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 Γp, n of spin dependent nucleon structure functions g1;P, n (x, Q2) is presented. The results of the calculations of twist-4 corrections to TP, n are discussed and their accuracy is estimated. The part of the proton spin ∑ 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 x'(0)-The comparison with the experimental data on ∑ gives x'(0) = (2.3 ±0.6) × 10−3 GeV2. The limits on ∑ and x'(0) are found from selfconsistency of the sum rule, ∑ = 0.32 ± 0.18, x'(0) = (2.4±0.6)10−3GeV2. The values of gA = 1.37±0.10 and g8 A =0.65±0.15 are also determined from the corresponding sum rules.