Nuclear Physics of Double Beta Decay

Study of the neutrinoless ββ decay allows us to put a stringent limit on the effective neutrino Majorana mass, a quantity of fundamental importance. To test our ability to evaluate the nuclear matrix elements that govern the decay rate, it is desirable to be able to describe the allowed two-neutrino decay. It is argued that only low-lying virtual intermediate states are important for that process, and thus it appears that the large-scale shell model evaluation, free from the various difficulties of QRPA, is the preferred method. In the 0ν decay, it is shown that there is a substantial cancellation between the paired and broken pair pieces of the nuclear wave function. Thus, despite the popular claim to the contrary, one expects that the 0ν rate is also sensitive to the details of nuclear structure. This is reflected in the spread of the theoretical values, leading to the uncertainty of about factor 2-3 in the deduced 〈mν〉 limit. Finally, brief comment about the 0ν decay with the exchange of a very heavy neutrino are made.