Extended quasiparticle RPA and double-beta-decay nuclear matrix elements

The quasiparticle random-phase-approximation model is extended toward restoration of fermion anticommutation relations for quasiparticle operators, beyond the quasi-boson approximation, within the framework of equation-of-motion method. The restoration of broken symmetries avoids an unreasonable enhancement of quasiparticle correlations in the ground state, and enables to make more reliable predictions. By the extended model, somewhat larger bounds are deduced on the neutrino mass and right-handed current parameters, compared to ordinary quasiparticle calculations, for a given half-life limit of the neutrinoless double beta decay.