Interpretation of angular distributions of Z-boson production at colliders

Abstract

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, qT, up to ∼300 GeV. Pronounced qT 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 qT 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 qT dependencies of λ and ν from the CMS and the earlier CDF data can be well described using this approach.

Keywords

• Drell–Yan process
• Lam–Tung relation
• QCD