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

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 and 40Ca. 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 1p1h sector contribute only weakly. This behavior contrasts with that of one body excitations, where coupling between 1p1h and 2p2h configurations is essential for generating collectivity. In the full SSRPA calculation, which incorporates the residual interaction among 2p2h 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, resulting in a resonance around twice the peak energy of the single giant dipole resonance. 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 2p2h mixing, as provided by SSRPA, is essential.

Comments: Accepted for publication in Physical Review C. 17 pages, 14 figures, 7 tables