Twist-4 contribution to unpolarized structure functions F-L and F-2 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 el ements of a twist-4 quark-gluon operator, which are of the order of the inv erse instanton size, 1/<(rho)over bar>(2) = (600 MeV)(2). The matrix elemen ts 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 agre ement 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 o f 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.