Systematic study of neutrinoless double beta decay to excited 0+ states Original Research Article

A systematic study of neutrinoless double beta (0νββ) decay to excited 0+ states, described as monopole-vibrational states or as composed of two collective quadrupole-phonon states, has been performed. Both double β− and double β+/EC decays have been analyzed within the framework of the multiple commutator model (MCM) by using a realistic nuclear many-body Hamiltonian and realistic mean-field single-particle bases. It is found that the associated 0νββ-decay transitions are suppressed relative to the decay to the final ground state both by the available phase space and especially by the smallness of the involved transition matrix elements. For completeness, also the double β+/EC decays to the final ground state have been discussed, a matter of growing experimental interest at the present.