Twist-4 nuclear parton distributions from photoproduction

We analyze anomalous nuclear enhancement in the photoproduction of jets as a higher-twist process in perturbative QCD. We use the Fermilab E683 data on dijet momentum imbalance to estimate the size of the relevant twist-4 parton distributions. We find that twist-4 matrix elements are of the order of 0.05-0.1 ${\mathrm{GeV}}^2$ times typical twist-2 parton distributions. We discuss a physical interpretation of the nuclear enhancement as a measure of the average net transverse color force on an off-shell parton moving through nuclear matter, and give an order-of-magnitude estimate for the typical squared transverse field strengths encountered by a fast moving parton.