If the lepton mixing angle theta_{ 23} is not maximal, there arises a problem of ambiguity in determining theta_{ 23} due to the existence of two degenerate solutions, one in the first and the other in the second octant. We discuss an experimental strategy for resolving the theta_{ 23} octant degeneracy by combining reactor measurement of theta_{ 13} with accelerator nu_{ mu} disappearance and nu_{ e} appearance experiments. The robustness of the theta_{ 23} degeneracy and the difficulty in lifting it only by accelerator experiments with conventional nu_{ mu} (and nu_{ mu}-bar) beam are demonstrated by analytical and numerical treatments. Our method offers a way to overcome the difficulty and can resolve the degeneracy between solutions sin^2 theta_{ 23} = 0.4 and sin^2 theta_{ 23} = 0.6 if sin^2 (2 theta_{ 13}) \gsim 0.05 at 95% CL by assuming the T2K phase II experiment and a reactor measurement with an exposure of 10 GW.kt.yr. The dependence of the resolving power of the octant degeneracy on the systematic errors of reactor experiments is also examined.
Categories: hep-ph hep-ex