Twist-4 contribution to unpolarized structure functions FL and F2 from instantons
Abstract
We compute in the instanton vacuum the nucleon matrix elements of the twist-4 QCD operators describing power corrections to the second moments of the unpolarized structure functions, FL and F2. Our approach takes into account the leading contribution in the packing fraction of the instanton medium, rho / R << 1. Parametrically leading are the matrix elements of a twist-4 quark-gluon operator, which are of the order of the inverse instanton size, 1/rho2 = (600 MeV)2. The matrix elements of the four-fermion (diquark) operators are suppressed by a factor (rho / R)4 and numerically small. These results are in agreement with the pattern of phenomenological 1/Q2-corrections to R = sigmaL / sigmaT and F2 found in QCD fits to the data. In particular, the rise of R at low Q2 can be obtained from instanton-type vacuum fluctuations at a low scale.
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