Determine the neutrino mass ordering (normal vs inverted)

Determine whether the neutrino mass eigenstate ordering is normal (m1 < m2 < m3) or inverted (m3 < m1 < m2) by establishing the position of m3 relative to m1 and m2 using oscillation measurements of electron antineutrinos.

Background

In the three-flavor neutrino oscillation framework, flavor states are mixtures of mass eigenstates ν1,ν2,ν3\nu_1, \nu_2, \nu_3 with splittings Δm212\Delta m^2_{21} and Δm312\Delta m^2_{31}. While the ordering of m1m_1 and m2m_2 is established from solar neutrino data, the relative position of m3m_3 remains unresolved.

The JUNO experiment is designed to measure precise reactor antineutrino spectra at a 52.5 km baseline, where interference between solar- and atmospheric-scale oscillations imprints a fine spectral structure that is sensitive to the neutrino mass ordering. Resolving this unknown is a primary long-term goal of JUNO.

References

While ordering of the first two mass states ($m_2 > m_1$) is known, the ordering of $m_3$---normal or inverted---remains undetermined.

— First measurement of reactor neutrino oscillations at JUNO  (2511.14593 - Abusleme et al., 18 Nov 2025) in Methods, Subsection "Oscillation of reactor \bar{\nu}_e"

The global fits leave both open, and they leave them correlated.

— Level-Four Modular Symmetry Selected by the Harmonic Pattern of Quark and Lepton Masses  (2609.09262 - Barger, 8 Sep 2026) in Section 6.5, subsection “Normal ordering with the first octant predicted”

Further, another important unresolved issue is the neutrino mass order, namely whether the neutrino masses follow the normal order ($m_1<m_2<m_3$) or the inverted order ($m_3<m_1<m_2$).

— Deviation from $μ-τ$ reflection symmetry under radiative corrections in the minimal seesaw framework  (2609.01196 - Pegu et al., 1 Sep 2026) in Section 1, Introduction

The neutrino mass ordering --- whether Normal Ordering (NO, Δm²₃₁ > 0, m₁ < m₂ < m₃) or Inverted Ordering (IO, Δm²₃₁ < 0, m₃ < m₁ < m₂) --- remains one of the central unresolved questions in neutrino physics, and its determination is a primary objective of DUNE.

— Probing Quantum Foundations in Long-Baseline Neutrino Oscillations: Wave Packet Approach from MINOS Data to DUNE Predictions  (2609.02975 - Farooq et al., 2 Sep 2026) in Section 4.2, “Mass Hierarchy for proposed DUNE model”