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 $\Gamma_{p,n}$ 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 $\Gamma_{p,n}$ are discussed and their accuracy is estimated. The part of the proton spin $\Sigma$ 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 $\chi' (0)$. The comparison with the experimental data on $\Sigma$ gives $\chi'(0)= (2.3 \pm 0.6) \times 10^{-3} GeV^2$. The limits on $\Sigma$ and $\chi'(0)$ are found from selfconsistency of the sum rule, \Sigma \ga 0.05, \chi' (0) \ga 1.6 \times 10^{-3} GeV^2. The values of $g_A = 1.37 \pm 0.10$ and $g^8_A = 0.65 \pm 0.15$ are also determined from the corresponding sum rules.Comment: Lecture at St.Petersburg Winter School on Theoretical Physics, Febr. 23-28, 1998 to be published in Proceedings, 14 pages, LaTeX 2e, 4 eps figures included using epsf.st