Consequences of a Z_2 Symmetry for Neutrino Oscillation

Abstract

A $Z_{2L}\times Z_{2R}$ generation symmetry in the neutrino sector predicts the atmospheric neutrino mixing to be maximal, and the MNS matrix element $U_{e3}$ to be zero, consistent with observations. Solar neutrino mixing may be maximal but is not required by the symmetry. Neutrino masses of the first two generations are predicted to vanish, providing a first approximation to the oscillation data. The consequence of a smaller $Z_2$ symmetry is also discussed. In that case, deviation from the $Z_{2L}\times Z_{2R}$ result is of the order of the neutrino mass ratio between the first two generations and the third generation.