Neutrino Mixing: $A_4$ Variations

Abstract

In the context of the non-Abelian discrete symmetry $A_4$, the neutrino mass matrix has been studied extensively. A brief update is presented to focus on the conceptual shift from tribimaximal mixing ($\theta_{13} = 0$, $\theta_{23} = \pi/4$, $\tan^2 \theta_{12} = 1/2$) to $\underline{\rm cobimaximal}$ mixing ($\theta_{13} \neq 0$, $\theta_{23} = \pi/4$, $\delta_{CP} = \pm \pi/2$) which agrees well with present data. Three specific realistic examples are proposed, two with three and the third with just two parameters.