Measurement of the Forward-Backward Charge Asymmetry of Electron-Positron Pairs in Proton anti-Proton Collisions at s**(1/2)=1.96-TeV

TL;DR

This study measures the mass-dependent forward-backward asymmetry $A_{FB}$ in dielectron events from proton-antiproton collisions at $\sqrt{s}=1.96$ TeV using 72 pb$^{-1}$ of CDF II data. By employing the Collins-Soper frame and unfolding techniques, it extracts $A_{FB}^{phys}$ in 15 $M_{ee}$ bins and simultaneously determines $Z$-quark couplings, $Z$-electron couplings, and the effective weak mixing angle $\sin^2\theta_W^{eff}$, comparing the results to SM expectations across a wide mass range up to 600 GeV/$c^2$. The analysis carefully accounts for detector acceptance, energy resolution, bremsstrahlung, and backgrounds (notably dijets) with two complementary unfolding approaches, one SM-constrained and one model-independent. The uncorrected distributions show excellent agreement with SM predictions, and the SM-constrained extraction yields couplings consistent with the SM within uncertainties, demonstrating the method's viability and setting the stage for improved precision with larger Run II datasets. Overall, the work provides a robust, data-driven test of electroweak couplings in the Drell-Yan channel at the Tevatron and a framework for precision EW parameter extraction from $A_{FB}$ measurements in hadron collisions.

Abstract

We present a measurement of the mass dependence of the forward-backward charge asymmetry (A_{FB}) for electron-positron pairs produced via an intermediate Z/gamma with mass Mee > 40 GeV/c**(2). We study the constraints on the Z-quark couplings imposed by our measurement. We analyze an integrated luminosity of 72 pb-1 collected by the CDF II detector in proton anti-proton collisions at s**(1/2) = 1.96 TeV at the Fermilab Tevatron. A comparison of the uncorrected A_{FB} between data and Standard Model Monte Carlo gives good agreement with a chi^2/DOF of 15.7/15. The couplings measurements are also consistent with Standard Model predictions.

Figures (27)

• Figure 1: One quadrant of the CDF II tracking and calorimetric detectors. The detectors have axial and reflective symmetry about z=0. CDF uses a cylindrical coordinate system with the $z$ (longitudinal) axis along the proton-beam direction; $r$ is the transverse coordinate, and $\phi$ is the azimuthal angle. The detector pseudorapidity is defined as $\eta_{det} \equiv {\rm -ln(tan}{\theta_{det} \over 2})$, where $\theta_{det}$ is the polar angle relative to the proton-beam direction measured from $z = 0$. The event pseudorapidity is defined as $\eta_{evt} \equiv {\rm -ln(tan}{\theta_{evt} \over 2})$, where $\theta_{evt}$ is the polar angle measured from the nominal $\overline{p}p$ collision point in $z$. The transverse momentum ($p_{_T}$) and energy ($E_{T}$) are the components projected onto the plane perpendicular to the beam axis ($p_{_T} \equiv p\cdot\sin\theta$ ; $E_{T} \equiv E\cdot\sin\theta$). The missing transverse energy, $\hbox{$\not\!\! E_{t}$}$, is defined as the magnitude of $-\Sigma_i E^i_T \hat{n}_i$, where $\hat{n}_i$ is a unit vector in the perpendicular plane that points from the beamline to the $i$th calorimeter tower.
• Figure 2: Cross section of upper part of plug calorimeter (top), and transverse segmentation, showing physical and trigger towers in a $30^{\circ}~\phi$ section (bottom). The logical segmentation for clustering purposes is the same except in the outer two rings ($\theta>30^{\circ}$), where two neighboring (in azimuth) $7.5^{\circ}$ towers are merged to match the $15^{\circ}$ segmentation of the central and wall calorimeters behind them.
• Figure 3: Diagram of $\hbox{$\overline{p}p$} \rightarrow \hbox{$Z/\gamma^*$}$, where one of the quarks radiates a gluon or photon imparting transverse momentum to the quark. Once the quarks annihilate to $\hbox{$Z/\gamma^*$}$, the source of the transverse momentum is ambiguous.
• Figure 4: Event electron identification efficiency, $(\epsilon_i^{\pm})_{ID}$, as a function of $M_{ee}$ measured from the $\hbox{$Z/\gamma^* \rightarrow\ e^+ e^-$}$ simulation. The dashed line is for forward events and the solid line is for backward events. The dip in efficiency below 90 GeV/$c^2$ and the differences between forward and backward efficiencies are due to radiation effects (see Sec. \ref{['s_event_sel']}).
• Figure 5: Distribution of dielectron invariant mass from $\hbox{$Z/\gamma^* \rightarrow\ e^+ e^-$}$ candidates in 72 pb$^{-1}$ of Run II data.