Measurement of leptonic asymmetries and top-quark polarization in $t\overline{t}$ production

We present measurements of lepton ($\ell$) angular distributions in top-quark ($t$) pair production and $t\overline{t}\to W^{+}b{W}^{-}\overline{b}\to {\ell }^{+}\nu b{\ell }^{-}\overline{\nu }\overline{b}$ decays produced in $p\overline{p}$ collisions at a center-of-mass energy of $\sqrt{s}=1.96\mathrm{TeV}$, where $\ell$ is an electron or muon. Using data corresponding to an integrated luminosity of $5.4{\mathrm{fb}}^{-1}$, collected with the D0 detector at the Fermilab Tevatron Collider, we measure for the first time the leptonic forward-backward asymmetry in dilepton final states and obtain $A_{\mathrm{FB}}^{\ell }=(5.8\pm 5.1(\mathrm{stat})\pm 1.3(\mathrm{syst}))\%$, corrected for detector acceptance. This is compared to the standard model prediction of $A_{\mathrm{FB}}^{\ell }(\mathrm{predicted})=(4.7\pm 0.1)\%$. A deviation from the standard model prediction as previously seen in a D0 measurement based on the analysis of the $\ell +\mathrm{\text{jets}}$ final state is not observed in these dilepton final states. The two results differ from each other by 1.4 standard deviations and are combined to obtain $A_{\mathrm{FB}}^{\ell }=(11.8\pm 3.2)\%$. Furthermore, we present a first study of the top-quark polarization.

Figure 1

Pseudorapidity distributions of the charged leptons for the combination of the $ee$, $e\mu$, and $\mu \mu$ final states after the selection criteria have been applied. The $\eta$ distribution of positively (a) and negatively (b) charged leptons, the distribution of $Q·\eta$ (c), and the distribution of $\Delta \eta ={\eta }_{{\ell }^{+}}-{\eta }_{{\ell }^{-}}$ (d) are shown. The vertical error bars indicate the statistical uncertainty. The $t\overline{t}$ contribution is normalized to the data after background subtraction.

Figure 2

The distribution of $\cos \theta$ is shown for the combination of the dilepton channels (a) and the $\ell +\mathrm{\text{jets}}$ channels (b). The data are compared to the SM predictions. The vertical error bars on the data points indicate the statistical uncertainty of the data. The distribution of $t\overline{t}$ pairs produced via a hypothetical $Z^{\prime }$ boson is also shown; the uncertainty due to the limited size of the MC sample is shown by the shaded band. The same $Z^{\prime }$ model as in [26, 27] is used.