The Drell-Yan Process and Factorization in Impact Parameter Space
  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 $\sigma(\rho)$, the cross section for the scattering of a $q\bar{q}$-pair with fixed transverse separation $\rho$ off a hadronic target. As in deep inelastic scattering the result can be given in terms of integrals of $\sigma(\rho)$. 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 $\sigma(\rho)$ can be obtained in the Drell-Yan process, which are not measurable in inclusive deep inelastic scattering. 
S.J. Brodsky, A. Hebecker, and E. Quack