· Oscillations · 2 min read
Normal vs. Inverted: The Neutrino Mass Ordering
The neutrino mass ordering asks whether the third mass state is heaviest or lightest. This is the difference between normal and inverted ordering, and why JUNO and DUNE aim to settle it.
The neutrino mass ordering is the question of how the three neutrino mass states are arranged from lightest to heaviest. Oscillation experiments measure the gaps between the masses but not their absolute values, and crucially not the sign of one gap. That leaves two possibilities, called normal ordering and inverted ordering. Which one nature chose is still unknown, and resolving it is a major goal of current neutrino physics.
What is the difference between the two orderings?
Two mass states sit close together and form what is called the solar pair, separated by a small splitting. A third state lies further away, across a much larger splitting. In normal ordering the lone third state is the heaviest, sitting above the solar pair. In inverted ordering it is the lightest, sitting below the pair. The sizes of the gaps are well measured; what remains undetermined is whether the large gap points up or down.
Why is the ordering still unknown?
The sign of the large splitting is hard to read because most oscillation measurements depend only on its magnitude. To expose the sign, experiments must exploit the matter effect, which treats the two orderings differently, or measure tiny interference patterns with great precision. Neither approach is easy, and present data only mildly prefer one option. The current global status is summarised on the parameters page and discussed among the field’s open questions.
How will JUNO and DUNE resolve it?
Two experiments are designed to settle the ordering. JUNO, a large liquid-scintillator detector in China, will measure reactor antineutrinos with such fine energy resolution that the subtle interference between the splittings reveals the sign. DUNE, sending a neutrino beam more than a thousand kilometres through the Earth in the United States, will use the matter effect along that long baseline. Their methods are independent, so together they should give a robust answer. You can explore how the orderings differ with the mass ordering tool.
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References
- 1.S. Navas et al. (Particle Data Group), Review of Particle Physics, Phys. Rev. D (2024). DOI: 10.1103/PhysRevD.110.030001.