A new release of the Monte Carlo program Herwig++ (version 2.0) is now
available. This is the first version of the program which can be used for
hadron-hadron physics and includes the full simulation of both initial- and
final-state QCD radiation.Comment: Source code and additional information available at
  http://hepforge.cedar.ac.uk/herwig

Gieseke, S.

Grellscheid, D.

Hamilton, K.

Ribon, A.

Richardson, P.

Seymour, M. H.

Stephens, P.

Webber, B. R.

English

arXiv

A new release of the Monte Carlo program Herwig++ (version 2.0) is now available. This is the first version of the program which can be used for hadron-hadron physics and includes the full simulation of both initial- and final-state QCD radiation.A new release of the Monte Carlo program Herwig++ (version 2.0) is now available. This is the first version of the program which can be used for hadron-hadron physics and includes the full simulation of both initial- and final-state QCD radiation

Seymour, M.H.

Webber, B.R.

CERN Document Server

arXiv:hep-ph/0609306 v1   29 Sep 2006Cavendish-HEP-06/18CERN-PH-TH/2006-182IFJPAN-IV-2006-6IPPP/06/55KA-TP-10-2006September 2006Herwig++ 2.0 Release NoteS. GiesekeInstitut fu¨r Theoretische Physik, Universita¨t KarlsruheE-mail: gieseke@particle.uni-karlsruhe.deD. GrellscheidIPPP, Department of Physics, Durham UniversityE-mail: david.grellscheid@durham.ac.ukK. HamiltonIPPP, Department of Physics, Durham UniversityE-mail: keith.hamilton@durham.ac.ukA. RibonPH Department, CERNE-mail: Alberto.Ribon@cern.chP. RichardsonIPPP, Department of Physics, Durham UniversityE-mail: Peter.Richardson@durham.ac.uk.ac.ukM.H. SeymourPH Department, CERNE-mail: M.Seymour@rl.ac.ukP. StephensInstitute of Nuclear Physics, CracowE-mail: stephens@hep.phy.cam.ac.ukB.R. WebberCavendish Laboratory, University of CambridgeE-mail: webber@hep.phy.cam.ac.ukAbstractA new release of the Monte Carlo program Herwig++ (version 2.0) is now available.This is the first version of the program which can be used for hadron-hadron physics andincludes the full simulation of both initial- and final-state QCD radiation.Contents1 Introduction 11.1 Availability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Hadron-Hadron Processes 13 Top Decay 24 Underlying Event 35 QED Radiation 36 Other Changes 47 Summary 61 IntroductionThe last major public version (1.0) of Herwig++ was reported in detail in [1], subsequently arelease (2.0β) with minimal hadron-hadron physics was made available for integration testingwithin experimental software frameworks, [2]. In this note we describe the main modificationsand new features included in the latest public version, 2.0. We consider this to be the firstversion which can be used for realistic physics studies of hadron-hadron collisions.Please refer to [1] and the present paper if using version 2.0 of the program. A full manualwill be released in the near future.The main new features of this version are that the limited functionality in the β-releasefor the simulation of hadron-hadron collisions has been extended to include: the simulationof processes other than Drell-Yan; final-state radiation of the time-like partons produced inthe space-like shower; initial- and final-state showers in the decay of heavy particles (e.g. thetop quark); the matrix element correction for Drell-Yan processes and top quark decays; thesimulation of QED radiation in particle decays using the YFS formalism [3]; and simulation ofthe underlying event using the model of the UA5 collaboration [4].1.1 AvailabilityThe new program, together with other useful files and information, can be obtained from thefollowing web site:http://hepforge.cedar.ac.uk/herwig/2 Hadron-Hadron ProcessesA number of matrix elements have been added for important processes in hadron collisionsincluding:1• Direct photon pair production, i.e. qq¯/gg → γγ;1The charge conjugate processes while included in the code have been omitted in the list for brevity.1Figure 1: The transverse momentum of the Z0 boson at the Tevatron calculated by HERWIG6.5and Herwig++ compared with run I CDF data taken from [7] with and without matrix elementcorrections.