Symmetry Breaking by Proton-Neutron Pairing

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The symmetries of the t=1 and t=0 pair-fields are different. The consequences for rotational spectra are discussed. For t=1, the concept of spontaneous breaking and subsequent restoration of the isospin symmetry turns out to be important. It permits us to describe the proton-neutron pair-correlation within the conventional frame of pairing between like particles. The experimental data are consistent with the presence of a t=1 field at low spin in N≈ Z nuclei. For a substantial t=0 field, the spectra of even-even and odd-odd N≈ Z nuclei become similar. The possibility of a rotationally induced J=1 pair-field at high spin is considered.