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Neutrinoless double-$β$ decay of ${}^{82}$Se in the shell model: beyond closure approximation

We recently proposed a new method to calculate the standard nuclear matrix elements for neutrinoless double-$β$ decay ($0νββ$) of ${}^{48}$Ca going beyond the closure approximation. Here we extend this analysis to the important case of ${}^{82}$Se, which was chosen as the base isotope for the upcoming SuperNEMO experiment. We demonstrate that using a mixed method that considers information from closure and non-closure approaches, one can get excellent convergence properties for the nuclear matrix elements, which allows one to avoid unmanageable computational costs. We show that in contrast with the closure approximation the mixed approach has a very weak dependence on the average closure energy. The matrix elements for the heavy neutrino-exchange mechanism that could contribute to the $0νββ$ decay of ${}^{82}$Se are also presented.