Colour Correlations and Multiplicities in Top Events

TL;DR

This work investigates color interconnection in near-threshold tt̄ events produced in e+e− collisions, focusing on how QCD radiation and fragmentation couple the production and decay stages of the top quark. It separates perturbative radiation (described by tb, t̄b̄ dipoles) from non-perturbative fragmentation, showing that interconnection is largely suppressed by the top width and a profile function chi(omega), with only soft gluons capable of enhancing interference. A dipole-based shower and fragmentation model across six scenarios reveals that interconnection effects appear as angular variations in multiplicity, especially when the bb̄ dipole or top-hadron formation is active, but the overall impact on inclusive observables and top-mass measurements remains modest. The study suggests that, with careful event reconstruction and fragmentation modelling, top masses can be extracted with uncertainties well below 100 MeV, while non-perturbative effects remain the dominant source of systematic uncertainty.

Abstract

In events of the type e+ e- -> t tbar -> b W+ bbar W-, particle production could depend in a non-trivial way on the kinematics of the process. Energetic perturbative gluon radiation can be generated (when kinematically allowed) by the original t tbar system and by the t -> b W+ and tbar -> bbar W- decays, with negligible interference between the production and decay stages and between the t and tbar decays. Soft perturbative gluon emission and non-perturbative fragmentation does introduce a correlation, however. To illustrate the size of these effects, we study the multiplicity as a function of the angle between the b and bbar jets, close to the t tbar threshold. Also potential uncertainties in top mass determinations are briefly addressed.