Collective 2+ states in the U(5) classical limit of the proton-neutron interacting boson model

The U(5) classical limit of the proton-neutron interacting boson model Hamiltonian, appropriate for the collective 2+ states, is investigated. The symmetric and mixed-symmetry 2+ states are identified as arising from the boson-number-weighted symmetric and antisymmetric vibrations in the beta degree of freedom, respectively. Also approximate formulas for the energies away from the U(5) limit and for the reduced matrix elements of the E2 and M1 transition operators are given.