Intruder band mixing in an <em>ab initio</em> description of ¹²Be

The spectrum of ¹²Be 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 SU(3) symmetry and are qualitatively consistent with Elliott model predictions.. Authors: McCoy, Anna E. [Argonne National Laboratory (ANL), Argonne, IL (United States); Univ. of Washington, Seattle, WA (United States); Michigan State Univ., East Lansing, MI (United States); Washington Univ., St. Louis, MO (United States)] (ORCID:0000000210331474); Caprio, Mark A. [University of Notre Dame, IN (United States)] (ORCID:0000000151383740); Maris, Pieter [Iowa State Univ., Ames, IA (United States)] (ORCID:0000000213517098); Fasano, Patrick J. [Argonne National Laboratory (ANL), Argonne, IL (United States); University of Notre Dame, IN (United States)] (ORCID:0000000324574976). DOE Contract: AC02-05CH11231; AC02-06CH11357; FG02-00ER41132; FG02-95ER40934; SC0013617; SC0021027; SC0023495. Subjects: 72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS