Abstract. We present a symmetry-based approach for shape coexistence in nuclei, founded on the concept of partial dynamical symmetry (PDS). The latter corresponds to a situation when only selected states (or bands of states) of the coexisting configurations preserve the symmetry while other states are mixed. We construct explicitly critical-point Hamiltonians with two or three PDSs of the type U(5), SU(3), SU(3) and SO(6), appropriate to double or triple coexistence of spherical, prolate, oblate and γ-soft deformed shapes, respectively. In each case, we analyze the topology of the energy surface with multiple minima and corresponding normal modes. Characteristic features and symmetry attributes of the quantum spectra and wave functions are discussed. Analytic expressions for quadrupole moments and E2 rates involving the remaining solvable states are derived and isomeric states are identified by means of selection rules.Keywords: dynamical symmetry, partial dynamical symmetry, shape coexistence in nuclei, interacting boson model.
IntroductionThe presence in the same nuclei, at similar low energies, of two or more sets of states which have distinct properties that can be interpreted in terms of different shapes, is a ubiquitous phenomena across the nuclear chart [1,2] [11,12], and the triple coexistence of spherical, prolate and oblate shapes in 186 Pb [13]. A detailed microscopic interpretation of nuclear shape-coexistence is a formidable task. In a shell model description of nuclei near shell-closure, it is attributed to the occurrence of multi-particle multi-hole intruder excitations across shell gaps. For medium-heavy nuclei, this necessitates drastic truncations of large model spaces, e.g., by Monte Carlo sampling [14,15] or by a bosonic approximation of nucleon pairs [16][17][18][19][20][21][22][23][24][25]. In a mean-field approach, based on energy density functionals, the coexisting shapes are associated with different minima of an energy surface calculated self-consistently. A detailed comparison with spectroscopic observables requires beyond mean-field methods, including restoration of broken symmetries and configuration mixing of angular-momentum and particle-number projected states [26,27]. Such extensions present a major computational effort and often require simplifying assumptions such as axial symmetry and/or a mapping to collective model Hamiltonians [23][24][25][26][27][28].A recent global mean-field calculation of nuclear shape isomers identified experimentally accessible regions of nuclei with multiple minima in their potential-energy surface [29,30]. Such