• Photon plus jet production, i.e. qq¯ → γg, qg → γq;• Higgs boson production, i.e. qq¯/gg → h0;• Higgs boson plus jet production, i.e. gg → gh0, qg → qh0 and qq¯ → gh0;• Heavy quark pair production, i.e. qq¯/gg → QQ¯;• QCD 2 → 2 scattering processes, i.e. gg → gg, gg → qq¯, qq¯ → gg, qg → qg, qq → qq,qq¯ → qq¯;in addition the W and Z Drell-Yan processes which were included in the previous version havebeen extended to include the hadronic decays of the boson.Currently the kinematic reconstruction of the parton showers in these processes does not usethe original approach advocated in [5] but a simpler method where the centre-of-mass energyand rapidity of the hard process are preserved in the showering process. This is the procedure of[5] for s-channel colour-singlet systems. The time-like shower of the ongoing partons producedin the initial-state shower is generated using the same choice of basis vectors as for the space-likeshower.The matrix element correction for the production of an s-channel gauge boson, either W±or Z0, has been included using the approach of [6]. The comparison of Herwig++ with andwithout the matrix element correction is shown in Fig. 1. Herwig++ gives better agreement withthe data for medium pT but peaks about 2 GeV below data in the low pT region. Herwig++does not yet include a model of intrinsic-kT in the incoming hadrons which could be tuned toreproduce the data.2Figure 2: Dalitz plot for gluonic radiation in top decay. In both plots both the soft and hardmatrix element corrections have been applied, but only one emission has been allowed. a)shows the radiation for the symmetric choice of [5] for emission from the top and bottom whileb) shows the radiation with the scales chosen to give the maximum amount of radiation fromthe bottom quark. The blue (innermost) line gives the limit for radiation from the bottom,the green (middle) line from the top and the red (outer) line the boundary of the phase-spaceregion.3 Top DecayOne of the main features of the new shower formalism [5] was the treatment of the QCDradiation in the decay of heavy particles. This is now implemented and includes the initial-state forward shower of the heavy particle in its decay together with the traditional time-likeshower of the decay products. This is essential in ensuring that the full phase-space for theemission of soft gluons is covered, in for example t→ bW .In addition, in order to have the same level of simulation as in the FORTRAN program, andto correct for the over emission of soft radiation from the outgoing particles, the matrix elementcorrection for t → bW has been included using the method of [8]. This will be described inmore detail in [9] and the forthcoming manual. The matrix element correction in HERWIGwas not infrared safe and required an arbitrary cutoff on the gluon energy however as the newalgorithm fills the whole region for soft emission such a cutoff is not required. Fig. 2 shows thedistribution of the radiation for two different choices of the maximal scales for radiation fromthe top and bottom quark.4 Underlying EventThe simulation of the underlying event currently uses the model of the UA5 collaboration [4]adapted as described in [10] to use the cluster model. This is the same as the FORTRANimplementation and is intended to give a simple model of the underlying event which will bereplaced by a more sophisticated approach based on multiple scattering in the next majorrelease.Fig. 3 shows the number of charged particles and transverse momentum transverse to thedirection of the leading jet at the Tevatron. In general there is good agreement betweenHerwig++ and HERWIG for the distributions of particles from the soft underlying event withHerwig++ producing more particles from the perturbative scattering.3 [GeV]ljetTp40 60 80 100ch,TN0246Hw++ No UA5Hw6.5 No UA5Hw++ UA5Hw6.5 UA5 [GeV]ljetTp40 60 80 100 [GeV]sum,TTp0510Hw++ No UA5Hw6.5 No UA5Hw++ UA5Hw6.5 UA5Figure 3: The number of charged particles and scalar sum of the transverse momentum inthe transverse azimuthal direction with respect to the direction of the leading jet at the Teva-tron as a function of the transverse momentum of the leading jet. The blue (upper) andblack (lower) points are the results from Herwig++ with/without the soft underlying event. Thelight green (upper) and dark green (lower) points were generated with HERWIG with/withoutthe soft underlying event. For both the number of charged particles and transverse the FOR-TRAN results lie slightly above those from Herwig++. The observables are defined in moredetail in [11].In addition, the simple model of the forced branching after the initial-state parton showers,to ensure that the evolution ends on the valence quarks in the incoming hadron, has beenreplaced with a more sophisticated approach based on that in the FORTRAN program [10].This model allows the forced branchings to take place between a scale QSpac (Default 2.5 GeV)and a multiple EmissionRange (Default 1.1) of the minimum scale. The energy fractions arethen determined using the perturbative result. This replaces the simple model in the betarelease which generated the scale at the minimum value and used a flat distribution for theenergy fraction which tended to give a large number of low mass soft collisions.5 QED RadiationThe simulation of QED radiation using the approach of [3] has been included for both particledecays and unstable s-channel resonances produced in the hard process. This approach is basedon the YFS formalism [12] which takes into account large soft photon logarithms to all orders.In addition, the leading collinear logarithms are included to O (α) by using the dipole splittingfunctions. A comparison of the results of Herwig++ and WINHAC [13] is shown in Figure 4 forleptonic W decays, more examples and a full description of the approach can be found in [3].By default this is switched off for both decays and hard processes but can easily be switchedon by including the QEDRadiationHandler as a PostSubProcessHandler for the hard process orusing the PhotonGenerator interface of the relevant Decayer.6 Other ChangesA number of other more minor changes have been made. The following changes have beenmade to improve the physics simulation:4Figure 4: The total energy of the photons radiated in W± → e±νe/ν¯e decays. In figure (a) thered (dashed) histogram was generated using the WINHAC [13] program, while the black (solid)line was generated using Herwig++. Figure (b) shows the difference between the spectra shownin (a) divided by the statistical error. The disagreement about 40 GeV is due to events withat least two hard photons which neither program is designed to model well.• Decayers have been added for top and electroweak gauge boson decays which are switchedon by default;• The default mass of the Higgs boson has been increased and the appropriate decay modesadded. In addition a number of Decayers have been added for the various Higgs decays.• Some changes have been made to give a more reasonable physical description of thesplitting of beam clusters.• The default pT cut for particles in the hard process has changed so that different cuts canbe used for different types of particles. The default cut for photons and jets (gluons andquarks other than top) has been raised to 20 GeV. The cut for top quarks and leptonshas been reduced to zero;• A number of AnalysisHandlers for the showering from different processes have been added;• The cut member of ClusterFissioner has been made virtual so that the model of [14] canbe implemented as an external package;2• An AnalysisHandler to print a tree history of the event using Graphviz has been added;• A new set of input files to run top quark pair production in e+e− collisions at 500 GeVhas been added;• The ShowerAlphaQCD class implementing strong coupling used in the shower has beenimproved so that the same options as in FORTRAN HERWIG can be used, the defaultremains as in the previous version.• The AlphaEM class implementing exactly the same running electromagnetic coupling asFORTRAN HERWIG has been included.The following more technical changes to the code structure have been made:2This is available from http://hepforge.cedar.ac.uk/herwig/.5• The Shower module has undergone a significant redesign in order to include the newfeatures in this version, to make implementing new matrix element corrections easier andin preparation for improvements such as the multi-scale shower and CKKW procedurewhich are foreseen in future versions;• The GlobalParameters object has been removed. The effective gluon mass is now taken tobe the constituent mass of the gluon from its ParticleData object. This should be set tozero if the Lund string model is used for the hadronization of the event.