Determination of the $\theta_{23}$ octant in long baseline neutrino experiments within and beyond the standard model

TL;DR

This paper assesses the ability of future long baseline neutrino experiments to determine whether the mixing angle θ23 is in the lower or higher octant, considering standard and nonunitary mixing scenarios.

Contribution

It provides a detailed numerical analysis of octant determination prospects, including the effects of potential nonunitarity in neutrino mixing.

Findings

Octant can be determined at 3σ confidence for certain θ23 and δ values.

Nonunitarity can affect the sensitivity of octant determination.

Contour plots illustrate the parameter regions where octant resolution is feasible.

Abstract

The recent data indicate that the neutrino mixing angle $\theta_{23}$ deviates from the maximal-mixing value of 45$^\circ$, showing two nearly degenerate solutions, one in the lower octant (LO) ($\theta_{23}<45^\circ$) and one in the higher octant (HO) ($\theta_{23}>45^\circ$). We investigate, using numerical simulations, the prospects for determining the octant of $\theta_{23}$ in the future long baseline oscillation experiments. We present our results as contour plots on the ($\theta_{23}-45^\circ$, $\delta$)--plane, where $\delta$ is the $CP$ phase, showing the true values of $\theta_{23}$ for which the octant can be experimentally determined at 3$\,\sigma$, 2$\,\sigma$ and 1$\,\sigma$ confidence level. In particular, we study the impact of the possible nonunitarity of neutrino mixing on the experimental determination of $\theta_{23}$ in those experiments.