Measurement of the Bjorken Sum at very low $Q^2$

We present new data on the Bjorken sum $\overline \Gamma_1^{p-n}(Q^2)$ at 4-momentum transfer $ 0.021 \leq Q^2 \leq 0.496$ GeV$^2$. The data were obtained in two experiments performed at Jefferson Lab: EG4 on polarized protons and deuterons, and E97110 on polarized $^3$He from which neutron data were extracted. The data cover the domain where chiral effective field theory ($\chi$EFT), the leading effective theory of the Strong Force at large distances, is expected to be applicable. We find that our data and the predictions from $\chi$EFT are only in marginal agreement. This is somewhat surprising as the contribution from the $\Delta(1232)$ resonance is suppressed in this observable, which should make it more reliably predicted by $\chi$EFT than quantities in which the $\Delta$ contribution is important. The data are also compared to a number of phenomenological models with various degrees of agreement.