Michael H. Seymour

HERWIG 6 : an event generator for hadron emission reactions with interfering gluons (including supersymmetric processes).

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 initialand 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.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.

A general algorithm for calculating jet cross sections in NLO QCD

We present a new general algorithm for calculating arbitrary jet cross sections in arbitrary scattering processes to next-to-leading accuracy in perturbative QCD. The algorithm is based on the subtraction method. The key ingredients are new factorization formulae, called dipole formulae, which implement in a Lorentz covariant way both the usual soft and collinear approximations, smoothly interpolating the two. The corresponding dipole phase space obeys exact factorization, so that the dipole contributions to the cross section can be exactly integrated analytically over the whole of phase space. We obtain explicit analytic results for any jet observable in any scattering or fragmentation process in lepton, lepton-hadron or hadron-hadron collisions. All the analytical formulae necessary to construct a numerical program for next-to-leading order QCD calculations are provided. The algorithm is straightforwardly implementable in general purpose Monte Carlo programs.

Longitudinally-invariant k⊥-clustering algorithms for hadron-hadron collisions☆

Abstract We propose a version of the QCD-motivated “k⊥” jet-clustering algorithm for hadron-hadron collisions which is invariant under boosts along the beam directions. This leads to improved factorization properties and closer correspondence to experimental practice at hadron colliders. We examine alternative definitions of the resolution variables and cluster recombination scheme, and show that the algorithm can be implemented efficiently on a computer to provide a full clustering history of each event. Using simulated data at √S = 1.8 TeV, we study the effects of calorimeter segmentation, hadronization and the soft underlying event, and compare the results with those obtained using a conventional cone-type algorithm.

The Dipole Formalism for the Calculation of QCD Jet Cross Sections at Next-to-Leading Order

In order to make quantitative predictions for jet cross sections in perturbative QCD, it is essential to calculate them to next-to-leading accuracy. This has traditionally been an extremely laborious process. Using a new formalism, imaginatively called the dipole formalism, we are able to construct a completely general algorithm for next-to-leading order calculations of arbitrary jet quantities in arbitrary processes. In this paper we present the basic ideas behind the algorithm and illustrate them with a simple example.

General-purpose event generators for LHC physics

We review the physics basis, main features and use of general-purpose Monte Carlo event generators for the simulation of proton-proton collisions at the Large Hadron Collider. Topics included are: the generation of hardscattering matrix elements for processes of interest, at both leading and nextto-leading QCD perturbative order; their matching to approximate treatments of higher orders based on the showering approximation; the parton and dipole shower formulations; parton distribution functions for event generators; non-perturbative aspects such as soft QCD collisions, the underlying event and diractive processes; the string and cluster models for hadron formation; the treatment of hadron and tau decays; the inclusion of QED radiation and beyond-Standard-Model processes. We describe the principal features of the Ariadne, Herwig++, Pythia 8 and Sherpa generators, together with the Rivet and Professor validation and tuning tools, and discuss the physics philosophy behind the proper use of these generators and tools. This review is aimed at phenomenologists wishing to understand better how parton-level predictions are translated into hadron-level events as well as experimentalists wanting a deeper insight into the tools available for signal and background simulation at the LHC.

Jet Substructure at the Tevatron and LHC: New results, new tools, new benchmarks

In this paper, we review recent theoretical progress and the latest experimental results in jet substructure from the Tevatron and the LHC. We review the status of and outlook for calculation and simulation tools for studying jet substructure. Following up on the report of the Boost 2010 workshop, we present a new set of benchmark comparisons of substructure techniques, focusing on the set of variables and grooming methods that are collectively known as ‘top taggers’. To facilitate further exploration, we have attempted to collect, harmonize and publish software implementations of these techniques.

Small x phenomenology: Summary and status

Abstract. The aim of this paper is to summarize the general status of our understanding of small-x physics. It is based on presentations and discussions at an informal meeting on this topic held in Lund, Sweden, in March 2001. This document also marks the founding of an informal collaboration between experimentalists and theoreticians with a special interest in small-x physics. This paper is dedicated to the memory of Bo Andersson, who died unexpectedly from a heart attack on March 4th, 2002.The aim of this paper is to summarize the general status of our understanding of small-x physics. It is based on presentations and discussions at an informal meeting on this topic held in Lund, Sweden, in March 2001. This document also marks the founding of an informal collaboration between experimentalists and theoreticians with a special interest in small-x physics. This paper is dedicated to the memory of Bo Andersson, who died unexpectedly from a heart attack on March 4th, 2002.

Herwig 7.0/Herwig++ 3.0 release note

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 HERWIG 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 process calculation for virtually all Standard Model processes, with matching to both angular-ordered and dipole shower modules via both subtractive (MC@NLO-type) and multiplicative (Powheg-type) algorithms; QED radiation and spin correlations in the angular-ordered shower; a consistent treatment 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.

Matrix-element corrections to parton shower algorithms

Abstract We discuss two ways in which parton shower algorithms can be supplemented by matrix-element corrections to ensure the correct hard limit: by using complementary phase-space regions, or by modifying the shower itself. In the former case, existing algorithms are self-consistent only if the total correction is small. In the latter case, existing algorithms are never self-consistent, a problem that is particularly severe for angular-ordered parton shower algorithms. We show how to construct self-consistent algorithms in both cases.

Searches for new particles using cone and cluster jet algorithms: A Comparative study

AbstractWe discuss the reconstruction of the hadronic decays of heavy particles using jet algorithms. The ability to reconstruct the mass of the decaying particles is compared between a traditional cone-type algorithm and a recently proposed cluster-type algorithm. The specific examples considered are the semileptonic decays of a heavy Higgs boson at