Spin-dependent twist-4 matrix elements from the instanton vacuum: Flavor-singlet and nonsinglet
Abstract
We estimate the twist-4 spin-1 nucleon matrix element f2 in an instanton-based description of the QCD vacuum. In addition to the flavor-nonsinglet we compute also the flavor-singlet matrix element, which appears in next-to-leading order of the (1/Nc)-expansion. The corresponding twist-3 spin-2 matrix elements d2 are suppressed in the packing fraction of the instanton medium, ( )/( R) << 1. We use our results to estimate the leading (1/Q2) power corrections to the first moment of the proton and neutron spin structure functions G1, as well as the intrinsic charm contribution to the nucleon spin.
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