Direct Evidence for Proton-Neutron Out-of-Phase Coupling in Mixed-Symmetry States from Scattering Experiments on ⁹²Zr

The coupling of the giant quadrupole resonance to valence space configurations is shown to dominantly contribute to the formation of low-lying quadrupole collective structures in vibrational nuclei with symmetric and mixed-symmetric character with respect to the proton-neutron degree of freedom. Experimental evidence is obtained from electron- and proton scattering experiments on the nucleus ⁹²Zr that are directly sensitive to the relative phase of valence space amplitudes by quantum interference.