Intruder Band Mixing in an Ab Initio Description of 12Be

The spectrum of 12Be exhibits exotic features, e.g., an intruder ground stateand shape coexistence, normally associated with the breakdown of a shellclosure. While previous phenomenological treatments indicated the ground statehas substantial contributions from intruder configurations, it is only withadvances in computational abilities and improved interactions that thisintruder mixing is observed in ab initio no-core shell model (NCSM)predictions. In this work, we extract electromagnetic observables and symmetrydecompositions from the NCSM wave functions to demonstrate that the low-lyingpositive parity spectrum can be explained in terms of mixing of rotationalbands with very different intrinsic structure coexisting within the low-lyingspectrum. These observed bands exhibit an approximate SU(3) symmetry and arequalitatively consistent with Elliott model predictions.