PERSISTENCE OF ROTATIONAL STRUCTURE IN NUCLEI AND QUASI-DYNAMICAL SYMMETRY

Abstract

The existence of quasi-dynamical symmetry (QDS) in physical systems and its significance for understanding the persistence of rotational structure in nuclei is explained in terms of the mathematical concept of an embedded representation. We consider the spectra obtained by coupling two SU (3) irreps by means of a quadrupole-quadrupole interaction. For a particular large value of this interaction, the two irreps combine to form a single (strongly-coupled) irrep while for zero interaction the weakly-coupled results are mixtures of many irreps. A notable result is the persistence of the rotor character of the low-energy states for a wide range of the interaction strength which can be explained by coherent mixing of SU (3) irreps. Such a coherent mixing of representations is an indication that the model has a QDS. Also notable is the fact that, for very weak interaction strengths, the rotational states of the yrast band approach those of a vibrational sequence while the B( E 2) transition strengths remain close to those of an axially symmetric rotor.