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 ($\sin^22\theta_{13}$, $1/\sin^2\theta_{23}$)-plane, through quadratic curves defined by $P(\nu_\mu\to\nu_e)=$ const. and $P(\bar{\nu}_\mu\to\bar{\nu}_e)=$ const., along with a straight line $P(\nu_\mu\to\nu_\mu)=$ const. After $\theta_{13}$ was determined by reactor neutrino experiments, the intrinsic degeneracy in $\theta_{13}$ transforms into an alternative octant degeneracy in $\theta_{23}$, which can potentially be resolved by incorporating the value of $P(\nu_\mu\to\nu_\mu)$. 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$\nu$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.