0νββ and 2νββ nuclear matrix elements, quasiparticle random-phase approximation, and isospin symmetry restoration

Within the quasiparticle random-phase approximation (QRPA) we achieve partial restoration of the isospin symmetry and hence fulfillment of the requirement that the 2νββ Fermi matrix element M F vanishes, as it should, unlike in the previous version of the method. This is accomplished by separating the renormalization parameter gpp of the particle-particle proton-neutron interaction into isovector and isoscalar parts. The isovector parameter gT=1 pp needs to be chosen to be essentially equal to the pairing constant gpair, so no new parameter is needed. For the 0νββ decay the Fermi matrix element M F is substantially reduced, while the full matrix element M0ν is reduced by ≈10%. We argue that this more consistent approach should be used from now on in the proton-neutron QRPA and in analogous methods.