• The splitting of the hadronic remnant has been moved to be part of the Hadronizationmodule;• The libraries of AnalysisHandlers supplied with the release have been restructured to sep-arate those which depend on KtJet from those which do not;• The original obsolete matrix elements have been deleted;• The default number of events outputted to the log file has been reduced to 100 due tothe larger files created when the UA5 underlying event model is switched on;• Numerous improvements have been made to the DOXYGEN documentation;• Several fixes have been made to correct memory leaks in the initialisation of the eventgenerator;• The Amegic interface which has not been used or tested in some time has been removedfrom the release;• The structure of the default input files has been changed and cleaned up;• Most classes have been cleaned up to remove unused member functions;• Vector and Tensor meson decay base classes have been removed to speed up code as littlefunctionality remained in them.The following bugs have been fixed:• A bug affecting the gluon splitting function for time-like radiation;• A number of bugs affecting the backward evolution of antiquarks;• A bug affecting the azimuthal distribution of the gauge boson in Drell-Yan processes;• Corrections to particle data tables so that gauge and Higgs boson spins are correctly set;• A bug in the VSSVertex where the off-shell scalar wavefunction was not correctly included;• A bug to the multi-channel decay phase-space integrator which affected a vertex with twooff-shell particles, which had not previously been encountered.7 SummaryHerwig++2.0 is the first version of the Herwig++ program with a complete simulation of hadron-hadron physics. We look forward to feedback and input from users, especially from the Tevatronand LHC experiments.Our next major milestone is the release of version 3.0 which will be at least as completeas HERWIG in all aspects of LHC and linear collider simulation. The major new features forthis version will be the inclusion of Beyond the Standard Model physics, a multiple scatteringmodel for the underlying event and the full inclusion of the new hadron decay module. Inaddition depending on the time scale for the release a number of improvements to the partonshower may also be included. Following the release of Herwig++3.0 we expect that support forthe FORTRAN program will cease.AcknowledgementsWe would like to thank Manuel Ba¨hr, Martyn Gigg, Seyi Latunde-Dada, Simon Pla¨tzer, KosukeOdagiri and Alexander Sherstnev for commenting on preliminary versions of the program andManuel Ba¨hr for help in testing the simulation of the underlying event and producing the plotsof the results of the UA5 model.References[1] S. Gieseke, A. Ribon, M. H. Seymour, P. Stephens and B. Webber, JHEP 0402 (2004)005 [arXiv:hep-ph/0311208].[2] S. Gieseke, D. Grellscheid, A. Ribon, P. Richardson, M. H. Seymour, P. Stephens andB. R. Webber, arXiv:hep-ph/0602069.[3] K. Hamilton and P. Richardson, JHEP 0607 (2006) 010 [arXiv:hep-ph/0603034].[4] G. J. Alner et al. [UA5 Collaboration], Nucl. Phys. B 291 (1987) 445.[5] S. Gieseke, P. Stephens and B. Webber, JHEP 0312 (2003) 045 [arXiv:hep-ph/0310083].[6] G. Corcella and M. H. Seymour, Nucl. Phys. B 565 (2000) 227 [arXiv:hep-ph/9908388].[7] A. A. Affolder et al. [CDF Collaboration], Phys. Rev. Lett. 84 (2000) 845 [arXiv:hep-ex/0001021].[8] G. Corcella and M. H. Seymour, Phys. Lett. B 442 (1998) 417 [arXiv:hep-ph/9809451].[9] K. Hamilton, P. Richardson, in preparation.[10] HERWIG 6.5, G. Corcella, I.G. Knowles, G. Marchesini, S. Moretti, K. Odagiri,P. Richardson, M.H. Seymour and B.R. Webber, JHEP 0101 (2001) 010 [arXiv:hep-ph/0011363]; [arXiv:hep-ph/0210213].[11] A. A. Affolder et al. [CDF Collaboration], “Charged jet evolution and the underlying eventin pp¯ collisions at Phys. Rev. D 65 (2002) 092002.[12] D. R. Yennie, S. C. Frautschi and H. Suura, Annals Phys. 13 (1961) 379.7[13] W. Placzek and S. Jadach, Eur. Phys. J. C 29 (2003) 325 [arXiv:hep-ph/0302065].[14] K. Odagiri, arXiv:hep-ph/0606315.8

