TWIST-4 NUCLEAR PARTON DISTRIBUTIONS FROM PHOTOPRODUCTION

We analyze anomalous nuclear enhancement in the photoproduction of jet s as a higher-twist process in perturbative QCD. We use the Fermilab E 683 data on dijet momentum imbalance to estimate the size of the relev ant twist-4 parton distributions. We find that twist-4 matrix elements are of the order of 0.05-0.1 GeV2 times typical twist-2 parton distri butions. We discuss a physical interpretation of the nuclear enhanceme nt as a measure of the average net transverse color force on an off-sh ell parton moving through nuclear matter, and give an order-of-magnitu de estimate for the typical squared transverse field strengths encount ered by a fast moving parton.