Precision of the leptonic mixing angle θ23 and its implications for the flavor models

Among three leptonic mixing angles, θ23, which characterizes the fractional contribution of two flavor eigenstates νμ and ντ to the third mass eigenstate ν3, is known to be the largest but least precisely measured. The work investigates the possible reach of θ23 precision with two forthcoming gigantic accelerator-based long-baseline neutrino experiments, namely, Hyper-Kamiokande (T2HK) and the Deep Underground Neutrino Experiment (DUNE), as well as a possible joint analyses of future neutrino facilities. Our simulation yields that each experiment will definitely establish the octant of θ23 for all values within a 1σ parameter interval, while considering the current limitation. However, if the actual value is 0.48≤sin2θ23≤0.54, it becomes challenging for these two experiments to reject the maximal (θ23=π/4) hypothesis and conclude its octant. This octant-blind region can be further explored with the proposed facilities European Spallation Source Neutrino Super Beam and Neutrino Factory. The accurate determination of the mixing angle θ23, as well as the accuracy of δCP, is crucial for examining a certain category of discrete non-Abelian leptonic flavor models. Specifically, if CP is conserved in the leptonic sector, the combined analysis of T2HK and DUNE will rule out the majority of these models. However, if the CP is maximally violated, a higher precision of δCP is necessary to test these flavor models. Published by the American Physical Society 2025