HERWIG 6.5: an event generator for Hadron Emission Reactions With Interfering Gluons (including supersymmetric processes)

TL;DR

HERWIG 6.5 introduces a comprehensive, Fortran-based Monte Carlo event generator that significantly extends previous capabilities with first-order matrix-element matching to parton showers, SUSY (MSSM) physics including R-parity violation, and spin-correlated decays. It integrates an elaborate parton-shower framework with angular ordering, heavy-quark dead-cone effects, and a cluster hadronization model along with a space-time event interpretation, enabling detailed final states across lepton, hadron, photon, and mixed collisions. The manual details extensive process catalogs (SM, MSSM, Higgs sectors, and beyond), SUSY-data input via ISAWIG, and interfaces to external tools (Les Houches, CIRCE, TAUOLA, MC@NLO, HDECAY, JIMMY, HERBVI). A key practical outcome is a flexible, highly-tunable framework that supports precise modeling of complex collider events at LHC energies, with an eye toward transitioning to HERWIG++ in C++. The combination of matrix-element corrections, spin correlations, and MSSM support positions HERWIG 6.5 as a versatile tool for precision Standard Model and beyond-Standard-Model phenomenology.

Abstract

HERWIG is a general-purpose Monte Carlo event generator, which includes the simulation of hard lepton-lepton, lepton-hadron and hadron-hadron scattering and soft hadron-hadron collisions in one package. It uses the parton-shower approach for initial- and final-state QCD radiation, including colour coherence effects and azimuthal correlations both within and between jets. This article updates the description of HERWIG published in 1992, emphasising the new features incorporated since then. These include, in particular, the matching of first-order matrix elements with parton showers, a more correct treatment of heavy quark decays, and a wide range of new processes, including many predicted by the Minimal Supersymmetric Standard Model, with the option of R-parity violation. At the same time we offer a brief review of the physics underlying HERWIG together with details of the input and control parameters and the output data, to provide a self-contained guide for prospective users of the program. This version of the manual (version 3) is updated to HERWIG version 6.5, which is expected to be the last major release of Fortran HERWIG. Future developments will be implemented in a new C++ event generator, HERWIG++.