A LARGE HILBERT-SPACE QRPA AND RQRPA CALCULATION OF NEUTRINOLESS DOUBLE-BETA DECAY

A large Hilbert space is used for the calculation of the nuclear matri x elements governing the light neutrino mass mediated mode of neutrino less double beta decay (O nu beta beta-decay) of Ge-76, Mo-100, Cd-116 , Te-128, and Xe-136 within the proton-neutron quasiparticle random ph ase approximation (pn-QRPA) and the renormalized QRPA with proton-neut ron pairing (full-RQRPA) methods. We have found that the nuclear matri x elements obtained with the standard pn-QRPA for several nuclear tran sitions are extremely sensitive to the renormalization of the particle -particle component of the residual interaction of the nuclear hamilto nian. Therefore the standard pn-QRPA does not guarantee the necessary accuracy to allow us to extract a reliable limit on the effective neut rino mass. This behavior already known from the calculation of the two -neutrino double beta decay matrix elements, manifests itself in the n eutrinoless double-beta decay but only if a large model space is used. The full-RQRPA, which takes into account proton-neutron pairing and c onsiders the Pauli principle in an approximate way, offers a stable so lution in the physically acceptable region of the particle-particle st rength. In this way more accurate values on the effective neutrino mas s have been deduced from the experimental lower limits of the half-lif es of neutrinoless double beta decay.