Results on spin sum rules and polarizabilities at low $Q^2$

We 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 $\delta_\mathrm{LT}(Q^2)$ (for the neutron). The data were taken at Jefferson Lab in Hall A by experiment E97110 (neutron) and in Hall B by experiments E03006 and E05111 (proton and deuteron, respectively). The experiments covered the very low $Q^2$ domain, down to $Q^2 \simeq 0.02$ GeV$^2$. This is well into the domain where Chiral Effective Field Theory ($\chi$EFT) predictions should be valid. Some measured observables agree with the state-of-the-art $\chi$EFT predictions but others are in tension, including $\delta_\mathrm{LT}^n(Q^2)$ which $\chi$EFT prediction was expected to be robust. This suggests that $\chi$EFT does not yet consistently describe nucleon spin observables, even at the very low $Q^2$ covered by the experiments.