  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.
