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

Abstract

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 ${\chi }^{\prime }(0)$. The comparison with the experimental data on $\Sigma$ gives ${\chi }^{\prime }(0)=(2.3\pm 0.6)\times 10^{-3}Ge{V}^2$. The limits on $\Sigma$ and ${\chi }^{\prime }(0)$ are found from selfconsistency of the sum rule, $\Sigma \text{ga}0.05,{\chi }^{\prime }(0)\text{ga}1.6\times 10^{-3}Ge{V}^2$. 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.

Comments: 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.sty