Direct evidence for two-neutrino double-beta decay in 82Se.

The two-neutrino mode of double-beta decay in Se has been observed in a time-projection chamber at a half-life of (1.1 — +Q) X 1020 yr (68% confidence level). This result from direct counting confirms the earlier geochemical measurements and helps provide a standard by which to test the double-beta-decay matrix elements of nuclear theory. It is the rarest natural decay process ever observed directly in the laboratory. Double-beta decay was suggested by Wigner in the 1930's as a second-order weak transition between isobars diA'ering by two units in atomic number. Assuming the emission of two electrons and two neutrinos (as required by the theory of Dirac), Geoppert-Mayer ' in 1935 made the first theoretical estimate of the extremely low rates for this process [pp(2v)]. Four years later, Furry invoked the theory of Majorana (in which the neutrino and antineutrino are identical) to propose an alternative mode of double-beta decay having no neutrinos in the final state [pp(Ov)]. Experimentally the two modes can be distinguished by the spectra of their sum energies; that is, the sum of the kinetic energies of the two emitted electrons. For PP(2v) a broadly distributed spectrum is expected since a portion of the energy is carried oA by unseen neutrinos. Essentially all of the transition energy is available to the electrons in PP(Ov). The resulting sharp energy spike would be typically 100 times easier to detect. Furry pointed out that because of the greater phase space available to PP(Ov), this mode could proceed at a faster rate than PP(2v) by many orders of magnitude. "According to the older theory, " he remarked, "it seemed certain that double-beta disintegration could never be observed because of its extremely minute probability , but the Majorana theory indicates that this is by no means necessarily the case. " Ironically, it is PP(2v) that has now been seen while Furry's potentially faster and experimentally much more distinctive pp(Ov) is yet to be detected. Possibly neutri-nos are indeed Dirac particles, but the Majorana alternative is not excluded by any known evidence. It is recognized today that parity nonconservation exerts a strong inhibiting influence, but need not exclude PP(Ov) entirely. An observation of PP(Ov) would have profound consequences for particle physics. The two electrons appearing alone would constitute a violation of lepton-number conservation — a symmetry breakdown expected in some theories of grand unification. A neutrinoless decay would further indicate a nonzero mass …