Jet Radius Dependence of Systematic Uncertainties in Semileptonic Top Pair Production

The dependence of modelling uncertainties on the jet radius parameter has been studied in semileptonic top pair production events at particle level using Rivet. Variations of the Pythia A14 tune, including initial- and final-state radiation variations, as well as colour reconnection and recoil models, were examined. Final-state radiation is found to be the dominant source of uncertainty for small jet radii, reaching the level of 10%, while initial-state radiation and scale variations on the cross section computation of the hard scattering have a much smaller impact of around 1%. The generator dependence, in particular the difference between Pythia and Herwig, remains one of the largest uncertainties in top measurements, with effects of up to 20% for small jet radii. Studies of jet constituents and baryon production were performed as a first step towards understanding these differences. The results indicate that theoretical modelling uncertainties are strongly dependent on the chosen jet radius, with larger radii mitigating these uncertainties in jet calibration and top-quark measurements.