Calculation of double-beta-decay rates and the neutrino mass

Predictions for 2nu and 0nu double-beta-decay rates are given for all nuclei with A = 70 or greater for which double-beta decay is energetically allowed. These predictions are based on detailed nuclear-structure studies of the beta strength distribution and replace earlier estimates based mostly on phase-space considerations. New and more stringent limits on the Majorana neutrino mass are deduced from existing double-beta-decay experiments. It is found that a proper treatment of the collective effects arising from spin-isospin as well as quadrupole-quadrupole forces leads to a strong reduction of the nuclear matrix elements for two-neutrino double-beta decay, but has only minor influence on the matrix elements for the neutrinoless decay mode. The smaller limits for neutrino mass result, therefore, mainly from the fact that the widely used scaling procedure underestimates the 0nu matrix elements. The extent to which interference between different neurinos affects the obtained mass limits is also discussed.