Forbidden β decays within the realistic shell model

For the first time, half-lives and energy spectra of forbidden decays are calculated within the realistic shell model. Namely, we approach this issue starting from a realistic nucleon-nucleon potential and deriving effective Hamiltonians and decay operators. Our goal is to explore the sensitivity of the shape of calculated energy spectra to the renormalization of forbidden -decay operators, an operation that allows to take into account those configurations that are not explicitly included in the chosen model space. The region that has been considered for this investigation are nuclei outside the ^78Ni core, more precisely we have studied the second-forbidden decays of ^94Nb and ^99Tc, and fourth-forbidden decays of ^113Cd and ^115In, that are currently of a renewed experimental interest in terms of novel spectroscopic techniques. Our results evidence that the introduction of a renormalized -decay operator leads to a marked improvement of the reproduction of experimental half-lives. As regards the spectra of both second-forbidden and fourth-forbidden decays, we have found that their calculated shapes are in good agreement with the observed ones, even if scarcely responsive to the renormalization of the decay operator. We carry out also a detailed inspection of the different components of the calculated spectra for a deeper insight about their role in reproducing the experimental shapes.