Interpretation of angular distributions of Z-boson production at colliders

High precision data of dilepton angular distributions in γ⁎/Zγ⁎/Z production were reported recently by the CMS Collaboration covering a broad range of the dilepton transverse momentum, qTqT, up to ∼300 GeV∼300 GeV. Pronounced qTqT dependencies of the λ and ν   parameters, characterizing the cos2⁡θcos2⁡θ and cos⁡2ϕcos⁡2ϕ angular distributions, were found. Violation of the Lam–Tung relation was also clearly observed. We show that the qTqT 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 qTqT dependencies of λ and ν from the CMS and the earlier CDF data can be well described using this approach.