Direct evidence for an orbital magnetic quadrupole twist mode in nuclei

The reactions 58Ni(e,e′) and 58Ni(p,p′) have been studied at kinematics favorable for the excitation of Jπ=2− states by isovector spin-flip transitions with ΔL=1. There are states at an excitation energy Ex≈10 MeV which are strongly excited in electron scattering but not in proton scattering, suggesting a predominantly orbital character. This is taken as direct evidence for the so-called twist mode in nuclei in which different layers of nuclear fluid in the upper and lower hemisphere counterrotate against each other. Microscopic quasiparticle-phonon model calculations which predict sizable orbital M2 strength at this excitation energy yield indeed a current flow pattern of the strongest transitions consistent with a twist-like motion.