Twist-4 contribution to unpolarized structure functions FL and F2 from instantons

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 F-2 Our approach takes into account the leading contribution in the packing fraction of the instanton medium, <(rho)over bar>/(R) over bar << 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/<(rho)over bar>(2) = (600 MeV)(2). The matrix elements of the four-fermion (diquark) operators are suppressed by a factor (<(P)over bar>/(K) over bar)(4) and numerically small. These results are in agreement with the pattern of phenomenological 1/Q(2)-corrections to R = sigma(L)/sigma(T) and F2 found in QCD fits to the data. In particular, the rise of R at low Q(2) can be obtained from instanton-type vacuum fluctuations at a low scare. (C) 2000 Elsevier Science B.V. All rights reserved.