Herwig++ is the successor of the event generator HERWIG. In its present version 2.2.1 it provides a program for full LHC event generation which is superior to the previous program in many respects. We briefly summarize its features and describe present work and some future plans

Bähr, M.

De Roeck, Albert

Gieseke, S.

Gigg, M.A.

Grellscheid, D.

Hamilton, K.

Jung, Hannes

Latunde-Dada, O.

Plätzer, S.

Richardson, P.

Seymour, M.H.

Sherstnev, A.

Tully, J.

Webberd, B.R.

English

arXiv

Herwig++ is the successor of the event generator HERWIG. In its present
version 2.2.1 it provides a program for full LHC event generation which is
superior to the previous program in many respects. We briefly summarize its
features and describe present work and some future plans.
 Comment: 5 pages, to appear in the proceedings of the HERA and the LHC
  worksho

Bahr, M.

Platzer, S.

Webber, B.R.

HERA and the LHC: 4th Workshop on the Implications of HERA for LHC Physics

DIAL UCLouvain

arXiv:0809.2002v1  [hep-ph]  11 Sep 2008Herwig++ Status Report∗CP3-08-42DCTP/08/134IPPP/08/67KA-TP-20-2008MCnet/08/10M. Ba¨hra, S. Giesekea, M.A. Giggb, D. Grellscheidb, K. Hamiltonc, O. Latunde-Dadad, S. Pla¨tzera,P. Richardsonb,e, M.H. Seymoure,f , A. Sherstnevd , J. Tullyb, B.R. Webberda Institut fu¨r Theoretische Physik, Universita¨t Karlsruhe,b IPPP, Department of Physics, Durham University,c Centre for Particle Physics and Phenomenology, Universite´ Catholique de Louvain,d Cavendish Laboratory, University of Cambridge,e Physics Department, CERN,f School of Physics and Astronomy, University of Manchester. †AbstractHerwig++ is the successor of the event generator HERWIG. In its present ver-sion 2.2.1 it provides a program for full LHC event generation which is su-perior to the previous program in many respects. We briefly summarize itsfeatures and describe present work and some future plans.1 IntroductionWith the advent of the LHC era it was decided to completely rewrite the general purpose event generatorHERWIG [1, 2] in C++ under the name Herwig++, based on the package ThePEG [3, 4]. The goal is notonly to provide a simple replacement of HERWIG but to incorporate physics improvements as well [5].From 2001 until now Herwig++ has been continuously developed and extended [6–10]. The currentversion is 2.2.1, cf. [11]. The physics simulation of the current version is more sophisticated than the oneof Fortran HERWIG in many respects. In this report we will briefly summarize the status of the differentaspects of the simulation. These are the hard matrix elements available, initial and final state partonshowers, the hadronization, hadronic decays and the underlying event. We conclude with an outlook toplanned future improvements.2 Physics simulation steps2.1 Matrix elementsThe event generation begins with the hard scattering of incoming particles or partons in the case ofhadronic collisions. We have included a relatively small number of hard matrix elements. These includee+e− annihilation to qq¯ pairs or simply to Z0 bosons and deep inelastic scattering. In addition thereis the Higgsstrahlung process e+e− → h0Z0. For hadron–hadron collisions we have the QCD 2 → 2processes including heavy quark production. For colourless final states we have the following matrixelements,hh→ (γ, Z0)→ ℓ+ℓ− , hh→W± → ℓ±νℓ(ν¯ℓ) , hh→ h0 , hh→ h0Z0 , hh→ γγ .We also provide matrix elements for processes with additional jets in the final state, likehh→ (γ, Z0,W±) + jet , hh→ h0 + jet .∗to appear in the proceedings of the HERA and the LHC workshop.†herwig@projects.hepforge.orgIn addition, there are matrix elements for perturbative decays of the top quark, which will be simu-lated including spin correlations (see below). There will be some more matrix elements added in futureversions, e.g. for hh→ qqh0. Despite the rather small number of matrix elements, there is no real limita-tion to the processes that may be simulated with Herwig++. In practice, one may use any matrix elementgenerator to generate a standard event file [12] which in turn can be read and processed by Herwig++.For processes with many legs in the final state we follow a different strategy. When the numberof legs becomes large — typically larger than 6–8 particles in the final state — it will be increasinglydifficult to achieve an efficient event generation of the full matrix element. For these situations wehave a generic framework to build up matrix elements for production and decays of particles in orderto approximate