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

Abstract

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 ($q_T$) distributions of $λ$ and $ν$ show a clear dependence on the dimuon mass ($Q$) while those of $μ$ have a strong rapidity ($x_F$) dependence. Furthermore, $λ$ and $ν$ are found to scale with $q_T/Q$. 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.

published version, two-column format, 10 pages, 14 figures