6 - Double Beta Decay

Summary

This Chapter is devoted to the exploration of double beta decay, a remarkable rare transition between two nuclei with the same mass number, which involves a change of the nuclear charge number by two units. It has long been recognized that a particular mode of double beta decay, in which two electrons and no neutrinos are emitted, is a powerful tool for the study of lepton conservation in general, and neutrino properties in particular. The description of the neutrinoless double beta decay involves an intricate mixture of elementary particle physics and physics of the nucleus. Among the particle physics issues which we wish to address with the help of double beta decay are the questions: is the neutrino a Majorana particle and if so, what is its mass? and, is there evidence that the weak interactions contain leptonic currents with a small right-handed component? The principal nuclear physics issues have to do with the identification and evaluation of the nuclear matrix elements responsible for the decay rate. As it is our goal to arrive at a quantitative answer for the mentioned particle properties, we have no choice but to learn first how to understand the nuclear mechanisms.

Our principal concern, as everywhere in this book, is the problem of extracting information on neutrino mass from these nuclear experiments. Accordingly, we begin with Section 6.1 on the phenomenology of double beta decay and how it relates to neutrino mass. This is followed in Section 6.2 with a discussion on the nuclear physics aspects. We then proceed with a description of the experimental results in Section 6.3, both from laboratory work and from studies of geological samples.