any tree level matrix element as a simple production process with subsequent two orthree body decays. This is a good approximation whenever the widths of the intermediate particles aresmall. The spin correlations among these particles can be restored with the algorithm described in [13].Also finite width effects are taken into account [14]. The full simulation of several processes of manymodels for physics beyond the standard model (MSSM, UED, Randall–Sunrum model) is thus possible inHerwig++ [15]. Here, all necessary matrix elements for production and decay processes are constructedautomatically from a model file.2.2 Parton Showers and matching with matrix elementsAfter the hard process has been generated, typically at a large scale ∼ 100GeV–1 TeV, the coloured par-ticles in the process radiate a large number of additional partons, predominantly gluons. As long as theseare resolved by a hard scale of ∼ 1GeV this is simulated with a coherent branching algorithm, as out-lined in [16] which generalizes the original algorithm [17–19] used in HERWIG. The main improvementswith respect to the old algorithm are boost invariance along the jet axis, due to a covariant formulation,and the improved treatment of radiation off heavy quarks. We are using mass–dependent splitting func-tions and a description of the kinematics that allows us to dynamically generate the dead–cone effect.In addition to initial and final state parton showers there are also parton showers in the decay of heavyparticles, the top quark in our case.When extrapolating to hard, wide–angle emissions, the parton shower description is not suffi-ciently accurate in situations where observables depend on large transverse momenta in the process. Inthese cases we supply so–called hard matrix element corrections that describe the hardest parton emis-sion, usually a hard gluon, with the full matrix element for the process that includes that extra parton. Inorder to consistently describe the whole phase space one has to apply soft matrix element corrections.Matrix element corrections are available for Drell–Yan type processes, Higgs production in gg fusionand e+e− annihilation to qq¯–pairs. In addition, we apply a matrix element correction in top–quark de-cays [20].From the point of view of perturbation theory, the hard matrix element correction is only one partof the next–to–leading order (NLO) correction to the Born matrix element. The full NLO calculation alsoincludes the virtual part with the same final state as the Born approximation. When trying to match NLOcalculations and parton shower algorithms systematically, we have to avoid double counting of the realemission contributions. Two systematic approaches are being successfully discussed and applied in eventgenerators: MC@NLO [21–23] and the POWHEG approach [24, 25]. In Herwig++ we have includedworking examples of matching in both approaches. The MC@NLO method, adopted to Herwig++ isdescribed in [26]. Whereas the POWHEG method has already been applied for several processes ine+e− annihilation [27, 28] and also for Drell–Yan production [29]. Parts of these implementations willbecome available in future releases.Another viable possibility to improve the description of QCD radiation in the event generation isthe matching to multiple tree–level matrix elements, that describe the radiation of n additional jets withrespect to the Born level. Theoretically most consistent is the CKKW approach [30] which has beenstudied in the context of an angular ordered parton shower in [31].2.3 Hadronization and decaysThe hadronization model in Herwig++ is the cluster hadronization model which has not been changedmuch from its predecessor in HERWIG. After the parton shower, all gluons are split nonperturbativelyinto qq¯ pairs. Then, following the colour history of the parton cascade, all colour triplet–antitriplet pairsare paired up in colourless clusters which still carry all flavour and momentum information of the originalpartons. While these are heavier than some threshold mass they will fission into lighter clusters until allclusters are sufficiently light. These light clusters will then decay into pairs of hadrons.The hadrons thus obtained are often heavy resonances that will eventually decay on timescales thatare still irrelevant for the experiment. These hadronic decays have been largely rewritten and are modeledin much greater detail in Herwig++. While in HERWIG they were often simply decayed according to theavailable phase space only, we