Drell-Yan process and factorization in impact parameter space

The cross section for Drell-Yan pair production in the limit of small $x_{\mathrm{target}}$ is derived in the rest frame of the target hadron. Our calculation is based on the fundamental quantity $\sigma \left(\rho \right)$, the cross section for the scattering of a $q\mathrm{q¯}$ 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 $\sigma \left(\rho \right)$. 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 $\sigma \left(\rho \right)$ can be obtained in the Drell-Yan process, which are not measurable in inclusive deep inelastic scattering.