Abstract
The operator expansion method for the evaluation of the 2ν ββ decay nuclear matrix element is extended to include the Coulomb interaction and nuclear central and tensor interactions. It is shown that the Coulomb interaction, which represents a part of one-body terms of Hamiltonian, plays a decisive role in taking into account effects of ground-state correlations, and a significant improvement is achieved in simulating the ‘‘exact’’ calculation based on the quasiparticle RPA. However, this method does not reproduce the strong suppression of the nuclear matrix element, probably because of the two-body approximation.

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