Theoretical investigation of two-particle two-hole effect on spin-isospin excitations through charge-exchange reactions

TL;DR

This paper investigates the impact of two-particle two-hole (2p2h) excitations on spin-isospin nuclear responses using charge-exchange reactions, revealing that 2p2h effects reduce cross section magnitudes without altering angular distributions.

Contribution

It introduces a microscopic framework incorporating 2p2h effects via the second Tamm-Dancoff approximation for analyzing charge-exchange reactions, improving upon conventional models.

Findings

2p2h effects decrease the magnitude of spin-flip cross sections.

The shape of angular distributions remains unaffected by 2p2h effects.

The approach better reproduces experimental data than the Lane model.

Abstract

Coherent one-particle one-hole (1p1h) excitations have given us effective insights into general nuclear excitations. However, the two-particle two-hole (2p2h) excitation beyond 1p1h is now recognized as critical for the proper description of experimental data of various nuclear responses. The spin-flip charge-exchange reactions $^{48}{\rm Ca}(p,n)^{48}{\rm Sc}$ are investigated to clarify the role of the 2p2h effect on their cross sections. The Fermi transition of $^{48}{\rm Ca}$ via the $(p,n)$ reaction is also investigated in order to demonstrate our framework. The transition density is calculated microscopically with the second Tamm-Dancoff approximation, and the distorted-wave Born approximation is employed to describe the reaction process. A phenomenological one-range Gaussian interaction is used to prepare the form factor. For the Fermi transition, our approach describes the experimental behavior of the cross section better than the Lane model, which is the conventional method. For spin-flip excitations including the GT transition, the 2p2h effect decreases the magnitude of the cross section and does not change the shape of the angular distribution. The $\Delta l=2$ transition of the present reaction is found to play a negligible role. The 2p2h effect will not change the angular-distributed cross section of spin-flip responses. This is because the transition density of the Gamow-Teller response, the leading contribution to the cross section, is not significantly varied by the 2p2h effect.