A Revisit to Top Quark Forward-Backward Asymmetry

TL;DR

This paper reviews various theoretical models for the top quark forward-backward asymmetry at the Tevatron, comparing their compatibility with recent experimental data and constraints, and identifies the $W'$ model as the most promising candidate.

Contribution

It provides a comprehensive analysis of multiple models explaining $A^t_{FB}$, highlighting the viability of the $U(1)_d$ flavor symmetry axigluon and $W'$ models under current experimental constraints.

Findings

The axigluon model is largely ruled out by dijet searches.

The $Z'$ and diquark models can fit $A_{FB}$ within 1 sigma but face other constraints.

The $W'$ model offers the best overall fit and predicts observable signals at the LHC.

Abstract

We analyze various models for the top quark forward-backward asymmetry ($A^t_{FB}$) at the Tevatron, using the latest CDF measurements on different $A^t_{FB}$s and the total cross section. The axigluon model in Ref. \cite{paul} has difficulties in explaining the large rapidity dependent asymmetry and mass dependent asymmetry simultaneously and the parameter space relevant to $A^t_{FB}$ is ruled out by the latest dijet search at ATLAS. In contrast to Ref. \cite{cp}, we demonstrate that the large parameter space in this model with a $U(1)_d$ flavor symemtry is not ruled out by flavor physics. The $t$-channel flavor-violating $Z^{\prime}$ \cite{hitoshi}, $W^{\prime}$\cite{waiyee} and diquark \cite{tim} models all have parameter regions that satisfy different $A_{FB}$ measurements within 1 $\sigma$. However, the heavy $Z^{\prime}$ model which can be marginally consistent with the total cross section is severely constrained by the Tevatron direct search of same-sign top quark pair. The diquark model suffers from too large total cross section and is difficult to fit the $t \bar{t}$ invariant mass distribution. The electroweak precision constraints on the $W'$ model based on $Z'$-$Z$ mixings is estimated and the result is rather weak ($m_{Z'} > 450$ GeV). Therefore, the heavy $W^{\prime}$ model seems to give the best fit for all the measurements. The $W^{\prime}$ model predicts the $t\bar{t}+j$ signal from $tW^{\prime}$ production and is 10%-50% of SM $t\bar{t}$ at the 7 TeV LHC. Such $t+j$ resonance can serve as the direct test of the $W^{\prime}$ model.