Symmetry Breaking by Proton-Neutron Pairing

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 $Napprox Z$ nuclei. For a substantial $t=0$ field, the spectra of even-even and odd-odd $Napprox Z$ nuclei become similar. The possibility of a rotationally induced $ J=1$ pair-field at high spin is considered.