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arXiv · hep-ph/9906444

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

B. Dressler·M. Maul·C. Weiss
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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.

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B. Dressler, M. Maul, C. Weiss. 1999-06-23. Twist-4 contribution to unpolarized structure functions F_L and F_2 from instantons. https://doi.org/10.1016/s0550-3213(00)00024-9

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