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 2 nu beta beta Fermi matrix element M-F(2 nu) vanishes, as it should, unlike in the previous version of the method. This is accomplished by separating the renormalization parameter g(pp) of the particle-particle proton-neutron interaction into isovector and isoscalar parts. The isovector parameter g(pp)(T=1) needs to be chosen to be essentially equal to the pairing constant g(pair), so no new parameter is needed. For the 0 nu beta beta decay the Fermi matrix element M-F(0 nu) is substantially reduced, while the full matrix element M-0 nu is reduced by approximate to 10%. We argue that this more consistent approach should be used from now on in the proton-neutron QRPA and in analogous methods. DOI: 10.1103/PhysRevC.87.045501