Threshold effects in multichannel coupling and spectroscopic factors in exotic nuclei

Abstract

In the threshold region, the cross section and the associated overlap integral obey the Wigner threshold law that results in the Wigner-cusp phenomenon. Due to flux conservation, a cusp anomaly in one channel manifests itself in other open channels, even if their respective thresholds appear at a different energy. The shape of a threshold cusp depends on the orbital angular momentum of a scattered particle; hence, studies of Wigner anomalies in weakly bound nuclei with several lowlying thresholds can provide valuable spectroscopic information. In this work, we investigate the threshold behavior of spectroscopic factors in neutron-rich drip-line nuclei using the Gamow Shell Model, which takes into account many-body correlations and the continuum effects. The presence of threshold anomalies is demonstrated and the implications for spectroscopic factors are discussed.In 1948, based on general principles, Wigner predicted [1] a characteristic behavior (a cusp) of scattering and reaction cross sections in the vicinity of a reaction threshold. This particular behavior (often referred to as the Wigner's threshold law or the Wigner-cusp phenomenon) was given a quantitative explanation a decade later [2,3,4,5,6,7]. In particular, it has been noted [3,4,5,6,8] that, due to the unitarity of the scattering matrix and the resulting flux conservation, the presence of a threshold anomaly in an opening reaction channel can trigger an appearance of Wigner cusps in other open channels with lower thresholds. As the shape of the cusp strongly depends on orbital angular momentum (is strongest in s and p waves), it was early realized that the presence of cusp anomaly could provide structural information about reaction products [3,4,9].The Wigner-cusp phenomenon has been studied experimentally and theoretically in various areas of physics: pion-nucleus scattering [9, 10], electron-molecule scattering [11], electron-atom scattering [12], and ultracold atom-diatom scattering [13]. In low-energy nuclear physics, threshold effects have been investigated in, e.g., charge-exchange reactions [14], neutron elastic scattering [15], and deuteron stripping [16]. Abramovich et al. [17] reviewed threshold phenomena in nuclear reactions, including those with the light neutron-rich systems such as 6 He, 10 Be, and 10 Li (see also Ref. [18]) that are of particular interest in the context of this work. The influence of threshold effects on the cross section and the strength function, hence the spectroscopic factor (SF) of a threshold state was pointed out in Refs. [8,19].The purpose of this study is many-fold. Firstly, we investigate whether the Wigner-cusp phenomenon appears naturally in a microscopic many-body approach rooted in an effective inter-nucleon interaction. The second goal is to investigate the influence of threshold effects and the coupling to the non-resonant continuum on overlap integrals, or spectroscopic factors. We demonstrate that the energy dependence of SFs caused by an opening of a reaction channel can be described at the shell-m...

Closed-shell properties ofO24withab initiocoupled-cluster theory

“…The reduction of spectroscopic factors will be enhanced for states close to a reaction channel threshold as discussed in Ref. [33]. Although the effect of the continuum on the spectroscopic factors is marginal in the cases we consider, the effect is crucial in order to obtain the correct asymptotic behavior of the overlap functions for one-neutron removal.…”

Reduction of spectroscopic strength: Weakly-bound and strongly-bound single-particle states studied using one-nucleon knockout reactions

“…When compared with the experimental inclusive cross section we obtain a reduction factor of R s = 0.90(7). In the future, threshold effects introduced by the proximity of the particle continuum [45] might be taken into account within the novel Gamow shell model [46] or other approaches that explicitly treat the continuum [47][48][49][50].…”

Four-body resonances of7B using the complex scaling method

“…It is also interesting to examine the effect of On the theoretical side, to treat unbound states explicitly several methods have been developed, such as the microscopic cluster model [12,13], the continuum shell model [14] and the Gamow shell model [15,16]. It is, however, difficult to satisfy multiparticle decay conditions correctly for all open channels.…”