Neutron spin-dependent structure function, Bjorken sum rule, and first evidence for singlet contribution at low $x$

We perform the isospin decomposition of proton and neutron SLAC data for moderately low $x$ in the region $0.01 \leq x \leq 0.1$. The isovector part is well described by a power behaviour $x^{\alpha}$, where $\alpha$ does not correspond to the intercept of single $a_1$ Regge trajectory, as expected. However, the observed power leads to the validity of Bjorken sum rule and it is consistent with the power extracted from all previous data using NLO evolution. At the same time, the isoscalar part behaviour may be interpreted as a partial cancellation between a positive non-singlet contribution and a strongly negative singlet one. Further experimental consequences are mentioned.