Herwig++ 2.0 Release Note

A new release of the Monte Carlo program HERWIG (version 6.5) is now
available. The main new features are: support for the Les Houches interface to
matrix element generators; additional SM and MSSM Higgs processes in lepton
collisions; additional matrix elements for the spin correlation algorithm; a
new version of the ISAWIG interface; interface to the MC@NLO program for heavy
quark, Higgs and vector boson production in hadron collisions. boson pair
production in hadron collisions. This is planned to be the last major release
of Fortran HERWIG. Future developments will be implemented in a new C++ event
generator, HERWIG++

Corcella, G

Knowles, IG

Marchesini, G

Moretti, S

Odagiri, K

Richardson, P

Seymour, MH

Webber, BR

Apollo (Cambridge)

arXiv:hep-ph/0609306v1  29 Sep 2006Cavendish-HEP-06/18CERN-PH-TH/2006-182IFJPAN-IV-2006-6IPPP/06/55KA-TP-10-2006September 2006Herwig++ 2.0 Release NoteS. GiesekeInstitut fu¨r Theoretische Physik, Universita¨t KarlsruheE-mail: gieseke@particle.uni-karlsruhe.deD. GrellscheidIPPP, Department of Physics, Durham UniversityE-mail: david.grellscheid@durham.ac.ukK. HamiltonIPPP, Department of Physics, Durham UniversityE-mail: keith.hamilton@durham.ac.ukA. RibonPH Department, CERNE-mail: Alberto.Ribon@cern.chP. RichardsonIPPP, Department of Physics, Durham UniversityE-mail: Peter.Richardson@durham.ac.uk.ac.ukM.H. SeymourPH Department, CERNE-mail: M.Seymour@rl.ac.ukP. StephensInstitute of Nuclear Physics, CracowE-mail: stephens@hep.phy.cam.ac.ukB.R. WebberCavendish Laboratory, University of CambridgeE-mail: webber@hep.phy.cam.ac.ukAbstractA new release of the Monte Carlo program Herwig++ (version 2.0) is now available.This is the first version of the program which can be used for hadron-hadron physics andincludes the full simulation of both initial- and final-state QCD radiation.Contents1 Introduction 11.1 Availability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Hadron-Hadron Processes 13 Top Decay 34 Underlying Event 35 QED Radiation 46 Other Changes 47 Summary 71 IntroductionThe last major public version (1.0) of Herwig++ was reported in detail in [1], subsequently arelease (2.0β) with minimal hadron-hadron physics was made available for integration testingwithin experimental software frameworks, [2]. In this note we describe the main modificationsand new features included in the latest public version, 2.0. We consider this to be the firstversion which can be used for realistic physics studies of hadron-hadron collisions.Please refer to [1] and the present paper if using version 2.0 of the program. A full manualwill be released in the near future.The main new features of this version are that the limited functionality in the β-releasefor the simulation of hadron-hadron collisions has been extended to include: the simulationof processes other than Drell-Yan; final-state radiation of the time-like partons produced inthe space-like shower; initial- and final-state showers in the decay of heavy particles (e.g. thetop quark); the matrix element correction for Drell-Yan processes and top quark decays; thesimulation of QED radiation in particle decays using the YFS formalism [3]; and simulation ofthe underlying event using the model of the UA5 collaboration [4].1.1 AvailabilityThe new program, together with other useful files and information, can be obtained from thefollowing web site:http://hepforge.cedar.ac.uk/herwig/2 Hadron-Hadron ProcessesA number of matrix elements have been added for important processes in hadron collisionsincluding:1• Direct photon pair production, i.e. qq¯/gg → γγ;1The charge conjugate processes while included in the code have been omitted in the list for brevity.1Figure 1: The transverse momentum of the Z0 boson at the Tevatron calculated by HERWIG6.5and Herwig++ compared with run I CDF data taken from [7] with and without matrix elementcorrections.