${}^6$He structure from semiclassical 3D models based on $\text{ab initio}$ spatial correlations

Abstract

We construct semiclassical 3D models based on $\text{ab initio}$ no-core shell model calculated two-body spatial correlations, which represent the preferred spatial configuration of nucleons for the ground state of ${}^6$He. We demonstrate that the presence of the two valence neutrons distorts and flattens the $\alpha$ core in ${}^6$He, marking a clear departure from the tetrahedral configuration of free ${}^4$He, and that the two valence neutrons and the center of mass of the $\alpha$ cluster ($\alpha$-CM) are roughly located at the three distinct vertices of an equilateral triangle. We compute the geometric parameters that commonly characterize ${}^6$He structure, 1) ${\theta }_{nn}$, the opening angle of the two valence neutrons with respect to $\alpha$-CM, and 2) $r_{nn}$, the root-mean-square separation between the two valence neutrons. Refining the model with uncertainty estimation, we obtain ${\theta }_{nn}=58.5_{-6.2}^{+5.4}$ degrees and $r_{nn}=4.12_{-0.43}^{+0.37}$ fm, agreeing with those calculated via a phenomenologically modified continuum-discretized coupled-channels method, while an interferometry experiment obtained a marginally higher $r_{nn}$.

Comments: 10 pages, 8 figures, 2 tables