Interpretation of angular distributions of Z-boson production at colliders

High precision data of dilepton angular distributions in γ*/Z production were reported recently by the CMS Collaboration covering a broad range of the dilepton transverse momentum, q_T, up to ~300 GeV. Pronounced q_T dependencies of the λ and ν parameters, characterizing the cos² θ and cos 2φ angular distributions, were found. Violation of the Lam-Tung relation was also clearly observed. We show that the q_T dependence of λ allows a determination of the relative contributions of the qq annihilation versus the qG Compton process. The violation of the Lam-Tung relation is attributed to the presence of a non-zero component of the q-q axis in the direction normal to the "hadron plane" formed by the colliding hadrons. The magnitude of the violation of the Lam-Tung relation is shown to reflect the amount of this 'non-coplanarity". The observed q_T dependencies of λ and ν from the CMS and the earlier CDF data can be well described using this approach.