A diagrammatic approach to quark exchange processes in meson-meson scattering is applied to the case of inelastic reactions of the type $(Q\barQ)+(q\barq)\rightarrow (Q\barq) + (q\barQ)$, where $Q$ and $q$ refer to heavy and light quarks, respectively. This string-flip process is discussed as a microscopic mechanism for charmonium dissociation (absorption) in hadronic matter. The cross section for the reaction $J/\psi + \pi \to D+ \bar D$ is calculated using a potential model, which is fitted to the meson mass spectrum. The temperature dependence of the relaxation time for the $\J/Psi$ distribution in a homogeneous thermal pion gas is obtained. The use of charmonium for the diagnostics of the state of hot hadronic matter produced in ultrarelativistic nucleus-nucleus collisions is discussed.Comment: 24 pages, 3 tables, 7 figure

The cross section for Drell-Yan pair production in the limit of small x target is derived in the rest frame of the target hadron. Our calculation is based on the fundamental quantity (), the cross section for the scattering of a qq pair with fixed transverse separation off a hadronic target. As in deep inelastic scattering the result can be given in terms of integrals of (). This is consistent with well-known factorization theorems and also relates higher-twist terms in both processes. An analysis of the angular distribution of the produced lepton shows that additional integrals of () can be obtained in the Drell-Yan process, which are not measurable in inclusive deep inelastic scattering. ͓S0556-2821͑97͒02005-5͔

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