Normal and Special Models of Neutrino Masses and Mixings

Abstract

One can make a distinction between "normal" and "special" models. For normal models $θ_{23}$ is not too close to maximal and $θ_{13}$ is not too small, typically a small power of the self-suggesting order parameter $\sqrt{r}$, with $r=Δm_{sol}^2/Δm_{atm}^2 \sim 1/35$. Special models are those where some symmetry or dynamical feature assures in a natural way the near vanishing of $θ_{13}$ and/or of $θ_{23}- π/4$. Normal models are conceptually more economical and much simpler to construct. Here we focus on special models, in particular a recent one based on A4 discrete symmetry and extra dimensions that leads in a natural way to a Harrison-Perkins-Scott mixing matrix.