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

Abstract

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^(2ν)_F 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^(T=1)_(pp) needs to be chosen to be essentially equal to the pairing constant g_pair, so no new parameter is needed. For the 0νββ decay the Fermi matrix element M^(0ν)_F is substantially reduced, while the full matrix element M^(0ν) 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