Effects of initial-state QCD interactions in the Drell-Yan process

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Abstract
We develop a simple, intuitive picture of the effects, in the Drell-Yan process, of initial-state interactions between the projectile and the target. For purposes of illustration, we present specific analyses in terms of a nonrelativistic QED model. However, our principal conclusions are valid in QCD as well. We show that initial-state interactions lead to an increase in the average of the square of the transverse momentum of the lepton pair that is proportional to the length of the target, and we also demonstrate that initial-state interactions can invalidate the parton-model (factorized) form of the cross section unless the beam energy is greater than a scale that grows with the length of the target.