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

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 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...

EffectiveK-factors for gg H WW at the LHC

“…4 could suggest an improper treatment of the effect of a jet veto in PYTHIA. In order to check the reliability of our reweighting procedure we have compared the efficiency of a jet veto with the one obtained with HERWIG [16], which is known to provide a better description of the p H T spectrum in the small-p H T region [17]. When p H T ∼ < 40 GeV, the efficiencies differ by less than 5%, thus confirming the validity of our approach.…”

Search for supersymmetry in events with four or more leptons ins=8 TeVppcollisions with the ATLAS detector

“…Details of the MC generation are listed in Table III. When the parton shower is generated with HERWIG-6.520 [40], the underlying event is simulated by JIMMY-4.31 [41]. All samples are processed using the full ATLAS detector simulation [42] based on GEANT4 [43], except for the tWZ, tZ and W=ZHð→ μμÞ samples, which are instead simulated with a parametrization of the performance of the ATLAS electromagnetic and hadronic calorimeters and with GEANT4 for other detector components [44].…”

Recent Results on Soft QCD at ATLAS

“…Figure 9 shows two distributions of ∆ n jets using a combined set of models: Alpgen [14], Herwig [15] and Jimmy [16]. In one distribution, DPI may take place and for the other DPI is explicitly turned off.…”