Top Quark Forward-Backward Asymmetry and $W'$-Boson with General Couplings

TL;DR

This paper investigates whether a hypothetical $W'$ boson with general couplings can explain the top quark forward-backward asymmetry observed at Fermilab, while remaining consistent with other experimental constraints.

Contribution

It introduces a comprehensive analysis of $W'$-boson models with various couplings and evaluates their viability against electric dipole moment and charge asymmetry constraints.

Findings

$W'$ with mass below 240 GeV is disfavored by electric dipole moment bounds.

Charge asymmetry measurements restrict the parameter space for $W'$ explanations.

Certain $W'$ models can still account for the asymmetry without conflicting with current data.

Abstract

The measured forward-backward asymmetry in top pair events at the Fermilab Tevatron collider deviates significantly from the standard model expectation. Several models have been proposed to describe the observed asymmetry which grows with the rapidity difference of the top pairs and also with the $t\bar{t}$ invariant mass. The presence of a heavy charged gauge boson $W'$ with the coupling $W'-t-d$ (left-handed, right-handed, and a mixture of left and right-handed couplings) could generate the desired top forward-backward asymmetry keeping the top pair cross section consistent with the standard model prediction. Such $W'$-boson makes contribution to the electric dipole moment of the neutron through contribution to the $d$-quark electric dipole moment, recently measured charge asymmetry ($A_{C}$) by the LHC experiments, and the total cross section of top pair at the LHC and Tevatron. We show that the upper bounds on neutron and top electric dipole moments disfavour any $W'$ with a mass below 240 GeV which could explain the Tevatron forward-backward asymmetry. It is shown that the charge asymmetry provides an allowed region in the parameters space with no overlap with the allowed region where the asymmetry could be described.