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 spin correlations and 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++.The program is written in Fortran and the user has to modify the main program HWIGPR to generate the type and number of events required. See section 8.1 for a sample main program. The program operates by setting up parameters in common blocks and then calling a sequence of subroutines to generate an event. Parameters not set explicitly in the block data HWUDAT or in HWIGPR are set to default values in the main initialisation routine HWIGIN. Output data are delivered in the LEP standard common block HEPEVT [25,26]. Note that all real variables accessible to the user, including those in HEPEVT, are of type DOUBLE PRECISION.Since version 6.3, to take account of the increased energy and complexity of interactions at LHC and future colliders, the default value of the parameter NMXHEP, which sets the array sizes in the standard /HEPEVT/ common block, has been increased to 4000.To generate events the user must first set up the beam particle names PART1, PART2 (type CHARACTER*8) and momenta PBEAM1, PBEAM2 (in GeV/c), a process code IPROC and the number of events required MAXEV.See section 4 for beams and processes available. All analysis of generated events (histogramming, etc.) should be performed by the user-provided routines HWABEG (to initialise the run), HWANAL (to analyse an event) and HWAEND (to terminate the run).A detailed event summary is printed out for the first MAXPR events (default MAXPR = 1). Setting IPRINT = 2 lists the particle identity codes, properties and decay schemes used in the program.The programming language is standard Fortran 77 as far as possible. However, the following may require modification for running on some computers • Most common blocks are inserted by INCLUDE 'HERWIG65.INC' statementsthe file HERWIG65.INC is part of the standard program package.• Many common blo...

We investigate modifications to the k ⊥ -clustering jet algorithm which preserve the advantages of the original Durham algorithm while reducing non-perturbative corrections and providing better resolution of jet substructure. We find that a simple change in the sequence of clustering (combining smaller-angle pairs first), together with the 'freezing' of soft resolved jets, has beneficial effects.

Abstract:The discovery by the ATLAS and CMS experiments of a new boson with mass around 125 GeV and with measured properties compatible with those of a Standard-Model Higgs boson, coupled with the absence of discoveries of phenomena beyond the Standard Model at the TeV scale, has triggered interest in ideas for future Higgs factories. A new circular e + e − collider hosted in a 80 to 100 km tunnel, TLEP, is among the most attractive solutions proposed so far. It has a clean experimental environment, produces high luminosity for top-quark, Higgs boson, W and Z studies, accommodates multiple detectors, and can reach energies up to the tt threshold and beyond. It will enable measurements of the Higgs boson properties and of Electroweak Symmetry-Breaking (EWSB) parameters with unequalled precision, offering exploration of physics beyond the Standard Model in the multi-TeV range. Moreover, being the natural precursor of the VHE-LHC, a 100 TeV hadron machine in the same tunnel, it builds up a long-term vision for particle physics. Altogether, the combination of TLEP and the VHE-LHC offers, for a great cost effectiveness, the best precision and the best search reach of all options presently on the market. This paper presents a first appraisal of the salient features of the TLEP physics potential, to serve as a baseline for a more extensive design study.

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