• Photon plus jet production, i.e. qq¯ → γg, qg → γq;• Higgs boson production, i.e. qq¯/gg → h0;• Higgs boson plus jet production, i.e. gg → gh0, qg → qh0 and qq¯ → gh0;• Heavy quark pair production, i.e. qq¯/gg → QQ¯;• QCD 2 → 2 scattering processes, i.e. gg → gg, gg → qq¯, qq¯ → gg, qg → qg, qq → qq,qq¯ → qq¯;in addition the W and Z Drell-Yan processes which were included in the previous version havebeen extended to include the hadronic decays of the boson.Currently the kinematic reconstruction of the parton showers in these processes does not usethe original approach advocated in [5] but a simpler method where the centre-of-mass energyand rapidity of the hard process are preserved in the showering process. This is the procedure of[5] for s-channel colour-singlet systems. The time-like shower of the ongoing partons producedin the initial-state shower is generated using the same choice of basis vectors as for the space-likeshower.The matrix element correction for the production of an s-channel gauge boson, either W±or Z0, has been included using the approach of [6]. The comparison of Herwig++ with andwithout the matrix element correction is shown in Fig. 1. Herwig++ gives better agreement withthe data for medium pT but peaks about 2 GeV below data in the low pT region. Herwig++does not yet include a model of intrinsic-kT in the incoming hadrons which could be tuned toreproduce the data.2Figure 2: Dalitz plot for gluonic radiation in top decay. In both plots both the soft and hardmatrix element corrections have been applied, but only one emission has been allowed. a)shows the radiation for the symmetric choice of [5] for emission from the top and bottom whileb) shows the radiation with the scales chosen to give the maximum amount of radiation fromthe bottom quark. The blue (innermost) line gives the limit for radiation from the bottom,the green (middle) line from the top and the red (outer) line the boundary of the phase-spaceregion.3 Top DecayOne of the main features of the new shower formalism [5] was the treatment of the QCDradiation in the decay of heavy particles. This is now implemented and includes the initial-state forward shower of the heavy particle in its decay together with the traditional time-likeshower of the decay products. This is essential in ensuring that the full phase-space for theemission of soft gluons is covered, in for example t→ bW .In addition, in order to have the same level of simulation as in the FORTRAN program, andto correct for the over emission of soft radiation from the outgoing particles, the matrix elementcorrection for t → bW has been included using the method of [8]. This will be described inmore detail in [9] and the forthcoming manual. The matrix element correction in HERWIGwas not infrared safe and required an arbitrary cutoff on the gluon energy however as the newalgorithm fills the whole region for soft emission such a cutoff is not required. Fig. 2 shows thedistribution of the radiation for two different choices of the maximal scales for radiation fromthe top and bottom quark.4 Underlying EventThe simulation of the underlying event currently uses the model of the UA5 collaboration [4]adapted as described in [10] to use the cluster model. This is the same as the FORTRANimplementation and is intended to give a simple model of the underlying event which will bereplaced by a more sophisticated approach based on multiple scattering in the next majorrelease.Fig. 3 shows the number of charged particles and transverse momentum transverse to thedirection of the leading jet at the Tevatron. In general there is good agreement betweenHerwig++ and HERWIG for the distributions of particles from the soft underlying event withHerwig++ producing more particles from the perturbative scattering.3 [GeV]ljetTp40 60 80 100ch,TN0246Hw++ No UA5Hw6.5 No UA5Hw++ UA5Hw6.5 UA5 [GeV]ljetTp40 60 80 100 [GeV]sum,TTp0510Hw++ No UA5Hw6.5 No UA5Hw++ UA5Hw6.5 UA5Figure 3: The number of charged particles and scalar sum of the transverse momentum inthe transverse azimuthal direction with respect to the direction of the leading jet at the Teva-tron as a function of the transverse momentum of the leading jet. The blue (upper) andblack (lower) points are the results from Herwig++ with/without the soft underlying event. Thelight green (upper) and dark green (lower) points were generated with HERWIG with/withoutthe soft underlying event. For both the number of charged particles and transverse the FOR-TRAN