Tri-bimaximal Mixing from Cascades

We study fermion mass matrices of the cascade form which naturally lead to the tri-bimaximal generation mixing in the lepton sector. The cascade neutrino matrix predicts a parameter-independent relation among the observables; the neutrino mixing angles and mass squared differences. The relation makes interesting predictions that the atmospheric neutrino mixing angle is close to maximal and the other two angles have a sharp correlation independently of neutrino mass eigenvalues. We also discuss phenomenological aspects of the cascade-form matrices; flavor-violating rare decays of charged leptons, thermal leptogenesis, and leptonic CP violation. It is pointed out that the cascades induce correlated predictions among the decay ratios, the baryon asymmetry of the Universe and the CP violation in neutrino oscillations. Several dynamical realizations of the cascade form are presented in U(1) flavor theory and in higher dimensions.