Explorations of the Top Quark Forward-Backward Asymmetry at the Tevatron

TL;DR

This work investigates whether $t$-channel exchange of color sextet or triplet scalars, with flavor-violating couplings to up-type quarks, can explain the Tevatron's enhanced top-quark forward-backward asymmetry $A_{FB}^t$ without distorting the measured $t\bar{t}$ cross section or invariant-mass distribution. Using an effective theory framework and MadGraph-based calculations, the authors show that sextet and triplet scalars can reproduce $A_{FB}^t$ within experimental constraints, while octet and singlet representations cannot without conflicting with other data. They identify viable benchmark points and discuss LHC implications, including $t\bar{t}$+jets signatures and direct scalar production, which motivate further searches for new colored scalars with sizable up-top flavor violation. The study highlights a potential path to new physics in the top sector, guiding future collider tests and theory refinements in the quest to resolve the Tevatron anomaly.

Abstract

We consider the recent measurement of the top quark forward-backward asymmetry at the Fermilab Tevatron, which shows a discrepancy of slightly more than 2$σ$ compared to the SM prediction. We find that $t$-channel exchange of a color sextet or triplet scalar particle can explain the measurement, while leaving the cross section for $t \bar{t}$ production within measured uncertainties. Such particles have good discovery prospects by study of the kinematic structure of $t \bar{t}$+jets at the LHC.

Figures (6)

• Figure 1: Forward-backward asymmetry (left panel) and $t \bar{t}$ inclusive cross section (right panel) for color triplet (red curves) and sextet (blue curves) scalars, as a function of the scalar mass $M_\phi$ for values of the coupling $y=2.0$ (solid curves),$y=4.0$ (dashed curves), and $y=6.0$ (dotted curves). Also shown are the range of values up to $1\sigma$ (yellow region) and $2\sigma$ (green region) from the experimental measurements.
• Figure 2: Forward-backward asymmetry (left panel) and $t \bar{t}$ inclusive cross section (right panel) for color singlet (red curves) and octet (blue curves) scalars, as a function of the scalar mass $M_\phi$ for values of the coupling $y=1.0$ (solid curves), $y=2.0$ (dashed curves) and $y=3.0$ (dotted curves). Also shown are the range of values up to $1\sigma$ (yellow region) and $2\sigma$ (green region) from the experimental measurements.
• Figure 3: The regions of parameter space (in the $M_\phi$ - $y$ plane) which predict both $A_{FB}^t$ and $\sigma_{t \bar{t}}$ consistently within one sigma (yellow region) and two sigma (green region) of the experimental measurements, for color sextet (left panel) and color triplet (right panel) scalars.
• Figure 4: $t \bar{t}$ invariant mass distribution for the SM, the theory with a color sextet scalar, and the theory with a color triplet scalar. The sextet ($M_{\phi}=610~\text{GeV}$, $y=3.65$) and triplet ($M_{\phi}=410~\text{GeV}$,$y=3.70$) benchmark points are within the 1$\sigma$ of both $A_{FB}^t$ and $\sigma_{t \bar{t}}$.
• Figure 5: Comparison of $t$-channel excange from models with vector bosons and scalars.