Double Beta Decay: Theory, Experiment, and Implications

Double beta decay is a rare spontaneous nuclear transition in which the nuclear charge changes by two units while the mass number remains the same. It has been long recognized as a powerful tool to study lepton number conservation in general, and neutrino properties in particular. Since the lifetimes of double beta decays are so long, the experiments on double beta decay are very challenging and have led to the development of many generally valuable techniques to achieve extremely low backgrounds. For the ββ decay to proceed, the initial nucleus must be less bound than the final one, but more bound than the intermediate nucleus. These conditions are realized in nature for a number of even-even nuclei (and never for nuclei with an odd number of protons or neutrons). Since the lifetime of the ββ decay is always much longer that the age of the Universe, both the initial and final nuclei exist in nature (some of the actinides being the only exceptions). In many of the “candidates” the transition of two neutrons into two protons is energetically possible, with the largest Q value just above 4 MeV. In a few cases the opposite transition, which decreases the nuclear charge, is also possible, but the Q values are typically smaller. The nuclear ββ transition can proceed in several ways. One of them, the 2ν decay