Double Beta-Decay

The first direct evidence of the double beta-decay of a nucleus has been reported by Elliott, Hahn and Moe in the issue of Physical Review Letters. Double beta-decay is an extremely rare process in which two electrons are emitted. Elliott et al observed the decay of ⁸²Se into ⁸²Kr plus two electrons and two electron antineutrinos with a measure of T = 1.1 ^+0.8 _-0.3 x 10²⁰ y, making i t the rarest natural decay process ever observed under laboratory conditions. More details are described in a review article on double beta-decay and in figure 5 b.1 we present the salient features that led to the above decay processes. The captions discuss the most important steps of the experimental set-up, as well as the observed electron energy spectra.

The ultimate goal is to find out if double beta-decay Mithout neutrinos can be observed. Such decay modes would violate lepton-number conservation, one of the few conservation laws thought to be rigidly fulfilled. Lepton number is defined +1 for an electron and neutrino, and -1 for their antiparticles! Therefore, in the standard model of particle physics, the emission of an electron must be accompanied by an antineutrino: the neutrino in this model is called a Dirac neutrino.

There exists a Majorana theory of neutrinos in which the neutrino and antineutrino are the same particle. The only distinction is that neutrinos are left-handed and antineutrinos right-handed. If neutrinos are exactly massless, there is no way to distinguish the Dirac neutrino from the Majorana neutrino. Thus, the observation of neutrinoless beta-decay would not only demonstrate lepton-number non-conservation but would prove that the neutrino's mass is not exactly zero!

The right- and left-handedness of the various beta-decay processes is illustrated in figure 5b.2, and is discussed in the section on parity violation in beta-decay.

The present stage of theory and experiment suggests that if electron neutrinos are Majorana particles, the effective mass is ≲ 1 eV. Now, a direct two-neutrino measurement exists, and as long as the possibility of a measurable neutrino mass exists, the quest for neutrinoless double beta-decay will go on.