Spin-flip M1 giant resonance as a challenge for Skyrme forces

Despite a great success of the Skyrme mean-field approach in the exploration of nuclear dynamics, it seems to fail in the description of the spin-flip M1 giant resonance. The results for different Skyrme parameterizations are contradictory and poorly agree with experiment. In particular, there is no parameterization which simultaneously describes the one-peak gross structure of M1 strength in doubly magic nuclei and two-peak structure in heavy deformed nuclei. The reason of this mismatch could lie in an unsatisfactory treatment of spin correlations and spin–orbit interaction. We discuss the present status of the problem and possible ways of its solution. In particular, we inspect (i) the interplay of the collective shift and spin–orbit splitting, (ii) the isovector M1 response versus isospin-mixed responses and (iii) the role of tensor and isovector spin–orbit interaction.