Neutrinoless double beta decay

A hypothetical nuclear decay in which a nucleus emits two electrons without emitting neutrinos. Its observation would show that lepton number is violated and neutrinos are Majorana particles.

Double beta decay: A nuclear transition in which a nucleus changes its proton number by two while emitting two electrons, with or without neutrinos. It is the broader process whose rare neutrino-free mode is being sought.
GERDA experiment: An experiment that searched for neutrinoless double beta decay using germanium-76 detectors immersed in liquid argon. Its germanium search set stringent limits on the decay rate.
Beta decay: A radioactive nuclear process in which a neutron changes into a proton or vice versa, emitting a charged lepton and a neutrino or antineutrino. The hypothesized mode combines two beta transitions while omitting their neutrinos.
Two-neutrino double beta decay: An observed nuclear decay in which two electrons and two antineutrinos are emitted. It has the same nuclear transformation but conserves lepton number and produces a continuous electron-energy spectrum.
Majorana particle: A particle that is identical to its antiparticle. A virtual Majorana neutrino could connect the two beta-decay vertices.
CUORE experiment: An experiment searching for neutrinoless double beta decay with cryogenic tellurium dioxide bolometers. Its tellurium-130 detectors test a different candidate isotope.
Neutrino: A neutral, weakly interacting elementary particle that comes in three known flavors. Neutrino properties determine whether the proposed exchange mechanism can operate.
Dirac particle: A particle whose antiparticle is a distinct state, as in the Dirac description of fermions. If neutrinos are Dirac particles, the standard light-neutrino exchange mechanism cannot produce this decay.
Lepton number: A quantum number that distinguishes leptons from antileptons in particle interactions. The hypothesized decay changes total lepton number by two units.
KamLAND-Zen: An experiment searching for neutrinoless double beta decay using xenon-136 dissolved in liquid scintillator. Its large xenon exposure provides a leading decay-rate constraint.

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