Twist-4 contribution to unpolarized structure functions F_L and F_2 from instantons

TL;DR

This paper calculates twist-4 contributions to unpolarized structure functions F_L and F_2 using the instanton vacuum model, highlighting the dominant quark-gluon operators and their agreement with experimental QCD fits.

Contribution

It provides a novel calculation of twist-4 matrix elements in the instanton vacuum, emphasizing the leading role of quark-gluon operators and their phenomenological relevance.

Findings

Matrix elements of twist-4 quark-gluon operators are of order (600 MeV)^2.

Four-fermion operator matrix elements are suppressed and numerically small.

Results align with observed 1/Q^2 corrections in experimental data.

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, F_L and F_2. 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/rho^2 = (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/Q^2-corrections to R = sigma_L / sigma_T and F_2 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 scale.