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Title: Is $\theta_{23}$ maximal, and if not, in which octant?
Plain: Muon-type and tau-type neutrinos are mixed in nearly equal amounts, but it is unknown whether the mixing angle is exactly 45 degrees or slightly above or below it. Exactly 45 degrees would point to a hidden symmetry between muons and taus.
Precise: Determine $\sin^{2}(\theta_{23})$ in the three-flavor PMNS framework to about $\pm 0.01$ and decide at $\ge 3\sigma$ whether $\theta_{23}=45\ \mathrm{deg}$ is excluded and, if so, whether $\theta_{23}<45\ \mathrm{deg}$ (lower octant) or $>45\ \mathrm{deg}$ (upper octant). NuFit-6.0 finds local minima near $\sin^{2}(\theta_{23})\sim 0.47$ and $\sim 0.56$ separated by $\Delta \chi^{2}<4$ for both mass orderings and both data variants (approximate).
Settled by: DUNE and Hyper-Kamiokande $\nu_{\mu}$ disappearance and $\nu_{e}$ appearance data, using the reactor value of $\theta_{13}$, combined with atmospheric data from IceCube and KM3NeT/ORCA.
Kind: well-posed
Answer type: classification
References: (empty)
Literature status: open
Status note: NuFit-6.0 (2024) leaves the octant unresolved, with the other-octant minimum always within $\Delta \chi^{2} < 4$.
Posed since: (empty)