Neutrinoless ββ decays to excited 0+ states and the Majorana-neutrino mass

Abstract

The nuclear matrix elements (NMEs) corresponding to the neutrinoless double-β (0νββ) decays to excited 0+ states of major experimental interest are calculated. All these decay transitions are electron emitting (0νβ−β− decays) and take place in the mass A = 76,82,96,100,110,116,124,130,136 nuclei. This work is an extension of our previous work [Phys. Rev. C 91, 024613 (2015)], where 0νββ decays to the ground states of the same nuclei were treated. We calculate the NMEs for transitions mediated by both the light (l-NMEs) and the heavy (h-NMEs) Majorana neutrinos. A higher-QRPA (quasiparticle random-phase approximation) framework, the multiple-commutator model, is adopted for the calculations, including a previously omitted contribution to the transitions to two-phonon states. A Bonn G-matrix-based effective nucleon-nucleon interaction is generated by exploiting the recently proposed isoscalar-isovector decomposition of the particle-particle proton-neutron interaction parameter, gpp. All the appropriate short-range correlations, nucleon form factors, and higher-order nucleonic weak currents are included to benchmark our calculations. The relevant nuclear spectroscopy was checked to validate the nuclear models used. The computed l-NMEs and h-NMEs are compared with the available other calculations and the relevance of the new included two-phonon term is discussed. The results are summarized by easy-to-use half-life-Majorana-mass interrelations.