Magnetic dipole excitations in nuclei: elementary modes of nucleonic motion

TL;DR

This paper reviews magnetic dipole excitations in nuclei, highlighting experimental findings of various organized modes of nucleonic motion and their implications for understanding nuclear structure and many-body systems.

Contribution

It synthesizes recent experimental data on magnetic dipole modes and discusses their significance in the broader context of many-body physics beyond nuclear systems.

Findings

Evidence for low-lying orbital scissors mode around 3 MeV

Observation of higher-lying spin-flip strength in 5-9 MeV range

Discovery of low-lying proton-neutron isovector quadrupole excitations

Abstract

The nucleus is one of the most multi-faceted many-body systems in the universe. It exhibits a multitude of responses depending on the way one 'probes' it. With increasing technical advancements of beams at the various accelerators and of detection systems the nucleus has, over and over again, surprised us by expressing always new ways of 'organized' structures and layers of complexity. Nuclear magnetism is one of those fascinating faces of the atomic nucleus we discuss in the present review. We shall not just limit ourselves to presenting the by now very large data set that has been obtained in the last two decades using various probes, electromagnetic and hadronic alike and that presents ample evidence for a low-lying orbital scissors mode around 3 MeV, albeit fragmented over an energy interval of the order of 1.5 MeV, and higher-lying spin-flip strength in the energy region 5 - 9 MeV in deformed nuclei, nor to the presently discovered evidence for low-lying proton-neutron isovector quadrupole excitations in spherical nuclei. To the contrary, we put the experimental evidence in the perspectives of understanding the atomic nucleus and its various structures of well-organized modes of motion and thus enlarge our discussion to more general fermion and bosonic many-body systems.