Octant of ${\theta }_{23}$ at long baseline neutrino experiments in the light of nonunitary leptonic mixing

The octant of the atmospheric mixing angle ${\theta }_{23}$ is still undetermined by neutrino oscillation experiments. Long-baseline experiments like $\mathrm{NO}\nu \mathrm{A}$, T2K, and DUNE offer good prospects for determining the octant of ${\theta }_{23}$. However, their capability to do so may be compromised by the possible presence of nonunitarity in the leptonic mixing matrix. In this paper, we study in detail the octant degeneracy with and without nonunitarity at the level of oscillation probabilities and events for these experiments. We also analyze their octant sensitivity and discovery reach and show that they are hampered in the presence of nonunitarity.

Figure 1

Bi-event plots for DUNE, $\mathrm{NO}\nu \mathrm{A}$ and T2K. The black solid ellipses correspond to the standard $3\nu$ case and are obtained by varying ${\delta }_{cp}\in [-\pi ,\pi ]$. The gray (green) and the cyan (dark red) band show the effect of NU for the two cases: HO-NH (HO-IH) and LO-NH (LO-IH). The dotted black ellipses represent the special case when ${\varphi }_{NP}=0^0$ in both the hierarchies.

Figure 2

Octant sensitivity plots for DUNE and $\mathrm{NO}\nu \mathrm{A}+\mathrm{T}2\mathrm{K}$ for assumed true NH (left) and IH (right). We consider the central values of the NU parameters as the true values.

Figure 3

Octant discovery reach of DUNE and $\mathrm{NO}\nu \mathrm{A}+\mathrm{T}2\mathrm{K}$ for assumed true NH (left) and IH (right). We consider the central values of the NU parameters as the true values.