Fermion Mass Hierarchies and Flavour Mixing from a Minimal Discrete Symmetry
Abstract
Description
We construct a simple model of fermion masses based on a spontaneously broken S3 X Z3 flavour group. At the leading order, in the neutrino sector S3 is broken down to a ν_μ-ν_τparity subgroup that enforces a maximal atmospheric mixing angle and a vanishing θ_{13}. In the charged lepton sector the ν_μ-ν_τparity is maximally broken and the resulting mass matrix is nearly diagonal. The charged lepton mass hierarchy is automatically reproduced by the S3 symmetry breaking parameter alone. A careful analysis shows that, after the inclusion of all relevant subleading effects, the model predicts θ_{23}=π/4+O(λ_c^2) and θ_{13}=O(λ_c^2), λ_c denoting the Cabibbo angle. A simple extension to the quark sector is also illustrated, where the mass spectrum and the mixing angles are naturally reproduced, with the exception of the mixing angle between the first two generations, that requires a small accidental enhancement.
18 pages
18 pages