Parton Shower and Matching Uncertainties in Top Quark Pair Production with Herwig 7

TL;DR

This paper assesses the uncertainties in top quark pair production predictions using Herwig 7, comparing different shower algorithms and matching schemes, and evaluates their agreement with LHC data.

Contribution

It introduces extensions to the Herwig dipole shower for quark mass effects and top decays, and systematically compares shower algorithms and matching schemes.

Findings

Large uncertainties affect predictions in certain regions.

Predictions with non-top-specific tunes show significant deviations from data.

Uncertainties are consistent across different shower and matching algorithms.

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 LHC 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.