First-forbidden contributions to 2νβ−β− decay amplitude

The nuclear matrix element for 2νβ−β− decay is obtained by including the virtual contributions coming from the first-forbidden excitations in intermediate nuclei. The forbidden contributions are calculated within proton-neutron quasi-particle random phase approximation (pnQRPA) with the separable schematic effective interaction. We try to reproduce available experimental data concerning the first-forbidden excitations in intermediate nuclei. A comparison of the first-forbidden contributions with the allowed ones is given here.