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In reactions with $q \bar q$ production or $q\bar q$ annihilation, initial-and final-state interactions give rise to large corrections to the lowest-ordercross sections. We evaluate the correction factor first for low relativekinetic energies by studying the distortion of the relative wave function. Wethen follow the procedure of Schwinger to interpolate this result with thewell-known perturbative QCD vertex correction factors at high energies, toobtain an explicit semi-empirical correction factor applicable to the wholerange of energies. The correction factor predicts an enhancement for $q\bar q$in color-singlet states and a suppression for color-octet states, the effectincreasing as the relative velocity decreases. Consequences on dileptonproduction in the quark-gluon plasma, the Drell-Yan process, and heavy quarkproduction processes are discussed.Comment: 25 pages (REVTeX), includes 2 uuencoded compressed postscript figure

Correction factors for reactions involvingqq¯annihilation or production