Lepton angular distributions of fixed-target Drell-Yan experiments in perturbative QCD and a geometric approach

The lepton angular distributions of the Drell-Yan process in fixed-target experiments are investigated by NLO and NNLO perturbative QCD. We present the calculated angular parameters λ, μ, ν and the degree of violation of the Lam-Tung relation, 1-λ-2ν, for the NA10, E615 and E866 experiments. Predictions for the ongoing COMPASS and SeaQuest experiments are also presented. The transverse momentum (qT) distributions of λ and ν show a clear dependence on the dimuon mass (Q) while those of μ have a strong rapidity (xF) dependence. Furthermore, λ and ν are found to scale with qT/Q. These salient features could be qualitatively understood by a geometric approach where the lepton angular distribution parameters are expressed in terms of the polar and azimuthal angles of the "natural axis" in the dilepton rest frame.