Disentangling data contributions to the precision measurement of the largest leptonic mixing angle

TL;DR

This paper analyzes neutrino oscillation data to improve the measurement of the largest leptonic mixing angle, revealing that appearance samples are more sensitive to the mixing angle's octant and maximality than disappearance samples.

Contribution

It demonstrates that appearance samples primarily determine the octant of θ23, and shows that combined experiments can exclude maximal mixing within a significant parameter range.

Findings

Appearance samples are more sensitive to θ23 octant.

Joint T2HK and DUNE analysis can exclude maximal mixing for 60% of the parameter space.

Measurement of θ23 is minimally affected by other unknown parameters.

Abstract

This study examines the precise measurement of the largest leptonic mixing angle $\theta_{23}$ through the analysis of neutrino oscillation data samples. Our findings indicate that, contrary to common understanding, the ${\nu}_{\mu}(\bar{\nu}_{\mu})\rightarrow {\nu}_{e}(\bar{\nu}_{e})$ appearance samples, rather than the ${\nu}_{\mu}(\bar{\nu}_{\mu})\rightarrow \nu_{\mu}(\bar{\nu}_{\mu})$ disappearance samples, are sensitive to test the hypothesis of maximal mixing $\theta_{23}=\pi/4$, particularly if $\theta_{23}$ resides in the higher octant and $\sin^2\theta_{23}<0.54$. The former serves as the primary source for determining the octant of the $\theta_{23}$ mixing angle; however, the latter remains relevant if $\theta_{23}$ is indeed in the lower octant with $\sin^2\theta_{23}<0.42$. In a joint T2HK and DUNE analysis, utilizing only appearance sub-samples can exclude the maximal-mixing and determine the actual octant for about 60\% of the currently allowed range of the $\theta_{23}$ angle. We argue that, despite the presence of parameter degeneracy, the precise measurement of $\theta_{23}$ exhibits minimal dependence on other unknown factors, including the CP-violation phase and neutrino mass ordering.