Resolving θ23 degeneracy by accelerator and reactor neutrino oscillation experiments

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) over bar (mu)) beams 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)greater than or similar to 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.