Gamow-Teller strength in ${}^{48}$Ca and ${}^{78}$Ni with the charge-exchange subtracted second random-phase approximation

Abstract

We develop a fully self-consistent subtracted second random-phase approximation for charge-exchange processes with Skyrme energy-density functionals. As a first application, we study Gamow-Teller excitations in the doubly-magic nucleus ${}^{48}$Ca, the lightest double-${\hat{I}}^2$ emitter that could be used in an experiment, and in ${}^{78}$Ni, the single-beta-decay rate of which is known. The amount of Gamow-Teller strength below 20 or 30 MeV is considerably smaller than in other energy-density-functional calculations and agrees better with experiment in ${}^{48}$Ca, as does the beta-decay rate in ${}^{78}$Ni. These important results, obtained without \textit{ad hoc} quenching factors, are due to the presence of two-particle -- two-hole configurations. Their density progressively increases with excitation energy, leading to a long high-energy tail in the spectrum, a fact that may have implications for the computation of nuclear matrix elements for neutrinoless double-${\hat{I}}^2$ decay in the same framework.