Double-β Decay OMITTED FROM SUMMARY TABLE NEUTRINOLESS DOUBLE-β DECAY

Neutrinoless double-beta (0νββ) decay would signal viola-tion of the total lepton number conservation. The process can be mediated by an exchange of a light Majorana neutrino, or by an exchange of other particles. However, the existence of 0νββ-decay requires Majorana neutrino mass, no matter what the actual mechanism is. As long as only a limit on the lifetime is available, limits on the effective Majorana neutrino mass, and on the lepton-number violating right-handed current or other possible mechanisms mediating 0νββ decay can be ob-tained, independently on the actual mechanism. These limits are listed in the next three tables, together with a claimed 0νββ-decay signal reported by part of the Heidelberg-Moscow collaboration. There, a 4σ excess of counts at the decay energy is used for a determination of the Majorana neutrino mass. This signal has not yet been independently confirmed. In the following we assume that the exchange of light Majorana neu-trinos (mi ≤ O(10 MeV)) contributes dominantly to the decay rate. Besides a dependence on the phase space (G0ν) and the nu-clear matrix element (M0ν), the observable 0νββ-decay rate is proportional to the square of the effective Majorana mass 〈mββ〉