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Eur. Phys. J. C (2016) 76:196

DOI 10.1140/epjc/s10052-016-4018-8

Special Article - Tools for Experiment and Theory

Herwig 7.0/Herwig++ 3.0 release note

Johannes Bellm

1,2

, Stefan Gieseke

, David Grellscheid

, Simon Plätzer

2,3,a

, Michael Rauch

, Christian Reuschle

1,4

Peter Richardson

2,5

, Peter Schichtel

, Michael H. Seymour

, Andrzej Siódmok

5,6

, Alexandra Wilcock

Nadine Fischer

, Marco A. Harrendorf

, Graeme Nail

, Andreas Papaefstathiou

, Daniel Rauch

Institute for Theoretical Physics, Karlsruhe Institute of Technology, Karlsruhe, Germany

IPPP, Department of Physics, Durham University, Durham, UK

Particle Physics Group, School of Physics and Astronomy, University of Manchester, Manchester, UK

HEP Theory Group, Department of Physics, Florida State University, Tallahassee, USA

CERN Theoretical Physics Department, Geneva, Switzerland

The Henryk Niewodniczanski Institute of Nuclear Physics in Cracow, Polish Academy of Sciences, Cracow, Poland

Institut für Experimentelle Kernphysik, Karlsruhe Institute of Technology, Karlsruhe, Germany

Received: 12 January 2016 / Accepted: 11 March 2016 / Published online: 11 April 2016

Abstract A major new release of the Monte Carlo event

generator Herwig++ (version 3.0) is now available. This

release marks the end of distinguishing Herwig++ and HER-

WIG development and therefore constitutes the ﬁrst major

release of version 7 of the Herwig event generator family. The

new version features a number of signiﬁcant improvements

to the event simulation, including: built-in NLO hard pro-

cess calculation for virtually all Standard Model processes,

with matching to both angular-ordered and dipole shower

modules via both subtractive (MC@NLO-type) and multi-

plicative (Powheg-type) algorithms; QED radiation and spin

correlations in the angular-ordered shower; a consistent treat-

ment of perturbative uncertainties within the hard process and

parton showering. Several of the new features will be covered

in detail in accompanying publications, and an update of the

manual will follow in due course.

1 Introduction

Herwig is a multi purpose particle physics event generator.

It is based on the experience gained with both the HERWIG

[1] and the Herwig++ [2] event generators. The latest ver-

sion of Herwig++, 3.0, marks the point at which the physics

capabilities of the HERWIG version 6 series are fully super-

seded, and thus the last point at which their development is

distinguished. Herwig++ 3.0 will henceforth be known as

Herwig 7.0. It replaces any prior HERWIG or Herwig++

versions.

e-mail: simon.platzer@durham.ac.uk

Herwig provides highly improved and extended physics

capabilities compared to both its predecessors, in particular

the ability to perform simulations at next-to-leading order

in QCD, while keeping the key physics motivations such

as coherent parton showers (including both angular-ordered

and dipole evolution), the cluster hadronization model, an

eikonal multiple interaction model, and highly ﬂexible BSM

capabilities.

The last major public version (2.7) of Herwig++ is

described in great detail in [2–7]. This release note summa-

rizes the major changes and improvements introduced since

then, which constitute the base for the Herwig 7series.The

physics questions addressed by the capabilities of Herwig

7 will be covered in detail in accompanying publications,

as well as comparisons with the other well-known general-

purpose event generators, Pythia [8,9] and Sherpa [10]. A

detailed manual covering all technical aspects will be pre-

pared in due course. Please refer to [2] and the present paper

if using Herwig 7.0.

1.1 Availability

The new program version, together with other useful ﬁles

and information, can be obtained from the web site https://

herwig.hepforge.org/. In order to improve our response to

user queries, all problems and requests for user support

should be reported via the bug tracker on our wiki. Requests

for an account to submit tickets and modify the wiki should

be sent to herwig@projects.hepforge.org.

Herwig is released under the GNU General Public License

(GPL) version 2 and the MCnet guidelines for the distribution

and usage of event generator software in an academic setting,

123

196 Page 2 of 8 Eur. Phys. J. C (2016) 76 :196

which are distributed together with the source, and can also

be obtained from http://www.montecarlonet.org/.

