The Binding Mechanism and a Low-Lying Resonance in 11Li

Abstract

Using a new 9 Li-n interaction taking into account the 9 Li-core excitation, it is shown that the 9 Li+n+n model reproduces the observed binding energy of 11 Li. This 9 Li-n interaction naturally explains the degeneracy of single particle s 1/2 -and p 1/2 -orbital states above the 9 Li+n threshold. Furthermore, by using the 9 Li-n interaction, a low-lying excited resonant state is obtained in 11 Li together with the ground state, and they are found to be partners in mixed configurations of (0p 1/2 ) 2 and (1s 1/2 ) 2 . §1. IntroductionRecently, much attention has been concentrated on the study of neutron-rich nuclei. 1) The first motivation for studying neutron-rich nuclei was provided by the observation of the large interaction cross-section of 11 Li. 2) It has been discussed in many works that the large interaction cross-section can be explained in terms of the so-called halo structure, which is a cloud of valence neutrons around the core nucleus.Early in the study of the exotic structure of 11 Li, calculations based on the 9 Li+n+n three-body model were carried out. In these models, the 9 Li-n interaction is chosen so as to reproduce the experimental p-wave resonance energy of 10 Li at E r ∼ 0.5 MeV. However, despite the success of this model in explaining the halo structure of 11 Li, the binding energy of 11 Li is not reproduced. For example, in the cluster orbital shell model (COSM), 3) which is one of the pioneering 9 Li+n+n models, the binding energy is too weak at least 1-2 MeV. 4), 5) Later, Thompson and Zhukov proposed a possibility of a virtual state, which is a low-lying s-state, in the 9 Li+n subsystem. Using a 9 Li-n potential with a strong state dependence, which produces the virtual state, they explained the experimental binding energy of 11 Li and also the 9 Li momentum distributions. 6) The essential point of their model is that the s-and p-states are almost energetically degenerate near the 9 Li+n threshold. This model is different from the previous model which assumes only the presence of low-lying p-states. When there is such a virtual state, since the state of two neutrons in the s-orbit can strongly couple to those in the p-state, the ground state should have the mixed configuration (s 1/2 ) 2 +(p 1/2 ) 2 . 6) However, in the analyses of Ref. 6), the mechanism of such a strong state dependence of the 9 Li-n interaction, which causes