S@INT Seminar: "(Un)welcome intruders: Getting your nucleus to come out of its shell (in ab initio calculations)"

Intruder states, involving shell model configurations in which nucleons are excited out of the valence shell, feature prominently in the excitation spectra of nuclei across the nuclear chart.  In comparison to "normal" states, for which the structure is well described by valence space configurations, "intruder" states have access to a much larger configuration space, allowing them to develop highly collective structure and greater deformation.  Their consequently larger correlation energy permits them to "intrude" into the low-lying spectrum. The proximity of weakly deformed normal states and highly deformed intruder states in the spectrum gives rise to "shape coexistence", with the concomitant possibility of shape mixing.  Intruder states may be found especially low in excitation energy near shell closures, where normal states can muster comparatively little correlation energy.  (In so-called "islands of inversion", an intruder can even take over as the ground state.)

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 welcome these intruders to their rightful place in the low-lying spectrum.  In this talk, we shall explore low-lying intruder structure in ab initio no-core configuration interaction calculations, for nuclei near the N=8 shell closure.  We shall find that mixing between normal and intruder configurations takes on a simple form, permitting extraction of a consistent mixing matrix element for two-state mixing, even though this mixing is entirely an emergent phenomenon. In the ab initio calculations, as in traditional phenomenology, we shall see that electric monopole (E0) transitions can play an important role in diagnosing normal-intruder mixing.