Intruder states and shape mixing at the N=8 shell closure from ab initio calculations

In contrast to “normal” states, for which the structure is well described by valence space configurations, in “intruder” states nucleons are excited out of the valence shell, providing access to a much larger configuration space. This allows intruder states to develop highly collective structure and greater deformation. Intruder states are notoriously challenging to describe in ab initio calculations, appearing too high in the excitation spectrum, if at all. However, with suitably soft interactions and sufficiently large-scale calculations, it becomes possible to reproduce intruders at realistic energies in the low-lying spectrum. In this talk, we explore low-lying intruder structure in ab initio no-core configuration interaction calculations, for nuclei at the neutron N=8 shell closure. We find that mixing between normal and intruder configurations, of different deformation (shape), can be described as two-state mixing, permitting extraction of a consistent mixing matrix element from the ab initio calculations.