Matching Parton Showers and Matrix Elements
Stefan Hoeche, Frank Krauss, Nils Lavesson, Leif Lonnblad, Michelangelo Mangano, Andreas Schaelicke, Steffen Schumann
TL;DR
The paper addresses how to merge fixed-order tree-level matrix elements with parton showers to accurately predict multijet final states, where a strict factorization is not available and multiple hard scales complicate the separation between hard processes and shower evolution. It compares three merging schemes—CKKW, Lönnblad, and MLM—using W production with jets as a test case, across different matrix‑element generators and parton cascades. The study finds broadly similar results across schemes but highlights meaningful differences that reflect how the calculation is partitioned between matrix elements and showers. These findings inform systematic uncertainties and guide practical choices in simulating multijet final states at hadron colliders.
Abstract
We compare different procedures for combining fixed-order tree-level matrix element generators with parton showers. We use the case of W-production at the Tevatron and the LHC to compare different implementations of the so-called CKKW scheme and one based on the so-called MLM scheme using different matrix element generators and different parton cascades. We find that although similar results are obtained in all cases, there are important differences.