Nucleon spin content and axial coupling constants in QCD sum rules approach

The review of current experimental situation in the measurements of the first moment ${\mathrm{\Gamma }}_{p,n}$ of spin dependent nucleon structure functions $g_{1;p,n}(x,Q^2)$ is presented. The results of the calculations of twist-4 corrections to ${\mathrm{\Gamma }}_{p,n}$ are discussed and their accuracy is estimated. The part of the proton spin $\mathrm{\Sigma }$ carried by $u,d,s$ quarks is calculated in the framework of the QCD sum rules in the external fields. The operators up to dimension 9 are accounted. An important contribution comes from the operator of dimension 3, which in the limit of massless $u,d,s$ quarks is equal to the derivative of QCD topological susceptibility ${\chi }^{\mathrm{\prime }}(0)$. The comparison with the experimental data on $\mathrm{\Sigma }$ gives $\chi’(0)= (2.3 \pm 0.6) \times 10^{-3} GeV^2$\mathrm{.}Thelimitson$\Sigma$and$\chi’(0)$ are found from selfconsistency of the sum rule, $\mathrm{\Sigma }\text{ga}0.05,{\chi }^{\mathrm{\prime }}(0)\text{ga}1.6\times {10}^{-3}Ge{V}^2$. The values of $g_A=1.37\pm 0.10$ and $g_A^8=0.65\pm 0.15$ are also determined from the corresponding sum rules.