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 ${f}_{2}$ 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 ${1/N}_{c}$-expansion. The corresponding twist-3 spin-2 matrix elements ${d}_{2}$ are suppressed in the packing fraction of the instanton medium, $\overline{\ensuremath{\rho}}/\overline{R}\ensuremath{\ll}1.$ We use our results to estimate the leading ${1/Q}^{2}$ power corrections to the first moment of the proton and neutron spin structure functions ${G}_{1},$ as well as the intrinsic charm contribution to the nucleon spin.