Charge asymmetries of top quarks at hadron colliders revisited

TL;DR

This work revisits the Standard Model prediction for the top-quark charge asymmetry at Tevatron and LHC by incorporating QED and weak corrections, finding a modest but meaningful increase over earlier results. It demonstrates that restricting the tt̄ system’s transverse momentum can significantly enhance the observed asymmetry, and it provides detailed hadronic predictions for Tevatron and LHC observables, including rapidity-based and cut-dependent definitions. The paper also explores beyond-Standard-Model implications at the LHC using axigluon-like models, showing how such scenarios could yield distinctive signatures in $A_{t\bar{t}}(Y)$ and related observables while respecting existing constraints. Collectively, the results emphasize the importance of precise SM calculations and carefully chosen kinematic selections for probing new physics in top-quark pair production.

Abstract

A sizeable difference in the differential production cross section of top- compared to antitop-quark production, denoted charge asymmetry, has been observed at the Tevatron. The experimental results seem to exceed the theory predictions based on the Standard Model by a significant amount and have triggered a large number of suggestions for "new physics". In the present paper the Standard Model predictions for Tevatron and LHC experiments are revisited. This includes a reanalysis of electromagnetic as well as weak corrections, leading to a shift of the asymmetry by roughly a factor 1.1 when compared to the results of the first papers on this subject. The impact of cuts on the transverse momentum of the top-antitop system is studied. Restricting the ttbar system to a transverse momentum less than 20 GeV leads to an enhancement of the asymmetries by factors between 1.3 and 1.5, indicating the importance of an improved understanding of the $t\bar t$-momentum distribution. Predictions for similar measurements at the LHC are presented, demonstrating the sensitivity of the large rapidity region both to the Standard Model contribution and effects from "new physics".

Figures (14)

• Figure 1: Origin of the QCD charge asymmetry in hadroproduction of heavy quarks: interference of final-state (a) with initial-state (b) gluon bremsstrahlung plus interference of the box (c) with the Born diagram (d). Crossed diagrams are omitted.
• Figure 2: Cut diagrams.
• Figure 3: Origin of the QCD charge asymmetry in hadroproduction of heavy quarks through flavor excitation.
• Figure 4: Representative diagrams contributing to the QCD-QED interference term.
• Figure 5: Representative diagrams contributing to the QCD-weak interference term.