Top Quark Mass Calibration for Monte Carlo Event Generators -- An Update

Abstract

We generalize and update our former top quark mass calibration framework for Monte Carlo (MC) event generators based on the e+ee^+e^- hadron-level 2-jettiness τ2\tau_2 distribution in the resonance region for boosted ttˉt\bar t production, that was used to relate the PYTHIA 8.205 top mass parameter mtMCm_t^{\rm MC} to the MSR mass mtMSR(R)m_t^{\rm MSR}(R) and the pole mass mtpolem_t^{\rm pole}. The current most precise direct top mass measurements specifically determine mtMCm_t^{\rm MC}. The updated framework includes the addition of the shape variables sum of jet masses τs\tau_s and modified jet mass τm\tau_m, and the treatment of two more gap subtraction schemes to remove the O(ΛQCD){\cal O}(\Lambda_{\rm QCD}) renormalon related to large-angle soft radiation. These generalizations entail implementing a more versatile shape-function fit procedure and accounting for a certain type of (mt/Q)2(m_t/Q)^2 power corrections to achieve gap-scheme and observable independent results. The theoretical description employs boosted heavy-quark effective theory (bHQET) at next-to-next-to-logarithmic order (N2^2LL), matched to soft-collinear effective theory (SCET) at N2^2LL and full QCD at next-to-leading order (NLO), and includes the dominant top width effects. Furthermore, the software framework has been modernized to use standard file and event record formats. We update the top mass calibration results by applying the new framework to PYTHIA 8.205, HERWIG 7.2 and SHERPA 2.2.11. Even though the hadron-level resonance positions produced by the three generators differ significantly for the same top mass parameter mtMCm_t^{\rm MC} value, the calibration shows that these differences arise from the hadronization modeling. Indeed, we find that mtMCm_t^{\rm MC} agrees with m_t^{\rm MSR}(1\,\mbox{GeV}) within 200200 MeV for the three generators and differs from the pole mass by 350350 to 600600 MeV.Comment: 70 pages, 15 figure

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