Abstract
We evaluate the theoretical uncertainties in next-to-leading order plus parton shower predictions for top quark pair production and decay in hadronic collisions. Our work is carried out using the Herwig 7 event generator and presents an in-depth study of variations in matching schemes with two systematically different shower algorithms, the traditional angular-ordered and alternative dipole shower. We also present all of the required extensions of the Herwig dipole shower algorithm to properly take into account quark mass effects, as well as its ability to perform top quark decays. The predictions are compared at parton level as well as to Large Hadron Collider data, including in the boosted regime. We find that the regions where predictions with a non-top-quark-specific tune differ drastically from data are plagued by large uncertainties which are consistent between our two shower and matching algorithms.
Highlights
Monte Carlo event generators [5,6,7] used for predictions of top quark pair production have seen several improvements, which mainly concentrated on combining next-to-leading order Quantum Chromodynamics (QCD) corrections with subsequent parton shower algorithms [8,9,10,11], and the production of additional jets using multi-jet merging algorithms, e.g. unitarized schemes [12,13,14,15] as employed inside Herwig 7, or approaches which share a similar spirit [16]
In this work we have presented a detailed study of next-to-leading order (NLO) plus parton shower matched predictions for top pair production at the Large Hadron Collider (LHC) in the Herwig 7 event generator
We have considered various sources of uncertainty, including the matching algorithms themselves for which two options, a subtractive (MC@NLO-type) and multiplicative (Powhegtype) paradigm can be used within Herwig 7, as well as all scale choices involved
Summary
Monte Carlo event generators [5,6,7] used for predictions of top quark pair production have seen several improvements, which mainly concentrated on combining next-to-leading order QCD corrections with subsequent parton shower algorithms [8,9,10,11], and the production of additional jets using multi-jet merging algorithms, e.g. unitarized schemes [12,13,14,15] as employed inside Herwig 7, or approaches which share a similar spirit [16]. The present work is centred around a thorough investigation of how reliable predictions by established paradigms, namely scale variations in matched predictions, are across phase space This question has not yet been answered by an in-depth comparison of similar, yet algorithmically very different, predictions and their associated variations which can be established to constitute a set of uncertainties when meeting well-defined constraints [24]. We use this study to introduce some improvements to both radiation from heavy quarks and the handling of their decays in the Herwig dipole shower module These changes enable us to perform this study between different matching and shower algorithms in a consistent way, using the same hadronization and underlying event models and with control over shower starting scales and resummation in the hard emission region
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