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Title: Is the neutrino mass ordering normal or inverted?
Plain: We know the mass differences between the three neutrinos but not whether the odd one out is heaviest (normal ordering) or lightest (inverted ordering).
Precise: Determine the sign of $\Delta m^2_{31}$ ($\left|\Delta m^2_{31}\right| \sim 2.5e-3\,\mathrm{eV}^2$) at $\ge 3\,\sigma$ from reactor oscillation spectra (JUNO), matter effects in long-baseline accelerator (DUNE, Hyper-K) and atmospheric data. Answer: normal or inverted.
Settled by: JUNO reactor spectral data over about 6 years combined with DUNE and atmospheric experiments.
Kind: well-posed
Answer type: classification
References: (empty)
Literature status: open
Status note: JUNO 207-day data combined with long-baseline accelerator $\Delta m^{2}_{31}$ constraints (arXiv:2609.37895, September 2026) favor normal ordering at one-sided $\ge 2.1\sigma$ for all $\delta_{\mathrm{CP}}$ (Bayes factor 10.18); global fits (NuFit-6.0, 2024) also lean normal; the 2025 NOvA plus T2K joint fit prefers neither.
Posed since: (empty)