Measurement of the forward-backward asymmetry in top quark-antiquark production in ppbar collisions using the lepton+jets channel

TL;DR

The paper measures the forward-backward asymmetry in top-quark pair production in proton–antiproton collisions using the full D0 Run II Tevatron dataset in the lepton+jets channel. It introduces new kinematic reconstruction for events with ≥4 jets and a partial reconstruction for 3-jet events, then unfolds the observed distributions to production level using regularized matrix inversion with TUNFOLD, including a 2D unfolding in $Δy$ and $m_{t\bar{t}}$. The result, $A_{FB} = 10.6 \pm 3.0\%$, is consistent with SM predictions and its dependencies on $|Δy|$ and $m_{t\bar{t}}$ agree with MC@NLO expectations, while the analysis benefits from improved reconstruction, channel-by-channel treatment, and expanded event selection. Overall, the work strengthens the SM baseline for $A_{FB}$ at the Tevatron and demonstrates robust methodology for precision asymmetry measurements in hadron colliders.

Abstract

We present a measurement of the forward--backward asymmetry in top quark-antiquark production using the full Tevatron Run II dataset collected by the D0 experiment at Fermilab. The measurement is performed in lepton+jets final states using a new kinematic fitting algorithm for events with four or more jets and a new partial reconstruction algorithm for events with only three jets. Corrected for detector acceptance and resolution effects, the asymmetry is evaluated to be 10.6+-3.0 %. Results are consistent with the standard model predictions which range from 5.0% to 8.8%. We also present the dependence of the asymmetry on the invariant mass of the top quark--antiquark system and the difference in rapidities of top quark and antiquark.

Figures (9)

• Figure 1: (Color online). The probability to correctly reconstruct the sign of $\Delta y$ as a function of the production-/level $\left|{\Delta y}\right|$ for the algorithm of Ref. bib:hitfit used to measure the $A_{\mathrm{FB}}$ in Ref. bib:ourPRD and the algorithms used to reconstruct $l$+$\geq$4 jet events and to partially reconstruct $l\textrm{+}3\textrm{\,jet}$ events in this paper.
• Figure 2: (Color online). The combined discriminant $D_{c}$. The region $D_{c}<1$ is not used to determine the signal $A_{\mathrm{FB}}$. The ratio between the data counts and the model expectation is shown in the lower panel, with the hashed area representing the systematic uncertainties. Figure is from Ref. bib:our_afbl.
• Figure 3: (Color online). Reconstructed difference between the rapidities of the top and antitop quarks, $\Delta y$. The left column shows $l\textrm{+}3\textrm{\,jet}$ events, and the right column shows $l$+$\geq$4 jet events. Rows from top to bottom display events with 0, 1, and $\ge$2 $b$ tags. Overflows are included in the edge bins. The ratio between the data counts and the model expectation is shown in the lower panels, with the hashed area representing the systematic uncertainties.
• Figure 4: (Color online). Reconstructed invariant mass of the top quark--antiquark pair, ${m_{t{\bar{t}}}}$. The left column shows $l\textrm{+}3\textrm{\,jet}$ events, and the right column shows $l$+$\geq$4 jet events. Rows from top to bottom display events with 0, 1, and $\ge$2 $b$ tags. The ratio between the data counts and the model expectation is shown in the lower panels, with the hashed area representing the systematic uncertainties.
• Figure 5: (Color online). Production-/level bins for the 2D measurement in the $({m_{t{\bar{t}}}},{\Delta y})$ plane, overlaid on the distribution in these variables predicted from MC@NLO. The shading reflects the predicted event density in arbitrary units. The solid and dashed lines denote the production-/level bins. The solid lines show bins that are used for the final result.