Higher-order QCD corrections to exclusive processes: The multiple scattering mechanism

The multiple-scattering diagram, calculated to lowest order in QCD, fits well topp elastic scattering data at larget. It is shown that higherorder QCD corrections seem to spoil this agreement: calculated as far as the insertion of two additional gluons, they give an exponential factor. This factor differs in detail from some previous conjectures. For colour-singlet bound states of quarks, the exponential factor is independent of whether or not the gluon mass is zero, unlike the case of simple quark-quark scatterin. Two régimes are considered:t ands of the same order, andt large but much less thans. Exponentiation seems to occur in both cases, but in a non-abelian theory more diagrams are important for the latter case.