Zero and one jet combined NLO analysis of the top quark forward-backward asymmetry

TL;DR

The paper tackles the Tevatron top-quark forward-backward asymmetry and its tension with SM predictions by constructing a merged NLO framework for tt and tt+jet production. It extends MC@NLO to massive particles and implements MEPS@NLO within SHERPA to preserve both NLO accuracy and parton-shower logarithmic accuracy. The authors present inclusive and differential A_FB predictions, finding good agreement for the p_T-dependent asymmetry but residual disparities in inclusive and mass/rapidity dependencies, with scale uncertainties reduced relative to LO. The work provides a reproducible, publicly usable framework and highlights the role of color-coherence, subleading color, and scale choices in shaping A_FB predictions, setting the stage for including electroweak corrections and top decays.

Abstract

We present an analysis of the forward-backward asymmetry in the production of top quark pairs at the Tevatron collider. We use novel Monte Carlo methods for merging matrix elements and parton showers to combine NLO QCD predictions for tt and tt+jet production. Theoretical uncertainties are quantified in detail. We find agreement with experimental data on the transverse momentum dependence of the asymmetry.

Figures (3)

• Figure 1: Transverse momentum spectrum of the $t\bar{t}$ pair (left) and $p_T$-dependent forward--backward asymmetry (right). We compare the MC@NLO prediction (red) and the parton-shower result (blue). Hard remainder terms have been set to zero in the MC@NLO simulation, while the parton shower has been reweighted with the local $K$-factor $\bar{\mathrm{B}}^{\rm(A)}/\mathrm{B}$ in order to make the two results comparable. Uncertainty bands stem from varying the scale of strong couplings in the resummation.
• Figure 2: Systematic uncertainty due to variation of the merging cut (left) and due to the scale choice (right). The dotted (dashed) lines in the left panel correspond to contributions from the zero (one) jet MC@NLO. The two bands in the right panel depict results from different choices of the functional form of the core scale, for more details see the main text. Each band has been obtained by varying the respective default scale by factors of two.
• Figure 3: Top quark forward--backward asymmetry in dependence on the transverse momentum (top), the absolute of the rapidity separation, $\Delta_{y,t\bar{t}}\equiv|y_t-y_{\bar{t}}|$ (bottom left), and the invariant mass (bottom right) of the $t\bar{t}$ system. MC@NLO zero plus one jet merged predictions -- together with their uncertainty bands -- are shown for both of the scale choices studied in this work, cf. Sec. \ref{['sec:scale_uncertainties']}. The comparison is against CDF background subtracted data (top right panel) and against parton-level corrected data (bottom panels) Aaltonen:2012it. The top left panel shows parton-level results.