Non-perturbative QCD Effects and the Top Mass at the Tevatron

TL;DR

The paper investigates how non-perturbative QCD effects, particularly colour reconnection and underlying-event modeling, can impact top quark mass calibrations at the Tevatron. It introduces color-annealing colour reconnection models implemented in Pythia 6.416, tunes them with UE data, and assesses their effect on a generator-level toy top-mass measurement. The study finds calibration offsets of about 0.8–1.0 GeV across models, with perturbative differences (old vs new shower schemes) contributing most to the spread and non-perturbative differences adding a smaller, but non-negligible, portion. These results indicate that non-perturbative effects can influence top mass extractions at the GeV level and provide a framework to quantify such uncertainties for future precision analyses.

Abstract

The modelling of non-perturbative effects is an important part of modern collider physics simulations. In hadron collisions there is some indication that the modelling of the interactions of the beam remnants, the underlying event, may require non-trivial colour reconnection effects to be present. We recently introduced a universally applicable toy model of such reconnections, based on hadronising strings. This model, which has one free parameter, has been implemented in the Pythia event generator. We then considered several parameter sets (`tunes'), constrained by fits to Tevatron minimum-bias data, and determined the sensitivity of a simplified top mass analysis to these effects, in exclusive semi-leptonic top events at the Tevatron. A first attempt at isolating the genuine non-perturbative effects gave an estimate of order +-0.5GeV from non-perturbative uncertainties. The results presented here are an update to the original study and include recent bug fixes of Pythia that influenced the tunings investigated.

Figures (4)

• Figure 1: Average transverse momentum in minimum bias events as function of the charged multiplicity. One can clearly see the change between the old default and Tune A.
• Figure 2: Generator level comparison of various models available in Pythia 6.416. Charged multiplicity distribution (right) and mean transverse momentum as function of the charged multiplicity.
• Figure 4: Calibration curve obtained with Tune A before (left) and after (right) JES rescaling. The inset shows the Gaussian fit to the distribution of reconstructed top masses for one specific nominal top mass, $m_\mathrm{top}^\mathrm{gen}=175\,\hbox{GeexV}$.
• Figure 5: Comparison of calibration offsets obtained for each model. The column on the left (dots) show the results obtained before $\mathrm{JES}$ rescaling, the right column (squares) after rescaling. The statistical precision due to the finite number of generated events is at the level of $\pm 0.15\,\hbox{GeexV}$.