Collective nuclear excitations with Skyrme-Second RPA

TL;DR

This paper employs second RPA calculations with Skyrme forces to improve the description of nuclear excited states in oxygen-16, accounting for complex configuration couplings and residual interactions, leading to more accurate excitation energies.

Contribution

It introduces a fully self-consistent second RPA approach that includes all relevant couplings and residual interactions without approximation, enhancing nuclear excitation modeling.

Findings

Second RPA shifts strength distributions to lower energies.

Second RPA results show increased fragmentation of strength.

Improved excitation energy predictions for low-lying states.

Abstract

Second RPA calculations with a Skyrme force are performed to describe both high- and low-lying excited states in $^{16}$O. The coupling between 1 particle-1 hole and 2 particle-2 hole as well as that between 2 particle-2 hole configurations among themselves are fully taken into account and the residual interaction is never neglected, not resorting therefore to a generally used approximate scheme where only the first kind of coupling is considered. The issue of the rearrangement terms in the matrix elements beyond standard RPA is addressed and discussed. As a general feature of second RPA results, a several-MeV shift of the strength distribution to lower energies is systematically found with respect to RPA distributions. A much more important fragmentation of the strength is also naturally provided by second RPA due to the huge number of 2 particle-2 hole configurations. A better description of the excitation energies of the low-lying 0$^+$ and 2$^+$ states is obtained with second RPA with respect to RPA.