Neutrino mass

Neutrinos have nonzero masses, established by flavor oscillations that depend on differences between their squared masses. Their absolute masses and ordering remain unknown.

Neutrino oscillation: The periodic change of a neutrino's flavor as it propagates, caused by mixing among states with different masses. Oscillations provide the direct evidence that neutrino mass states are not all degenerate.
KATRIN experiment: A tritium beta-decay experiment designed to measure or constrain the effective electron-neutrino mass. Its endpoint spectrum tests the absolute mass scale without relying on oscillations.
Dirac neutrino: A neutrino whose particle and antiparticle are distinct, with mass arising from a Dirac mass term. This possibility preserves lepton number and differs from a Majorana mass description.
Absolute neutrino mass scale: The individual neutrino masses or an equivalent measure of their overall scale. Oscillation data determine only mass-squared differences, not this absolute scale.
Bruno Pontecorvo: A physicist who developed early theories of neutrino flavor mixing and oscillation. His proposal connected neutrino transformation with the possibility of nonzero masses.
Neutrino mixing: The relationship between neutrino flavor states and neutrino mass states, described by a unitary mixing matrix. Mixing lets neutrinos produced in one flavor evolve into other flavors when their masses differ.
Neutrinoless double beta decay: A hypothetical nuclear decay that emits two electrons and no neutrinos, violating lepton-number conservation. If mediated by Majorana neutrinos, its rate can reveal an effective combination of neutrino masses.
Majorana neutrino: A neutrino that is its own antiparticle, permitted if neutrinos have Majorana mass terms. Oscillations establish nonzero mass but do not determine whether neutrinos are Majorana particles.
Effective electron-neutrino mass: A weighted combination of neutrino mass eigenvalues that affects the endpoint of beta-decay spectra. Direct beta-decay experiments constrain this mass combination independently of cosmology.
Raymond Davis Jr.: An American chemist and physicist who led a pioneering solar-neutrino detection experiment. His chlorine experiment found fewer solar neutrinos than expected, prompting the solar-neutrino problem.

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