Monte Carlo Top Quark Mass Calibration

Abstract

The most precise top quark mass measurements use kinematic reconstruction methods, determining the top mass parameter of a Monte Carlo event generator, mtMC. Due to the complicated interplay of hadronization and parton shower dynamics in Monte Carlo event generators relevant for kinematic reconstruction, relating mtMC to field theory masses is a non-trivial task. In this talk we report on a calibration procedure to determine this relation using hadron level QCD predictions for 2-Jettiness in e+e- annihilation, an observable which has kinematic top mass sensitivity and a close relation to the invariant mass of the particles coming from the top decay. The theoretical ingredients of the QCD prediction are reviewed. Fitting e+e- 2-Jettiness calculations at NLL/NNLL order to Pythia 8.205, we find that mtMC agrees with the MSR mass (Formula Presented.) within uncertainties. At NNLL we find (Formula Presented.). mtMC can differ from the pole mass mtpole by up to 600MeV, and using the pole mass generally leads to larger uncertainties. At NNLL we find mtMC = mtpole +(0.57±0.28)GeV as the fit result. In contrast, converting (Formula Presented.) obtained at NNLL to the pole mass gives a result for mtpole that is substantially larger and incompatible with the fit result. We also explain some theoretical aspects relevant for employing the C-parameter as an alternative calibration observable.