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We present a decomposition of the hadronic tensor for a general polarizedDrell-Yan process AB --> l^+ l^- X in terms of helicity structure functionsusing the helicity axes of the dilepton rest frame as a basis. Next, weconsider a QCD parton model and in the framework of a generalized QCDfactorization scheme, which applies when the dilepton invariant mass M is muchlarger than the transverse component of the photon momentum, q_T, in thehadronic center of mass frame. In this approximation we compute the angulardistribution of the unpolarized Drell Yan cross section and of the Siverseffect, in single polarized Drell-Yan scattering, by taking into account thetransverse motion of partons inside the initial hadrons, k_\perp. Interestingand simple results are found in the kinematical region of k_\perp similar toq_T << M, to first order in a q_T / M expansion. Finally, explicit analyticalexpressions, convenient for phenomenological studies, are obtained assuming afactorized Gaussian dependence on intrinsic momenta for the unpolarized andSivers distribution functions, in analogy to those obtained for semi-inclusivedeep inelastic scattering.

Polarized and unpolarized Drell-Yan angular distribution in the helicity formalism