Low-lying isovector monopole resonances
  The mass difference between the even-even isobaric nuclei having the valence nucleons on the same degenerate level is attributed to a Josephson-type interaction between pairs of protons and pairs of neutrons. This interaction can be understood as an isospin symmetry-breaking mean field for a four-particle interaction separable in the two particles-two holes channel. The strength of this mean field is estimated within an o(5) algebraic model, by using the experimental value of the inertial parameter for the collective isorotation induced by the breaking of the isospin symmetry. In superfluid nuclei, the presumed interaction between the proton and neutron condensates leads to coupled oscillations of the BCS gauge angles, which should appear in the excitation spectrum as low-lying isovector monopole resonances. 
M. Grigorescu