Intruder band mixing in an Description of Be
Anna E. McCoy (Physics Division, Argonne National Laboratory, Argonne, USA, Facility for Rare Isotope Beams, Michigan State University, East Lansing, USA, Institute for Nuclear Theory, University of Washington, Seattle, USA, Washington University in Saint Louis, Saint Louis, USA)
; Mark A. Caprio (Department of Physics and Astronomy, University of Notre Dame, Notre Dame, USA)
; Pieter Maris (Department of Physics and Astronomy, Iowa State University, Ames, USA)
; Patrick J. Fasano (Physics Division, Argonne National Laboratory, Argonne, USA, Department of Physics and Astronomy, University of Notre Dame, Notre Dame, USA)
The spectrum of Image 1 exhibits exotic features, e.g., an intruder ground state and shape coexistence, normally associated with the breakdown of a shell closure. While previous phenomenological treatments indicated the ground state has substantial contributions from intruder configurations, it is only with advances in computational abilities and improved interactions that this intruder mixing is observed in ab initio no-core shell model (NCSM) predictions. In this work, we extract electromagnetic observables and symmetry decompositions from the NCSM wave functions to demonstrate that the low-lying positive parity spectrum can be explained in terms of mixing of rotational bands with very different intrinsic structure coexisting within the low-lying spectrum. These observed bands exhibit an approximate symmetry and are qualitatively consistent with Elliott model predictions.