New QCD Effects at Large x

Abstract

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

Comments: (Lectures at the XXXII Cracow School of Theoretical Physics, Zakopane 1992), 22 pages, 7 figures not included (available on request), phyzzx.tex