Nucleon structure functions from the instanton vacuum: Leading and non-leading twists

Abstract

We review the description of nucleon structure functions in the instanton vacuum. This includes the calculation of the twist-2 parton distributions at a low normalization point as well as higher-twist matrix elements. The instanton vacuum with its inherent small parameter, the packing fraction of the instanton medium, $\bar\rho / R$, provides a consistent picture of the non-perturbative gluon degrees of freedom at the scale $\bar\rho^{-1} \simeq 600 MeV$. The twist-2 quark and antiquark distributions are of order unity, while the twist-2 gluon distribution is of order $(\bar\rho / R)^4$. Twist-4 matrix elements determining power corrections to the Bjorken, Ellis-Jaffe and Gross-Llewellyn-Smith sum rules are found to be of order $(\bar\rho / R)^0$. We present numerical estimates for the parametrically large quantities.