Neutrino mass ordering

Reactor antineutrinos can change type on their way to a detector. This change is called oscillation. Each type mixes components with three distinct masses, called mass states. Two of their masses lie close together; the third is separated by a larger gap. Is that third state above the close pair, called normal ordering, or below it, called inverted ordering? Experiments look for the different energy patterns these two arrangements predict.[2] [3]

A reactor’s electron-type antineutrinos do not all remain in that type on their way to a detector. Each type mixes three quantum components with definite masses, called mass states. Their interference changes the type probabilities during travel. Two states form a close pair; does the third lie above them or below them? The question is the sign of the larger mass-squared difference, $\Delta m_{32}^2=m_3^2-m_2^2$, within the three-flavour framework.

Oscillations encode differences of squared masses rather than directly weighing each state. At JUNO, interference between the oscillations associated with the two larger splittings produces fine structure in the reactor energy spectrum. The two orderings predict different structure, so energy resolution, detector response and the unoscillated reactor spectrum matter to the inference.

Accelerator experiments also use oscillations after travel through Earth. Matter affects those probabilities, and their ordering inference is correlated with other mixing parameters, including the CP phase controlling an intrinsic neutrino–antineutrino difference. The September 2026 JUNO preprint combines reactor information with external accelerator constraints; its reported preference is conditional on those inputs, rather than an ordering measurement from JUNO alone.[2] [3]

The experiments have not decisively selected which order nature uses.[1] [2]

June’s first JUNO data demonstrate measurement capability. A September preprint adds a conditional normal-ordering preference with accelerator inputs; the evidence remains preliminary and does not decisively determine the ordering.[1] [2]

Oscillations measure differences between masses, while other laboratory experiments constrain their overall size. The ordering tells us which mass patterns can satisfy both kinds of measurement. It also helps separate neutrino properties that are entangled in accelerator experiments.[3]

Different orderings connect the observed oscillation differences to different possible absolute mass spectra. They therefore change how laboratory mass constraints and rare-decay searches are interpreted together. The same ordering information helps distinguish matter effects from an intrinsic neutrino–antineutrino asymmetry in accelerator data; it does not itself determine every mass or prove that a rare decay occurs.[3]