Neutrinoless double beta decay ($(A,Z)\to (A,Z+2)+2e^{-}$)

A lepton-number-violating nuclear transformation emitting two electrons without neutrinos. A signal appears near the nuclear Q value in summed electron energy, after recoil and detector response are accounted for. Its interpretation requires the decay mechanism and nuclear matrix elements.

Two charged-current beta vertices can be connected by a virtual Majorana neutrino. With light exchanged masses relative to nuclear momentum scales, the chirality connection makes the amplitude proportional to ${\sum }_i{U}_{ei}^2{m}_i$, not an incoherent mass average. Squaring introduces the effective Majorana mass, nuclear matrix elements and phase space. Other lepton-number-violating operators can also mediate the same final state, so translating a rate into a mass requires the stated mechanism.

A contained neutrinoless event deposits approximately the nuclear Q-value in the two-electron sum, apart from recoil and losses. Ordinary two-neutrino double beta decay has a continuous electron-sum radiation spectrum. Resolution, escape losses and backgrounds broaden or mimic features, so a peak assignment requires calibrated response, sidebands and appropriate event topology. A null peak search is a limit on a decay rate under an efficiency/background model, not proof of Dirac neutrinos.

A search in which isotope-enriched detector material is also the decay source. germanium detectors combine a contained energy deposit with energy resolution, pulse information and material-background controls. Background peaks and resolution tails must be tested before assigning a signal.

The coherent electron-flavor mass combination entering the usual light-neutrino-exchange amplitude. Its phases can cancel contributions, so a null double-beta result does not imply vanishing individual masses.