Partial dynamical SU(3) symmetry and the nature of the lowest K=0 collective excitation in deformed nuclei

We discuss the implications of partial dynamical SU(3) symmetry (PDS) for the structure of the lowest [Formula Presented] collective excitation in deformed nuclei. We consider an interacting boson model Hamiltonian whose ground and [Formula Presented] bands have good SU(3) symmetry, while the [Formula Presented] band is mixed. It is shown that the double-phonon components in the [Formula Presented] wave function arise from SU(3) admixtures which, in turn, can be determined from absolute [Formula Presented] rates connecting the [Formula Presented] and ground bands. An explicit expression is derived for these admixtures in terms of the ratio of [Formula Presented] and [Formula Presented] bandhead energies. The SU(3) PDS predictions are compared with existing data and with broken-SU(3) calculations for [Formula Presented]Er.