QCD Coherence and the Top Quark Asymmetry

TL;DR

This paper investigates how coherent QCD radiation affects the top quark forward-backward asymmetry, revealing that parton showering can generate and modify asymmetries even at leading order, with implications for event generator modeling.

Contribution

It explains the origin of asymmetry features in QCD radiation, compares fixed-order and event generator results, and discusses the impact of showering treatments on top quark asymmetries.

Findings

Asymmetry grows more negative with higher transverse momentum of top pairs.

Showering can produce positive contributions to the inclusive asymmetry depending on recoil treatment.

Coherent parton showering reproduces key asymmetry features observed in fixed-order calculations.

Abstract

Coherent QCD radiation in the hadroproduction of top quark pairs leads to a forward--backward asymmetry that grows more negative with increasing transverse momentum of the pair. This feature is present in Monte Carlo event generators with coherent parton showering, even though the production process is treated at leading order and has no intrinsic asymmetry before showering. In addition, depending on the treatment of recoils, showering can produce a positive contribution to the inclusive asymmetry. We explain the origin of these features, compare them in fixed-order calculations and the Herwig++, Pythia and Sherpa event generators, and discuss their implications.