– 1– NEUTRINOLESS DOUBLE-β DECAY

Neutrinoless double-beta (0νββ) decay would signal vio-lation of 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, on the lepton-number violating right-handed current or other possible mechanisms mediating 0νββ-decay can be obtained, independently of 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 collabora-tion. There is tension between that claim and several recent experiments which did not find evidence for 0νββ decay. In the following we assume that the exchange of light Majorana neutrinos (mνi ≤ 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〉