Effects of ground-state correlations on dipole and quadrupole excitations of $^{40}$Ca and $^{48}$Ca

TL;DR

This paper investigates how ground-state correlations influence dipole and quadrupole excitations in calcium isotopes $^{40}$Ca and $^{48}$Ca, revealing significant effects on resonance fragmentation and low-lying strength.

Contribution

It applies the extended random phase approximation (ERPA) from density-matrix theory to analyze ground-state correlation effects on nuclear excitations, a novel approach in this context.

Findings

Ground-state correlations cause significant fragmentation of the giant quadrupole resonance in $^{40}$Ca.

Low-lying dipole strength in $^{48}$Ca is affected by partial neutron state occupation.

ERPA results qualitatively agree with experimental strength distributions below 10 MeV.

Abstract

The effects of ground-state correlations on the dipole and quadrupole excitations are studied for $^{40}$Ca and $^{48}$Ca using the extended random phase approximation (ERPA) derived from the time-dependent density-matrix theory. Large effects of the ground-state correlations are found in the fragmentation of the giant quadrupole resonance in $^{40}$Ca and in the low-lying dipole strength in $^{48}$Ca. It is discussed that the former is due to a mixing of different configurations in the ground state and the latter is from the partial occupation of the neutron single-particle states. The dipole and quadrupole strength distributions below 10 MeV calculated in ERPA are in qualitatively agreement with experiment.