Double-beta decay

In a double-beta event, two neutrons decay simultaneouly into two protons, two beta rays (electrons) and two antineutrinos. Experimentalists are now searching for another form of double-beta decay, one that does not produce neutrinos or antineutrinos. If such an event is found, it could unravelone of nature's great mysteries: What, if anything, is the mass of the neutrino? According to the Standard Model, the neutrino accompanying a negative beta ray is the distinct antiparticle of the one accompanying a positive beta ray. The theories that go beyond the Standard Model and assign a mass to the neutrino, however, predict that the particle emitted with a negative beta ray should be the same as the one emitted with a positive ray. In other words, the neutrino would be its own antiparticle. How can we tell whether these predictions are right? Double-beta decay is the ideal process in which to seek an answer to this question. If the neutrino has mass and is its own antiparticle, then the neutrino emitted in the first stage of the process might be reaborbed in the second, yielding a form of decay in which no neutrinos materialize. 8 figs