1.2 Prerequisites

Herwig 7.0 is based on ThePEG 2.0, which is available

along with the Herwig installation sources at https://herwig.

hepforge.org/downloads. Further requirements are BOOST

[11], gsl [12], fastjet [13] and LHAPDF [14], while a

number of other dependencies are necessary in order to

fully exploit the program’s capabilities. Amongst these are

HepMC and/or Rivet [15] to analyze simulated events, as

well as some or all of the external amplitude libraries dis-

cussed in Sect. 2.2.

In order to simplify the installation process, we provide

a bootstrap script to facilitate a consistent build and instal-

lation of Herwig in a convenient way. The script requires

a python installation, and is available from https://herwig.

hepforge.org/herwig-bootstrap.

1.3 Documentation

A signiﬁcant new feature is the online documentation, which

has been completely rewritten and greatly extended to reﬂect

the major changes introduced with this version and replaces

the wiki pages. It can be found at https://herwig.hepforge.

org/tutorials/. An update of the more detailed physics and

manual will be made available in a similar format in due

course. Code snippets are provided for a wide variety of con-

trol functions for easy inclusion into input ﬁles. Detailed doc-

umentation of the source code and input ﬁle interfaces gen-

erated with doxygen is available at https://herwig.hepforge.

org/doxygen/.

2 NLO event simulation

A key ingredient in the design and development of Her-

wig 7.0 was to provide event simulation at next-to-leading

order (NLO) accuracy in the strong coupling by default for as

many Standard Model processes as possible in an automated

way. The program, with the help of external libraries used

for amplitude calculation, is now able fully automatically

to assemble NLO QCD corrections to virtually all Standard

Model processes, including matching to both of its parton-

shower algorithms [16,17], via methods inspired by either the

MC@NLO [18]orPowheg[19] type algorithms, which we

refer to as subtractive and multiplicative matching, respec-

tively.

Based on extensions of the previously developed Match-

box module [20], NLO event simulation is now possible

without the requirement of separately running external codes

and/or dealing with intermediate event sample ﬁles. Slight

changes have been made to improve Herwig

’s steering at the

level of input ﬁles, and signiﬁcant improvements are provided

to integration and unweighting, including parallelization to

meet the requirements of more complex processes.

2.1 The matchbox module

The design of the Matchbox modules closely resembles the

structure of the NLO QCD cross section calculated within

a subtraction paradigm, including the matching subtrac-

tions required to consistently combine such calculations with

parton showering downstream. Subtraction terms are avail-

able in a ﬂexible way, though only Catani–Seymour dipoles

[21,22] are provided so far, including both massless and mas-

sive QCD as well as the subtraction terms required for super-

symmetric QCD corrections.

Parton-shower matching subtractions are provided on an

equally ﬂexible footing, including those required for the

angular-ordered shower [16], the dipole shower [17], as

well as matrix-element corrected showers forming the basis

of Powheg-type matching. For the latter, we provide addi-

tional functionality to sample the matrix-element correction

Sudakov using the adaptive method outlined in [23]. In order

to simplify the calculation of matching subtractions for the

angular-ordered shower, the kinematics reconstruction used

to work out the ﬁnal shower kinematics has been changed

to avoid additional Jacobian factors when compared to the

dipole parameterization in the case of a single (or in general,

the hardest) emission.

2.2 External amplitude providers

In order to set up the full calculation of a cross section,

Matchbox requires plug-ins to provide the respective tree

and one-loop amplitudes. These plug-ins can be interfaced

either at the level of matrix elements squared (or tree-loop

interferences, respectively), or at the level of helicity, colour-

ordered subamplitudes with both trace- and colour ﬂow bases

provided within the Matchbox core through adapted versions

of the ColorFull [24] and CVolver [25] libraries.

While we

provide built-in amplitudes for a limited number of processes,

the bulk of Standard Model processes can be simulated using

external amplitude plug-ins.

Based on extensions of the BLHA standard [26,27], Her-

wig currently supports interfaces to GoSam [28], Mad-

Graph [29], NJet [30], OpenLoops[31] and VBFNLO

[32,33]. Amplitudes for a limited number of LHC relevant

processes are directly provided along with the release, and

amplitudes for electroweak Higgs plus jets production are

Other choices of colour bases are straightforward to implement

through a very transparent interface.

123

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