Matrix element corrections in the Pythia8 parton shower in the context of matched simulations at next-to-leading order

TL;DR

This paper investigates matrix element corrections in Pythia8 for NLO matched simulations involving resonances, proposing alternative settings that include recoil effects without double counting, demonstrated through top-antitop production at the LHC.

Contribution

It introduces new MEC configurations in Pythia8 that avoid double counting and incorporate recoil effects in resonance decays within NLO matched simulations.

Findings

Disabling MEC is unnecessary with the narrow-width approximation.

Proposed MEC settings prevent double counting while including recoil effects.

Comparison shows improved simulation accuracy for top-antitop production.

Abstract

We discuss the role of matrix element corrections (MEC) to parton showers in the context of MC@NLO-type matchings for processes that feature unstable resonances, where MEC are liable to result in double-counting issues, and are thus generally not employed. By working with Pythia8, we show that disabling all MEC is actually unnecessary in computations based on the narrow-width approximation, and we propose alternative MEC settings which, while still avoiding double counting, allow one to include hard-recoil effects in the simulations of resonance decays. We illustrate our findings by considering top-antitop production at the LHC, and by comparing MadGraph_aMC@NLO predictions with those of POWHEG-BOX and standalone Pythia8.