Spontaneous and dynamical breaking of mean field symmetries in the proton-neutron quasiparticle random phase approximation and the description of double beta decay transitions

The breakdown of the isospin symmetry, at the level of the quasiparticle mean field approximation, and its partial restoration by effective interactions, at the quasiparticle random phase approximation ~QRPA! level of approximation, are studied. The method used to define effective symmetry breaking two-body interactions has been applied previously to particle-number and rotational symmetry violations. The connection between the present approach and the proton-neutron QRPA method with renormalized two-particle interactions is discussed. The formalism is applied to calculate nuclear matrix elements for Fermi double beta decay transitions. @S0556-2813 ~99!03101-5#