Measurement of the Inclusive Leptonic Asymmetry in Top-Quark Pairs that Decay to Two Charged Leptons at CDF

We measure the inclusive forward–backward asymmetry of the charged-lepton pseudorapidities from top-quark pairs produced in proton–antiproton collisions and decaying to final states that contain two charged leptons (electrons or muons). The data are collected with the Collider Detector at Fermilab and correspond to an integrated luminosity of $9.1{\mathrm{fb}}^{-1}$. We measure the leptonic forward–backward asymmetry, $A_{\mathrm{FB}}^{\ell }$, to be $0.072\pm 0.060$ and the leptonic pair forward–backward asymmetry, $A_{\mathrm{FB}}^{\ell \ell }$, to be $0.076\pm 0.082$. The measured values can be compared with the standard model predictions of $A_{\mathrm{FB}}^{\ell }=0.038\pm 0.003$ and $A_{\mathrm{FB}}^{\ell \ell }=0.048\pm 0.004$, respectively. Additionally, we combine the $A_{\mathrm{FB}}^{\ell }$ result with a previous determination from a final state with a single lepton and hadronic jets and obtain $A_{\mathrm{FB}}^{\ell }=0.090_{-0.026}^{+0.028}$.

Figure 1

Differential contribution to $A_{\mathrm{FB}}^{\ell }$ for the powheg simulation of $t\overline{t}$ production. The solid curve shows the estimation with Eq. (4) where $\mathcal{A}(q_{\ell }{\eta }_{\ell })$ is obtained with a fit of Eq. (6) on the asymmetric part of the $q_{\ell }{\eta }_{\ell }$ spectrum from the sample and $\mathcal{S}(q_{\ell }{\eta }_{\ell })$ is directly from the sample; the dashed curve is from the two-Gaussian model [27]. The vertical dashed line indicates the outer limits of the acceptance regions for charged leptons, which is $|q_{\ell }{\eta }_{\ell }|=2.0$.

Figure 2

(a) Comparison of the observed distribution of $q_{\ell }{\eta }_{\ell }$ with the SM expectations. (b) Asymmetric part of the distribution in (a) defined in Eq. (3b) from data after background subtraction together with the best fit with Eq. (6) and the expectations from the powheg MC model. The data points in (b) are placed at the bin centroids predicted by the powheg simulation. The inner bars on the data points represent the statistical uncertainties, while the outer bars represent the total uncertainties. The bands indicate the one standard deviation region for statistical and $\text{statistical}+\text{systematic}$ uncertainties.

Figure 3

The same figures as Fig. 2, but with $\mathrm{\Delta }\eta$ instead of $q_{\ell }{\eta }_{\ell }$.