neutrinomassat

Neutrino masses, or neutrinomassat in Finnish, refer to the nonzero rest masses of the neutrino flavor eigenstates. In the original Standard Model, neutrinos were massless, but observations of neutrino oscillations show that at least two neutrinos have finite masses and that flavor states are mixtures of mass eigenstates.

Experiments measuring neutrino oscillations determine two independent mass-squared differences and the mixing between flavors. The commonly quoted values are approximately Δm21^2 ≈ 7.4×10^-5 eV^2 and |Δm31^2| ≈ 2.5×10^-3 eV^2, with mixing angles describing how electron, muon, and tau neutrinos combine into mass states. The ordering of the masses—normal (m1 < m2 < m3) or inverted (m3 < m1 < m2)—remains unresolved.

The absolute neutrino mass scale is still unknown. Direct kinematic measurements of beta decay, such as those by the KATRIN experiment, constrain the effective electron neutrino mass mβ to below about 0.8–1.0 eV. Cosmological observations limit the sum of neutrino masses, Σmν, to roughly 0.1–0.2 eV depending on the data sets and models adopted. Combined, these results guide theories of mass generation and the possible hierarchy.

Mass generation beyond the Standard Model can involve Dirac masses, Majorana masses, or both. The seesaw mechanism explains small neutrino masses through heavy right-handed states and often implies Majorana neutrinos. A major experimental probe is neutrinoless double beta decay; its observation would signal lepton-number violation and a Majorana nature, with current limits constraining the effective Majorana mass to the tens to hundreds of meV range, depending on nuclear physics inputs.

Future experiments in oscillation studies, beta decay, neutrinoless double beta decay, and cosmology aim to determine the mass scale, hierarchy, and the fundamental character of neutrinos.