OPERATOR EXPANSION METHOD AND NUCLEAR BETA-BETA-DECAY

Recent developments in the theoretical description of double beta decay matrix elements are outlined. We present the formalism of the operator expansion method (OEM) and discuss its physical basis. The OEM has been combined with ground state wave functions obtained in a usual QRPA calculation and applied to the problem of evaluating nuclear matrix elements for double beta decay in the 2nubetabeta as well as in the 0nubetabeta decay mode. For 2nubetabeta decay we have calculated the half-lives for all possible beta-beta- decay candidates with A greater-than-or-equal-to 70. We confront our results with the existing experimental data and discuss the quality of the model. 0nubetabeta decay matrix elements are then used to extract upper limits for the effective neutrino mass from published experimental lower half-life limits. The presently sharpest limits come from experiments using isotopically enriched Ge-76. A short discussion is devoted to a recent measurement using U-238 in a radiochemical experiment. Our conclusion is that the experimental result should be interpreted as being due to 2nubetabeta decay, and lepton number violation is not necessary to explain the experimental finding.