The MC@NLO 3.3 Event Generator

TL;DR

The article presents MC@NLO 3.3, a practical framework for embedding NLO QCD matrix elements into a HERWIG-based parton shower, enabling precise hadroproduction predictions across a broad set of processes. It details the two-executable workflow (NLO and MC), the Les Houches interface for event handling, and the management of negative-weight events, while outlining process-specific conventions, decays, and spin correlations. The manual also covers the software packaging, environment setup, parton-density options, event-file generation, and user-configurable input parameters, providing a comprehensive guide for deploying MC@NLO in phenomenological studies. Overall, MC@NLO 3.3 offers updated Higgs matching, expanded process coverage (including single-top and vector-boson channels), and practical guidance for integrating NLO computations with showering within the HERWIG framework, improving the reliability of collider predictions. The documentation emphasizes careful handling of PDFs, event-file workflows, and decay treatments to ensure consistent, physically meaningful results.

Abstract

This is the user's manual of MC@NLO 3.3. 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, single top, lepton pairs, and Higgs bosons in association with a W or Z. Spin correlations are included for all processes except ZZ and WZ production. This document is self-contained, but we emphasise the main differences with respect to previous versions.