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 $\lambda$, $\mu$, $\nu$ and the degree of violation of the Lam-Tung relation, $1-\lambda-2\nu$, 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 $\lambda$ and $\nu$ show a clear dependence on the dimuon mass ($Q$) while those of $\mu$ have a strong rapidity ($x_F$) dependence. Furthermore, $\lambda$ and $\nu$ 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.
Highlights
The Drell-Yan (D-Y) process [1] is one of the important experimental approaches to explore the partonic structure of hadrons [2]
Overall our studies show that salient features of Qualitatively the transverse momentum (qT)=Q scaling and xF dependency for the λ, ν, μ parameters of fixed-target D-Y experiments evaluated by next-to-leading order (NLO) pQCD as well as the L-T violation 1 − λ − 2ν from the NNLO pQCD can be nicely understood using the geometric picture
We have presented a comparison of the measurements of the angular parameters λ, μ, ν and 1 − λ − 2ν of the D-Y process from the fixed-target experiments with the corresponding results from the NLO and NNLO pQCD calculations
Summary
The Drell-Yan (D-Y) process [1] is one of the important experimental approaches to explore the partonic structure of hadrons [2]. We interpreted the violation of the L-T relation as a consequence of the acoplanarity of the partonic subprocess [21,22] This acoplanarity can arise from intrinsic transverse momenta of partons inside the hadrons, or from the perturbative gluon radiation beyond OðαsÞ such that the axis of the annihilating quark-antiquark pair (natural axis) no longer necessarily resides on the colliding hadron plane. In addition to the violation of the L-T relation, other salient features of the qT dependence of the λ, μ and ν parameters of the Z production data from the collider experiments [21,22], as well as the rotational invariance properties of these parameters [23], could be well explained by this intuitive geometric approach.
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