QRPA and its extensions in a solvable model

An exactly solvable model is introduced, which is equivalent to the exact shell-model treatment of protons and neutrons in a single j-shell for Fermi-type excitations. Exact energies, quasiparticle numbers and double beta decay. Fermi amplitudes are computed and compared with the results of both the standard quasiparticle random phase approximation (QRPA) and the renormalized one (RQRPA), and also with those corresponding to the Hamiltonian in the quasiparticle basis (qp). A zero excitation energy state is found in the exact case, occurring at a value of the residual particle-particle interaction at which the QRPA collapse. The RQRPA and the qp solutions do not include this zero-energy eigenvalue in their spectra, probably due to spurious correlations. (Author)