The 10th International Workshop on Chiral Dynamics

We will report on recently published experimental results on spin sum rules, and particularly on the generalized spin polarizabilities $\gamma_0(Q^2)$ (for both the proton and neutron) and $\delta_{LT}(Q^2)$ (for the neutron). The data were taken at Jefferson Lab in Hall A (neutron) and B (proton and deuteron) by experiments E97-110 and EG4, respectively. They covered the very low $Q^2$ domain, down to $Q^2 \sim 0.02$ GeV$^2$, where Chiral Effective Field Theory ($\chi$EFT) predictions should be valid. While some obervables agree with the state-of-the-art $\chi$EFT theoretical predictions, others are in tensions, including $\delta_{LT}^n(Q^2)$ for which $\chi$EFT prediction was expected to be robust. This suggests that $\chi$EFT does not yet consistently describe nucleon spin observables, even in the very low $Q^2$ domain covered by the experiments.