Nuclear Double-Beta Decay and a New Limit on Lepton Nonconservation

Nuclear double-β decay is discussed on the assumptions (1) that the process, treated as second-order weak, occurs predominantly without neutrino emission and so violates lepton convervation, and (2) that the matrix element for the process is mainly due to the no-neutrino double-β decay of an isospin-3/2 nucleon resonance present, albeit with a small ({}1%) probability, in the parent or daughter nucleus. The theoretical lifetimes obtained on this basis are compared with the corresponding experimental lifetimes; the various lifetime ratios are correctly predicted, and a limit {}10^-4 is imposed on the relevant "lepton-non-conservation" parameter. A discussion is also given of the convergence problems associated with the matrix elements for second-order weak processes in general. In addition it is shown that in spite of the presence of virtual-neutrino closed loops, divergences never arise in the matrix elements for no-neutrino double-β decay if the basic nuclear constituents (i.e., quarks or nucleons) have isospin not exceeding {} and mean separation greater than zero.