The MC@NLO 2.3 Event Generator

TL;DR

The paper documents MC@NLO 2.3, a practical framework for embedding NLO QCD matrix elements into a HERWIG-based parton shower via the Les Houches interface, enabling consistent production of SM Higgs, vector bosons, lepton pairs, heavy-quark pairs, and vector-boson pairs. It details a two-executable workflow (NLO to generate an event file and MC to shower events), explains handling of negative-weight events, and specifies file structures, running environments, and PDF options (with LHAPDF not supported). The manual also introduces the new lepton-pair production mode, outlines pre-run configuration, event-file generation, decay handling (with spin correlations neglected at present), and the expected result artifacts, providing practical guidance for users to set up and run MC@NLO 2.3. Overall, it serves as a comprehensive reference for integrating NLO calculations with parton showers in a flexible, externally driven workflow. The guidance enables practitioners to perform accurate cross-section calculations and event generation for a broad set of SM processes within a standardized, two-step pipeline.

Abstract

This is the user's manual of MC@NLO 2.3. This package is a practical implementation, based upon the HERWIG event generator, of the MC@NLO formalism, which allows one to incorporate NLO QCD matrix elements consistently into a parton shower framework. Processes available in this version include the hadroproduction of single vector and Higgs bosons, vector boson pairs, heavy quark pairs, and lepton pairs. This document is self-contained, but we emphasise the main differences with respect to previous versions.