Consequences of Approximate $S_3$ Symmetry of the Neutrino Mass Matrix

Understand

Assuming that the neutrino mass matrix is dominated by a term with the permutation symmetry $S_{\scriptscriptstyle 3}$ it is possible to explain neutrino data only if the masses are quasi-degenerate.

• A sub-dominant term with an approximate $\mu -\tau$ symmetry leads to an approximate tri-bimaximal form.
• Experimental consequences are discussed.