On Precision of the Leptonic Mixing Angle $\theta_{23}$ and its Implications for the Flavor Models

TL;DR

This paper evaluates the potential of upcoming neutrino experiments to precisely measure the leptonic mixing angle θ23 and discusses implications for flavor models, emphasizing the importance of accurate measurements for testing theoretical frameworks.

Contribution

It provides a simulation-based analysis of the precision reach of Hyper-Kamiokande, DUNE, and other facilities in determining θ23's octant and implications for flavor models.

Findings

Hyper-K and DUNE can establish θ23 octant within current uncertainties.

Challenging to reject maximal θ23 hypothesis if sin²θ23 is between 0.48 and 0.54.

Additional facilities like ESSnuSB can explore octant-blind regions.

Abstract

Among three leptonic mixing angles, $\theta_{23}$ angle, which characterizes the fractional contribution of two flavor eigenstates $\nu_{\mu}$ and $\nu_{\tau}$ to the third mass eigenstate $\nu_3$, is known to be the largest but the least precisely measured. The work investigates possible reach of $\theta_{23}$ precision with two upcoming gigantic accelerator-based long-baseline neutrino experiments, namely Hyper-Kamiokande and DUNE experiments as well as a possible joint analyses of future neutrino facilities. Our simulation yields that each experiment will definitely establish the octant of $\theta_{23}$ angle for all values within 1$\sigma$ parameter interval, while considering the current limitation. However, if the actual value is $0.48\leq \sin^2\theta_{23}\leq 0.54$, it becomes challenging for these two experiments to reject the maximal ($\theta_{23}=\pi/4$) hypothesis and conclude its octant. This octant-blind region can be further explored with the proposed facilities ESSnuSB and a neutrino factory. Accurate determination of the mixing angle $\theta_{23}$, as well as the accuracy of $\delta_{CP}$, is crucial for examining a certain category of discrete non-Abelian leptonic flavor models. Specifically if CP is conserved in leptonic sector, the combined analysis of Hyper-K and DUNE will rule out the majority of these models. However, if the CP is maximally violated, higher precision of $\delta_{CP}$ is necessary for testing these flavor models.