Neutrino mass ordering

Neutrino mass ordering is the arrangement of the three neutrino mass eigenstates by mass. It remains unknown whether the third state is heavier or lighter than the two closely spaced states.

Neutrino oscillation: The phenomenon in which neutrinos change flavor as they propagate because flavor states are mixtures of mass states. Oscillations provide the measurements from which mass ordering can be inferred.
JUNO: A Chinese liquid-scintillator observatory designed to measure reactor antineutrino oscillations with high precision. Its fine energy spectrum can resolve ordering-dependent interference at a medium baseline.
Normal ordering: The neutrino mass arrangement in which the third mass eigenstate is heavier than the other two. This is one of the two alternatives that experiments must distinguish.
Cosmological neutrino mass: Constraints on the combined neutrino mass from their effects on cosmic expansion and structure formation. Cosmology measures a mass sum whose minimum differs between the two orderings.
Neutrino mass eigenstates: The three states that propagate with definite masses and combine to form neutrino flavor states. Their relative masses are precisely what the ordering question concerns.
IceCube: A cubic-kilometer neutrino observatory embedded in Antarctic glacial ice. Its DeepCore detector studies ordering-sensitive matter effects in atmospheric neutrinos.
Inverted ordering: The neutrino mass arrangement in which the third mass eigenstate is lighter than the other two. This is the alternative to normal ordering, with a different matter-effect signature.
Effective Majorana mass: The mass combination governing the rate of neutrinoless double beta decay if neutrinos are Majorana particles. Its allowed range depends on the ordering and on unknown neutrino phases.
PMNS matrix: The unitary matrix relating neutrino flavor states to neutrino mass eigenstates. Its mixing angles determine how strongly each mass state shapes oscillation probabilities.
Deep Underground Neutrino Experiment: A long-baseline experiment sending neutrinos from Fermilab to detectors in South Dakota. Its appearance and disappearance measurements are designed to determine the ordering.

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