NEW QCD PRODUCTION MECHANISMS FOR HARD PROCESSES AT LARGE-X

We study the QCD production mechanisms for lepton and quark pairs of high mass M that carry a large fraction x of the projectile momentum, We show that the dominant contribution comes from peripheral processes in which low-x spectator quarks interact with the target, with the hardness scale of the collision being given by Q2 = M2(1 - x). In the high-M2 limit with fixed M2(1 - x) we identify new leading-order perturbative contributions from the hadron wave function which involve more than one constituent. These "intrinsic" contributions cannot be expressed in terms of the usual single-parton structure functions, implying a breakdown of QCD factorization. In a numerical study of a simple gauge theory model. we show that such contributions can dominate the standard single-parton factorizable terms. These results appear to explain several anomalies seen in the data: the excess production and the anomalous nuclear-number of dependence of open and bound charm at large x; the "cumulative" effects (x > 1) observed in hadron production from nuclei; and the large target-polarization asymmetry observed for hadron production at high x. The intrinsic multi-parton processes provide new mechanisms for the hadro-production of the J/psi directly in a color-singlet state and also for the production of heavy flavor systems in lepto-production at high momentum.