a neutrino mass spectrum with two degenerate and one massless neutrino and also constrain mixing among them. We derive possible mixing patterns following from the choices $S_\nu=Z_2,~ Z_2\times Z_2$ and $T_l=Z_2, ~ Z_2\times Z_2, ~Z_3, ~Z_5$ as subgroups of $A_5$. One predicts the maximal atmospheric neutrino mixing angle $\theta_{23}$ and $\mu$-$\tau$ reflection symmetry in large number of cases but it is also possible to obtain non-maximal values for $\theta_{23}$. Only the third column of the neutrino mixing matrix can be obtained at the leading order due to degeneracy in masses of two of the neutrinos. We take up a specific example within $A_5$ group and identify Higgs vacuum expectations values which realize the above assumptions. Non-leading terms present in this example are shown to lead to splitting among degenerate pairs and a consistent description of both neutrino masses and mixing angles.