Longitudinal Spin Dependence of Massive Lepton Pair Production

We show that massive lepton-pair production, the Drell-Yan process, should be a good source of independent constraints on the polarized gluon density, free from the experimental and theoretical complications of photon isolation that beset studies of prompt photon production. We provide predictions for the spin-averaged and spin-dependent differential cross sections as a function of transverse momentum $Q_T$ at energies relevant for the Relativistic Heavy Ion Collider (RHIC) at Brookhaven.