Proton spin content: links to QCD topological susceptibility

The part of the proton spin Σ 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 χ'(0). The comparison with the experimental data on Σ gives χ'(0) = (2.3 ± 0.6) x 10-3 GeV2. The limits on Σ and χ'(0) are found from selfconsistency of the sum rule, Σ≥0.05, χ'(0) ≥ 1.6 x 10-3 GeV2. The values of gA =1.37 ± 0.10 and gA8 = 0.65 ± 0.15 are also determined. (author)