Impact of ground-state correlations on the multipole response of nuclei: Ab initio calculations of moment operators

We develop a framework that allows us to calculate integrated properties of the nuclear response from first principles. Using the in-medium similarity renormalization group (IMSRG), we calculate the expectation values of moment operators that are linked to the multipole response of nuclei. This approach is applied to the isoscalar mono- and quadrupole as well as the isovector dipole response of closed-shell nuclei from He 4 to Ni 78 for different chiral two- and three-nucleon interactions. We find that the inclusion of many-body correlations in the nuclear ground state significantly impacts the multipole response when going from the random-phase approximation to the IMSRG level. Our IMSRG calculations lead to an improved description of experimental data in O 16 and Ca 40 , including a good reproduction of the Thomas-Reiche-Kuhn enhancement factor. These findings highlight the utility of the moment method as a benchmark for other approaches that describe nuclear response functions through the explicit treatment of excited states.