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Top Quark Forward-Backward Asymmetry

Kingman Cheung, Wai-Yee Keung, Tzu-Chiang Yuan

TL;DR

The paper addresses the Tevatron top-quark forward-backward asymmetry that challenges Standard Model predictions. It proposes a W' boson coupling to down and top quarks, mediating a t-channel process that can enhance the asymmetry while keeping the ttbar cross section within experimental limits. The authors derive the relevant amplitudes, compute A_fb and cross sections, and scan the parameter space (M_W', g') under data constraints, finding viable regions especially for a predominantly right-handed W'. They also discuss alternative explanations such as non-universal Z', KK gluons, and axigluons, and examine collider implications and observable signatures.

Abstract

The recent forward-backward asymmetry recorded by the CDF Collaboration for the top and anti-top quark pair production indicates more than $2σ$ deviation from the Standard Model prediction, while its total production cross section remains consistent. We propose a $W'$ boson that couples to down and top quarks. We identify the parameter space that can give rise a large enough forward-backward asymmetry without producing too many top and anti-top quark pairs. Other models presented erstwhile in the literature that can produce such effects are also discussed.

Top Quark Forward-Backward Asymmetry

TL;DR

The paper addresses the Tevatron top-quark forward-backward asymmetry that challenges Standard Model predictions. It proposes a W' boson coupling to down and top quarks, mediating a t-channel process that can enhance the asymmetry while keeping the ttbar cross section within experimental limits. The authors derive the relevant amplitudes, compute A_fb and cross sections, and scan the parameter space (M_W', g') under data constraints, finding viable regions especially for a predominantly right-handed W'. They also discuss alternative explanations such as non-universal Z', KK gluons, and axigluons, and examine collider implications and observable signatures.

Abstract

The recent forward-backward asymmetry recorded by the CDF Collaboration for the top and anti-top quark pair production indicates more than deviation from the Standard Model prediction, while its total production cross section remains consistent. We propose a boson that couples to down and top quarks. We identify the parameter space that can give rise a large enough forward-backward asymmetry without producing too many top and anti-top quark pairs. Other models presented erstwhile in the literature that can produce such effects are also discussed.

Paper Structure

This paper contains 8 sections, 13 equations, 3 figures.

Table of Contents

  1. Introduction
  2. The Formulas
  3. Results
  4. Discussion
  5. Non-universal $Z'$
  6. Kaluza-Klein gluons
  7. Axigluon
  8. $W'$ in this work

Figures (3)

  • Figure 1: The contour of the asymmetry in $t\bar{t}$ production in the plane of $(M_{W'},\; g')$. The lines shown are for $A_{fb} = 0.12,\; 0.19$ and $0.26$. The chiral couplings for $W' - d -t$ are $g_L = g_R = 1$.
  • Figure 2: The contour of the asymmetry in $t\bar{t}$ production in the plane of $(M_{W'},\; g')$. The lines shown are for $A_{fb} = 0.12,\; 0.19$ and $0.26$. The chiral couplings for $W' - d -t$ are $g_L=0,\; g_R = 1$.
  • Figure 3: The $M_{t \bar{t}}$ invariant mass distribution. Data taken from Table III of cdf-mtt is shown along with the Standard Model result. Also shown are the results of $(M_{W'} , g') =$ (150, 0.7), (250, 1.0), (400, 1.4) and (570, 2). Each pair of these values are chosen within the $1 \sigma$ range of $\sigma (t \bar{t})$ in Fig. \ref{['2']}.