Limitations of the number self-consistent random phase approximation - art. no. 054301

The quasiparticle random phase approximation (QRPA) equations are solved ta king into account the Pauli principle at the expectation value level, and a llowing changes in the mean field occupation numbers to minimize the energy while having the correct number of particles in the correlated vacuum. The study of Fermi pn excitations in Ge-76 using a realistic Hilbert space sho ws that the pairing energy gaps in the modified mean field are diminished u p to one half of the experimental value when strong proton-neutron correlat ions are present. Additionally, the Ikeda sum rule for Fermi transitions is violated due to the lack of scattering terms in the phonon operators. Thes e results call for a critical revision of the double beta decay half-lives estimated using the QRPA extensions when standard QRPA calculations collaps e.