Forward-backward asymmetry in top quark-antiquark production

We present a measurement of forward-backward asymmetry in top quark-antiquark production in proton-antiproton collisions in the final state containing a lepton and at least four jets. Using a data set corresponding to an integrated luminosity of $5.4{\mathrm{fb}}^{-1}$, collected by the D0 experiment at the Fermilab Tevatron Collider, we measure the $t\overline{t}$ forward-backward asymmetry to be $(9.2\pm 3.7)\%$ at the reconstruction level. When corrected for detector acceptance and resolution, the asymmetry is found to be $(19.6\pm 6.5)\%$. We also measure a corrected asymmetry based on the lepton from a top quark decay, found to be $(15.2\pm 4.0)\%$. The results are compared to predictions based on the next-to-leading-order QCD generator mc@nlo. The sensitivity of the measured and predicted asymmetries to the modeling of gluon radiation is discussed.

Figure 1

Input variables to the discriminant between $t\overline{t}$ and background events. Overflows are added to the highest bins.

Figure 2

The discriminant for events with (a) $\Delta y<0$ and (b) $\Delta y>0$.

Figure 3

The reconstructed $\Delta y$. Bin widths correspond to about half of the detector resolution in $\Delta y$.

Figure 4

The reconstructed charge-signed lepton rapidity.

Figure 5

The reconstructed (a) $q_l{y}_l$ and (b) $\Delta y$ in events with no $b$-tagged jets. In (b), the bin widths correspond to about half of the detector resolution in $\Delta y$.

Figure 6

The $t\overline{t}$ asymmetry versus $p_T^{t\overline{t}}$ as predicted by mc@nlo+herwig. For comparison, the predictions from pythia with different tunes [35] are also shown.

Figure 7

The reconstructed $p_T^{t\overline{t}}$. The hatched band represents systematic uncertainties due to the jet energy scale and resolution. Data points are compared to predictions based on (a) mc@nlo and (b) pythia with ISR off. Bin widths correspond to about half of the detector resolution in $p_T^{t\overline{t}}$.