Nonunitary deviation from the tribimaximal lepton mixing and its implications on neutrino oscillations

Abstract

We propose a new pattern of the neutrino mixing matrix which can be parametrized as the product of an arbitrary Hermitian matrix and the well-known tribimaximal mixing matrix. In this scenario, nontrivial values of the smallest neutrino mixing angle ${\ensuremath{\theta}}_{13}$ and the $CP$-violating phases entirely arise from the nonunitary corrections. We present a complete set of series expansion formulas for neutrino oscillation probabilities both in vacuum and in matter of constant density. We do a numerical analysis to show the nonunitary effects on neutrino oscillations. The possibility of determining small nonunitary perturbations and $CP$-violating phases is discussed by measuring neutrino oscillation probabilities and constructing ``deformed unitarity triangles.'' Some brief comments on the nonunitary neutrino mixing matrix in the type-II seesaw models are also given.