results lie slightly above those from Herwig++. The observables are defined in moredetail in [11].In addition, the simple model of the forced branching after the initial-state parton showers,to ensure that the evolution ends on the valence quarks in the incoming hadron, has beenreplaced with a more sophisticated approach based on that in the FORTRAN program [10].This model allows the forced branchings to take place between a scale QSpac (Default 2.5 GeV)and a multiple EmissionRange (Default 1.1) of the minimum scale. The energy fractions arethen determined using the perturbative result. This replaces the simple model in the betarelease which generated the scale at the minimum value and used a flat distribution for theenergy fraction which tended to give a large number of low mass soft collisions.5 QED RadiationThe simulation of QED radiation using the approach of [3] has been included for both particledecays and unstable s-channel resonances produced in the hard process. This approach is basedon the YFS formalism [12] which takes into account large soft photon logarithms to all orders.In addition, the leading collinear logarithms are included to O (α) by using the dipole splittingfunctions. A comparison of the results of Herwig++ and WINHAC [13] is shown in Figure 4 forleptonic W decays, more examples and a full description of the approach can be found in [3].By default this is switched off for both decays and hard processes but can easily be switchedon by including the QEDRadiationHandler as a PostSubProcessHandler for the hard process orusing the PhotonGenerator interface of the relevant Decayer.6 Other ChangesA number of other more minor changes have been made. The following changes have beenmade to improve the physics simulation:4Figure 4: The total energy of the photons radiated in W± → e±νe/ν¯e decays. In figure (a) thered (dashed) histogram was generated using the WINHAC [13] program, while the black (solid)line was generated using Herwig++. Figure (b) shows the difference between the spectra shownin (a) divided by the statistical error. The disagreement about 40 GeV is due to events withat least two hard photons which neither program is designed to model well.• Decayers have been added for top and electroweak gauge boson decays which are switchedon by default;• The default mass of the Higgs boson has been increased and the appropriate decay modesadded. In addition a number of Decayers have been added for the various Higgs decays.• Some changes have been made to give a more reasonable physical description of thesplitting of beam clusters.• The default pT cut for particles in the hard process has changed so that different cuts canbe used for different types of particles. The default cut for photons and jets (gluons andquarks other than top) has been raised to 20 GeV. The cut for top quarks and leptonshas been reduced to zero;• A number of AnalysisHandlers for the showering from different processes have been added;• The cut member of ClusterFissioner has been made virtual so that the model of [14] canbe implemented as an external package;2• An AnalysisHandler to print a tree history of the event using Graphviz has been added;• A new set of input files to run top quark pair production in e+e− collisions at 500 GeVhas been added;• The ShowerAlphaQCD class implementing strong coupling used in the shower has beenimproved so that the same options as in FORTRAN HERWIG can be used, the defaultremains as in the previous version.• The AlphaEM class implementing exactly the same running electromagnetic coupling asFORTRAN HERWIG has been included.The following more technical changes to the code structure have been made:2This is available from http://hepforge.cedar.ac.uk/herwig/.5• The Shower module has undergone a significant redesign in order to include the newfeatures in this version, to make implementing new matrix element corrections easier andin preparation for improvements such as the multi-scale shower and CKKW procedurewhich are foreseen in future versions;• The GlobalParameters object has been removed. The effective gluon mass is now taken tobe the constituent mass of the gluon from its ParticleData object. This should be set tozero if the Lund string model is used for the hadronization of the event.