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

摘要

Coherent one-particle one-hole (1p1h) excitations have given us effectiveinsights into general nuclear excitations. However, the two-particle two-hole(2p2h) excitation beyond 1p1h is now recognized as critical for the properdescription of experimental data of various nuclear responses. The spin-flipcharge-exchange reactions $^{48}{\rm Ca}(p,n)^{48}{\rm Sc}$ are investigated toclarify the role of the 2p2h effect on their cross sections. The Fermitransition of $^{48}{\rm Ca}$ via the $(p,n)$ reaction is also investigated inorder to demonstrate our framework. The transition density is calculatedmicroscopically with the second Tamm-Dancoff approximation, and thedistorted-wave Born approximation is employed to describe the reaction process.A phenomenological one-range Gaussian interaction is used to prepare the formfactor. For the Fermi transition, our approach describes the experimentalbehavior of the cross section better than the Lane model, which is theconventional method. For spin-flip excitations including the GT transition, the2p2h effect decreases the magnitude of the cross section and does not changethe shape of the angular distribution. The $\Delta l=2$ transition of thepresent reaction is found to play a negligible role. The 2p2h effect will notchange the angular-distributed cross section of spin-flip responses. This isbecause the transition density of the Gamow-Teller response, the leadingcontribution to the cross section, is not significantly varied by the 2p2heffect.