The Physics of Herwig 7

TL;DR

Herwig 7 presents a modular, ThePEG-based event generator that combines automated, external-amplitude hard-process calculations with dual parton-shower schemes (angular-ordered and dipole) and a comprehensive hadronization model. The framework centers on Matchbox for LO/NLO calculations, subtraction-based real emission, and fixed-order–to–shower matching and merging, including KrkNLO variants; it also supports multiple external amplitude providers and a Les Houches interface. The paper details the architectural components, phase-space generation strategies, and internal checks that ensure IR and pole cancellation, along with sophisticated features for EW/QED evolution, heavy-quark effects, and minimum-bias/underlying-event modeling. Together, these elements yield a flexible, accurate platform for SM and BSM collider phenomenology, with extensive tooling for tuning, uncertainty estimation, and cross-validation against data. The combination of two complementary showers, advanced matching/merging schemes, and broad external-amplitude support positions Herwig 7 as a robust tool for current and future collider phenomenology, including precise predictions for multi-jet and EW-rich final states.

Abstract

We present the physics foundations and recent developments of Herwig 7, the modern successor of the original HERWIG and Herwig++ series. Herwig 7 provides a flexible and systematically improvable framework for the simulation of high-energy lepton and hadron collisions, with particular emphasis on QCD and EW effects. Hard scattering processes are generated within the automated Matchbox framework, which integrates external amplitude providers, supports tree-level, next-to-leading-order (NLO) and loop-induced matrix elements, and implements subtraction schemes, multi-channel phase-space sampling, dynamic scale choices and both POWHEG- and MC@NLO-type matching algorithms. Consistent multijet merging at LO and NLO is provided, enabling precise predictions across a wide range of SM processes. Parton radiation is simulated using two complementary showers: an angular-ordered shower incorporating QCD coherence and the heavy-quark dead-cone effect, and a dipole shower optimised for NLO matching and multijet merging. Higher-order corrections are included through matrix-element corrections and dedicated reweighting techniques, while QED and EW radiation are treated using a YFS formalism and EW showering algorithms. The modelling of non-perturbative physics employs an advanced cluster hadronization framework with improved cluster formation, fission and decay, as well as colour reconnection models, heavy-quark effects and interfaces to alternative hadronization schemes. An extended eikonal multiple-partonic-scattering model, incorporating semi-hard and soft components together with diffractive interactions, enables realistic descriptions of minimum-bias and underlying-event data. Herwig 7 thus represents a versatile event generator, providing a coherent, modular and extensible platform for Standard Model and beyond-the-Standard-Model collider phenomenology at current and future facilities.

Figures (30)

• Figure 1: Sample results from fixed-order calculations of VBF processes obtained with Matchbox. The left panel shows the spectrum of the third jet in Higgs-plus-three-jet events at NLO QCD using the full EW process. The right panel shows a comparison of the rapidity difference of the tagged jets, comparing the VBF approximation to the full calculations in events with one Higgs and three jets.
• Figure 2: A schematic diagram of a $2 \rightarrow n$ processes that can be simulated via the “blob” matrix elements.
• Figure 3: An overview of the classes in Matchbox and their interaction.
• Figure 4: Performance test for $W^{\pm} \to W^{\pm} Z^0$ EW branching. In order to suppress the non-RS-related contributions, we have imposed the following cuts: $\Delta R_{W^{\pm},V} > 1$, $\Delta R_{W^{\pm},jet} < 1$ and $\Delta R_{V,jet}< 1$.
• Figure 5: Angular distribution of $W^{\pm}$ bosons with high transverse momentum jets at $\sqrt{s}=8$ TeV (ATLAS data ATLAS:2016jbu).