Single and low-lying states dominance in two-neutrino double-beta decay

A theoretical analysis of the single-state dominance hypothesis for the two-neutrino double-beta decay rates is performed on the examples of the double-beta decays of 100Mo, 116Cd, and 128Te. We also test the validity of an extended low-lying-state dominance that takes into account the contributions of the low-lying excited states in the intermediate nucleus to the double-beta decay rates. This study has been accomplished for all the double-beta emitters for which we have experimental information on their half-lives. The theoretical framework is a proton-neutron quasiparticle random-phase approximation based on a deformed Skyrme Hartree-Fock mean field with pairing correlations. Our calculations indicate that there are not clear evidences for single- or low-lying-state dominance in the two-neutrino double-beta decay. Finally, we investigate the single electron energy distributions of the outgoing electrons in the double-beta decay processes with an exact treatment of the energy denominators, which could help to a more comprehensive analysis of NEMO-3 data.