• The splitting of the hadronic remnant has been moved to be part of the Hadronizationmodule;• The libraries of AnalysisHandlers supplied with the release have been restructured to sep-arate those which depend on KtJet from those which do not;• The original obsolete matrix elements have been deleted;• The default number of events outputted to the log file has been reduced to 100 due tothe larger files created when the UA5 underlying event model is switched on;• Numerous improvements have been made to the DOXYGEN documentation;• Several fixes have been made to correct memory leaks in the initialisation of the eventgenerator;• The Amegic interface which has not been used or tested in some time has been removedfrom the release;• The structure of the default input files has been changed and cleaned up;• Most classes have been cleaned up to remove unused member functions;• Vector and Tensor meson decay base classes have been removed to speed up code as littlefunctionality remained in them.The following bugs have been fixed:• A bug affecting the gluon splitting function for time-like radiation;• A number of bugs affecting the backward evolution of antiquarks;• A bug affecting the azimuthal distribution of the gauge boson in Drell-Yan processes;• Corrections to particle data tables so that gauge and Higgs boson spins are correctly set;• A bug in the VSSVertex where the off-shell scalar wavefunction was not correctly included;• A bug to the multi-channel decay phase-space integrator which affected a vertex with twooff-shell particles, which had not previously been encountered.67 SummaryHerwig++2.0 is the first version of the Herwig++ program with a complete simulation of hadron-hadron physics. We look forward to feedback and input from users, especially from the Tevatronand LHC experiments.Our next major milestone is the release of version 3.0 which will be at least as completeas HERWIG in all aspects of LHC and linear collider simulation. The major new features forthis version will be the inclusion of Beyond the Standard Model physics, a multiple scatteringmodel for the underlying event and the full inclusion of the new hadron decay module. Inaddition depending on the time scale for the release a number of improvements to the partonshower may also be included. Following the release of Herwig++3.0 we expect that support forthe FORTRAN program will cease.AcknowledgementsWe would like to thank Manuel Ba¨hr, Martyn Gigg, Seyi Latunde-Dada, Simon Pla¨tzer, KosukeOdagiri and Alexander Sherstnev for commenting on preliminary versions of the program andManuel Ba¨hr for help in testing the simulation of the underlying event and producing the plotsof the results of the UA5 model.References[1] S. Gieseke, A. Ribon, M. H. Seymour, P. Stephens and B. Webber, JHEP 0402 (2004)005 [arXiv:hep-ph/0311208].[2] S. Gieseke, D. Grellscheid, A. Ribon, P. Richardson, M. H. Seymour, P. Stephens andB. R. Webber, arXiv:hep-ph/0602069.[3] K. Hamilton and P. Richardson, JHEP 0607 (2006) 010 [arXiv:hep-ph/0603034].[4] G. J. Alner et al. [UA5 Collaboration], Nucl. Phys. B 291 (1987) 445.[5] S. Gieseke, P. Stephens and B. Webber, JHEP 0312 (2003) 045 [arXiv:hep-ph/0310083].[6] G. Corcella and M. H. Seymour, Nucl. Phys. B 565 (2000) 227 [arXiv:hep-ph/9908388].[7] A. A. Affolder et al. [CDF Collaboration], Phys. Rev. Lett. 84 (2000) 845 [arXiv:hep-ex/0001021].[8] G. Corcella and M. H. Seymour, Phys. Lett. B 442 (1998) 417 [arXiv:hep-ph/9809451].[9] K. Hamilton, P. Richardson, in preparation.[10] HERWIG 6.5, G. Corcella, I.G. Knowles, G. Marchesini, S. Moretti, K. Odagiri,P. Richardson, M.H. Seymour and B.R. Webber, JHEP 0101 (2001) 010 [arXiv:hep-ph/0011363]; [arXiv:hep-ph/0210213].[11] A. A. Affolder et al. [CDF Collaboration], “Charged jet evolution and the underlying eventin pp¯ collisions at Phys. Rev. D 65 (2002) 092002.[12] D. R. Yennie, S. C. Frautschi and H. Suura, Annals Phys. 13 (1961) 379.7[13] W. Placzek and S. Jadach, Eur. Phys. J. C 29 (2003) 325 [arXiv:hep-ph/0302065].[14] K. Odagiri, arXiv:hep-ph/0606315.8

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