Neutrino mass ordering and absolute mass scale

Determine the ordering of the three neutrino mass eigenstates (normal or inverted) and ascertain the absolute neutrino mass scale, using cosmological and laboratory measurements that are sensitive to neutrino properties such as mass-squared differences, the sum of neutrino masses, and related cosmological signatures.

Background

Neutrino oscillation experiments have established that neutrinos are massive by measuring mass-squared differences, but they do not fix the absolute mass scale or the ordering of the mass eigenstates. Two possibilities are considered: the normal ordering (with the smallest mass splitting between the lowest mass states) and the inverted ordering (with the smallest splitting between the highest mass states).

Cosmological observations such as those from DESI constrain the sum of neutrino masses through their effects on the expansion history and the growth of structure, while laboratory experiments set complementary bounds (e.g., on the effective electron antineutrino mass). Despite increasingly stringent cosmological limits, the absolute mass scale and mass ordering remain undetermined.