$2νββ$-decay to first $2^+$ state with partial isospin symmetry restoration from spherical QRPA calculations

With partially restored isospin symmetry, we calculate the nuclear matrix element for a special decay mode of $2νββ$ (two neutrino double beta decay) -- the decay to the first $2^+$ excited states. With the realistic CD-Bonn nuclear force, we analyze the dependence of the nuclear matrix elements on the iso-vector and iso-scalar parts of proton-neutron particle-particle interaction. The dependence on the different nuclear matrix element is observed and the results are explained. We also give the phase space factors with numerical electron wave functions and properly chosen excitation energies. Finally we give our results for the half-lives of this decay mode for different nuclei.

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