Is the neutrino mass ordering normal or inverted?
In plain words
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 statement
Determine the sign of $\Delta m^2_{31}$ ($\left|\Delta m^2_{31}\right| \sim 2.5 \times 10^{-3}\,\mathrm{eV}^2$) 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.
What would settle it
JUNO reactor spectral data over about 6 years combined with DUNE and atmospheric experiments.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
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.
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See also
- Prerequisite for Is the cosmological neutrino mass limit consistent with oscillation data?
- Related Is $\theta_{23}$ maximal, and if not, in which octant?
- Related Does neutrino flavor conversion change supernova explosion outcomes?
- Related Cosmological measurement of the neutrino mass sum to 0.015 eV
- Related What is the mass of the lightest neutrino?
- Related Do neutrino oscillations violate CP symmetry?
- Related Are neutrinos Majorana particles, their own antiparticles?
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