Matching Matrix Elements and Parton Showers with HERWIG and PYTHIA

TL;DR

The paper tackles the problem of reliably merging fixed-order matrix elements with parton-shower simulations in HERWIG and PYTHIA to improve predictions of hard jet production. It implements and tests several matching strategies, notably the CKKW approach, a pseudo-shower variant, and a Mangano-inspired method, across e+e- and hadron-hadron processes, exploring scale and clustering dependencies. The results show that with carefully chosen Sudakov treatments and scales, CKKW-HERWIG can achieve good parton-level agreement, while pseudo-shower and MLM approaches offer robust, less-tuned alternatives with manageable hadronization effects. Together, they provide a framework for estimating systematic uncertainties in multijet predictions at current and future colliders and guide practical implementations of matrix-element–shower matching.

Abstract

We report on our exploration of matching matrix element calculations with the parton-shower models contained in the event generators HERWIG and Pythia. We describe results for e+e- collisions and for the hadroproduction of W bosons and Drell--Yan pairs. We compare methods based on (1) a strict implementation of ideas proposed by Catani, et al., (2) a generalization based on using the internal Sudakov form factors of HERWIG and Pythia, and (3) a simpler proposal of M. Mangano. Where appropriate, we show the dependence on various choices of scales and clustering that do not affect the soft and collinear limits of the predictions, but have phenomenological implications. Finally, we comment on how to use these results to state systematic errors on the theoretical predictions.