Measuring lepton angular distributions of Drell-Yan process provides a powerful tool to explore the reaction mechanisms and the parton distributions of colliding hadrons. For example, the Lam-Tung relation has been proposed as a benchmark of the pQCD effect in Drell-Yan process. Violations of Lam-Tung relation were observed in the measurements of fixed-target experiments as well as $\gamma^*$/$Z$ production in the large transverse-momentum regions recently measured by the CMS and ATLAS experiments at LHC. It is important to understand the origin of these violations.
In this talk, we present a comparison of data with the fixed-order pQCD calculations by which the baseline of pQCD effects is illustrated. Then using an intuitive geometric approach, we show that these dependencies can be readily understood. The violation of the Lam-Tung relation, appearing at large transverse-momentum region, is attributed to the presence of a “non-coplanarity” effect. This interpretation is consistent with the appearance of violation beyond LO-QCD effect. Furthermore, we predict that $A_0$ and $A_2$ for $Z$ plus single gluon-jet events are very different from that of $Z$ plus single quark-jet events, allowing a new experimental tool for checking various algorithms which attempt to discriminate quark jets from gluon jets.
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