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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 $\M^2$\ and the other by $Λ_{QCD}^2/(1-x)$. When these two scales are comparable, the twist expansion of Perturbative QCD breaks down. We discuss the dynamics in this new QCD limit, where $μ^2=\M^2(1-x)$ is held fixed as $\M^2\to \infty$. 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 $μ^2$. This leads, in particular, to a nuclear target dependence of $A^{2/3}$ at small $μ$. Qualitatively, the dynamics in the new limit agrees with earlier phenomenological models of ``intrinsic'' heavy quark production.