Questioning model-independent estimates of 2 νββ decay rates

In this work we discuss the validity of recently published results, by Rumyantsev and Urin, concerning nuclear matrix elements of the two-neutrino double-beta decay transitions. These authors claim that these matrix elements can be calculated in a model-independent way. We have re-analyzed their results and extended their formalism to account for proton-neutron correlations at the QRPA level of approximation. We have found that the formalism fails in describing the double beta decay observables.