Nuclear Responses to Two-Body External Fields Studied with the Second Random-Phase-Approximation

TL;DR

This paper explores nuclear double-phonon excitations in 16O using the SSRPA method, revealing significant redistribution of strength and emphasizing the necessity of a microscopic 2p-2h treatment for accurate modeling.

Contribution

It demonstrates that SSRPA provides a detailed microscopic description of double-phonon excitations, highlighting differences from simpler models and the importance of residual interactions among 2p-2h configurations.

Findings

Strength distributions are significantly modified in SSRPA.

Double-phonon modes show energy shifts and additional strength.

High-lying resonances are dominated by neutron-proton configurations.

Abstract

This study investigates nuclear responses to two-body external fields, interpreted as double-phonon excitations, within the subtracted second random-phase approximation (SSRPA) for 16O. To clarify the underlying characteristics of these modes, Hartree-Fock (HF) and SSRPA with the diagonal approximation are first examined. The resulting strength distributions are nearly identical, indicating that residual interactions in the 1p-1h sector contribute only weakly. This behavior contrasts with that of one-body excitations, where coupling between 1p-1h and 2p-2h configurations is essential for generating collectivity. In the full SSRPA calculation, which incorporates the residual interaction among 2p-2h configurations, the strength distributions are substantially modified. The double IS 0+ and 2+ modes show pronounced redistribution, with peaks shifted to lower energies and additional strength emerging at higher energies, whereas the double IV $1^{-}$ mode shifts predominantly to higher energies due to a largely repulsive interaction. Analysis of single transition amplitudes reveals that low-lying resonances are formed coherently through constructive neutron-neutron, proton-proton, and neutron-proton configurations, while high-lying resonances are dominated by neutron-proton configurations, reflecting their higher state density. These results demonstrate that double-phonon excitations cannot be described by simple folding of one-body responses; a fully microscopic treatment of 2p-2h mixing, as provided by SSRPA, is essential.