Double beta decay

The problem of double beta decay is reviewed with emphasis on its relevance to lepton number conservation. Recently, the ratio of the double beta-decay half-lives of ${}_{}{}^{128}{}_{}{}^{}\mathrm{Te}$ and ${}_{}{}^{130}{}_{}{}^{}\mathrm{Te}$ has been measured in a geological experiment and a limit for the ratio of the neutrinoless rate to the total rate for ${}_{}{}^{82}{}_{}{}^{}\mathrm{Se}$ decay has been obtained from a direct-detection experiment. For the first time, these results show conclusively that double beta decay is not primarily a lepton-number-violating neutrinoless process. However, they also do not agree with calculations which assume that only lepton-number-conserving two-neutrino double beta decay occurs. The conclusion that lepton number conservation is violated is suggested by limited experimental information. By considering contributions to the total rate from both the two-neutrino and the neutrinoless channels, we obtain data which are consistent with a lepton nonconservation parameter of order $\eta =3.5\mathrm{}\times {10}^{-5\mathrm{}}$. Roughly the same value of $\eta$ is obtained by assuming that the decay occurs either via lepton emission from two nucleons or via emission from a resonance in the nucleus.