First Measurement of the Angular Coefficients of Drell-Yan $e^{+}e^{-}$ pairs in the Z Mass Region from $p\bar{p}$ Collisions at $\sqrt{s}$ = 1.96 TeV

TL;DR

The angular distributions are studied as a function of the transverse momentum of the electron-positron pair and show good agreement with the Lam-Tung relation, consistent with a spin-1 description of the gluon.

Abstract

We report on the first measurement of the angular distributions of final state electrons in $p\bar{p}\to γ^{*}/Z\to e^{+}e^{-}+X$ events produced in the $Z$ boson mass region at $\sqrt{s}=1.96$ TeV. The data sample collected by the CDF II detector for this result corresponds to 2.1 fb$^{-1}$ of integrated luminosity. The angular distributions are studied as a function of the transverse momentum of the electron-positron pair and show good agreement with the Lam-Tung relation, consistent with a spin-1 description of the gluon, and demonstrate that at high values of the transverse momentum, $Z$ bosons are produced via quark anti-quark annihilation and quark-gluon Compton processes.

Figures (3)

• Figure 1: Di-electron $P_{T}$ spectrum of data, default (CDF Tuned) pythia prediction, and backgrounds (QCD and electroweak process). The mass range corresponds to $66 < M_{ee} < 116$ GeV/c$^{2}$.
• Figure 2: The $\cos\theta$ distribution of data and default (CDF Tuned) pythia prediction.
• Figure 3: Comparison of the measured values of $A_0$, $A_2$, $A_3$ and $A_4$ (for $66<M_{ee}<116$ GeV/c$^2$), shown with statistical and systematic uncertainties combined in quadrature, to theory predictions. The data are plotted at the mean $P_T$ of the events for each bin. The last bin corresponds to $P_T > 55$ GeV/c with no upper limit. The horizontal uncertainty is RMS of the transverse momenta in each bin. Agreement chi is found with the predictions of fewz and powheg (shown) , and also with dyrad , madgraph , and pythia $~Z$ +1-jet MC (not shown). The data do not favor chi the predictions of default pythia , and vbp . Also shown are the pure $q\bar{q}\to \gamma^{*}/Z~G$ annihilation diagram prediction and the $qG \to \gamma^{*}/Z~q$ Compton process prediction from the pythia $~Z$ +1-jet MC.