Octant degeneracy and plots of parameter degeneracy in neutrino oscillations revisited

The three kinds of parameter degeneracy in neutrino oscillation, the intrinsic, sign, and octant degeneracy, form an eight-fold degeneracy. The nature of this eight-fold degeneracy can be visualized on the ( sin 2 2 θ 13 , 1 / sin 2 θ 23 )-plane, through quadratic curves defined by P ( ν μ → ν e ) = const and P ( ν ¯ μ → ν ¯ e ) = const , along with a straight line P ( ν μ → ν μ ) = const . After θ 13 was determined by reactor neutrino experiments, the intrinsic degeneracy in θ 13 transforms into an alternative octant degeneracy in θ 23 , which can potentially be resolved by incorporating the value of P ( ν μ → ν μ ) . In this paper, we analytically discuss whether this octant parameter degeneracy is resolved or persists in the future long baseline accelerator neutrino experiments, such as T2HK, DUNE, T2HKK, and ESS ν SB . It is found that the energy spectra near the first oscillation maximum are effective in resolving the octant degeneracy, whereas those near the second oscillation maximum are not.