New QCD effects at large x

In heavy quark production at large Feynman x there are two hardness scales,one given by the heavy quark pair mass M2 and the other by ΛQCD2/(1-x). When the two scales are comparable, the twist expansion of Perturbative QCD breaks down. I discuss the dynamics in this new QCD limit, where μ2 = M2(1-x) is held fixed as M2 → ∞. New diagrams are found to contribute, which can enhance the cross section above that expected for leading twist. The heavy quarks are produced by a peripheral scattering on the target of hardness μ2. This leads, in particular, to a nuclear target dependence of A2/3 at small μ. Qualitatively, the dynamics in the new limit agrees with earlier phenomenological models of ''intrinsic'' heavy quark production. 7 figs., 45 refs. (author)