now take into account more experimental information, like form factors,that allow for a realistic modeling of decay matrix elements [32,33]. In a major effort, a large fraction ofthe decay channels described in the particle data book [34] have been included into Herwig++.2.4 Underlying eventThe underlying event model of Herwig++ is a model for multiple hard partonic interactions, based on aneikonal model, similar to JIMMY [35]. In addition to the signal process there are a number of additionalQCD scatters, including full parton showers, that contribute to the overall hadronic activity in the finalstate and eventually also give rise to a (relatively soft) jet substructure in the underlying event. Themodel has two important parameters, one parameter µ, describing the spatial density of partonic matterin the colliding protons. Secondly, there is one cut off parameter p⊥,min that gives a lower bound on thedifferential cross section for QCD 2→ 2 jet production. The model has been carefully tuned to Tevatrondata [36]. Further possible bounds on the model parameters have been studied in [37]. An alternativemodeling of the underlying event on the basis of the UA5 model [38] is also available for historic reasons.Currently, the multiple partonic interaction model is limited to hard scattering while a soft com-ponent is simply not present. For a realistic simulation of minimum bias events a soft component is,however, very important. An extension into the soft region, allowing us the simulation of minimum biasevents is currently being studied and is likely to be included in the next release of Herwig++.3 AvailabilityThe latest version of Herwig++ is always available from hepforge:http://projects.hepforge.org/herwigThere one can also find wiki pages to help with questions concerning installation, changing particularparameters and other frequently asked questions. The installation process is straightforward on anymodern variant of linux. The physics details of the program are now documented in great detail in ourmanual [33]. The pdf version of the manual contains addional links to the online documentation of thecode. All important parameters have been carefully tuned to a wealth of available data and the code isshipped with default paramters that give the best overall description of the data that we have tuned to.Details of the tune can also be found in the manual [33].AcknowledgmentsThis work has been supported in part by the EU Marie Curie Research and Training Network MCnetunder contract MRTN-CT-2006-035606.References[1] G. Corcella et. al., HERWIG 6: An event generator for hadron emission reactions with interferinggluons (including supersymmetric processes), JHEP 01 (2001) 010, [hep-ph/0011363].[2] G. Corcella et. al., HERWIG 6.5 release note, hep-ph/0210213.[3] L. Lo¨nnblad, ThePEG, Pythia7, Herwig++ and Ariadne, Nucl. Instrum. Meth. A559 (2006)246–248.[4] M. Bertini, L. Lo¨nnblad, and T. Sjo¨strand, Pythia version 7-0.0: A proof-of-concept version,Comput. Phys. Commun. 134 (2001) 365–391, [hep-ph/0006152].[5] S. Gieseke, Event generators: New developments, hep-ph/0210294.[6] S. Gieseke, A. Ribon, M. H. Seymour, P. Stephens, and B. Webber, Herwig++ 1.0: An eventgenerator for e+ e- annihilation, JHEP 02 (2004) 005, [hep-ph/0311208].[7] S. Gieseke et. al., Herwig++ 2.0 beta release note, hep-ph/0602069.[8] S. Gieseke et. al., Herwig++ 2.0 release note, hep-ph/0609306.[9] M. Ba¨hr et. al., Herwig++ 2.1 Release Note, arXiv:0711.3137.[10] M. Ba¨hr et. al., Herwig++ 2.2 Release Note, arXiv:0804.3053.[11] http://projects.hepforge.org/herwig.[12] J. Alwall et. al., A standard format for Les Houches event files, Comput. Phys. Commun. 176(2007) 300–304, [hep-ph/0609017].[13] P. Richardson, Spin correlations in Monte Carlo simulations, JHEP 11 (2001) 029,[hep-ph/0110108].[14] M. A. Gigg and P. Richardson, Simulation of Finite Width Effects in Physics Beyond the StandardModel, arXiv:0805.3037.[15] M. Gigg and P. Richardson, Simulation of beyond standard model physics in Herwig++, Eur.Phys. J. C51 (2007) 989–1008, [hep-ph/0703199].[16] S. Gieseke, P. Stephens, and B. Webber, New formalism for QCD parton showers, JHEP 12(2003) 045, [hep-ph/0310083].[17] G. Marchesini and B. R. Webber, Simulation of QCD Jets Including Soft Gluon Interference, Nucl.Phys. B238 (1984) 1.[18] B. R. Webber, A QCD Model for Jet Fragmentation Including Soft Gluon Interference, Nucl. Phys.B238 (1984) 492.[19] G. Marchesini and B. R. Webber, Monte Carlo Simulation of General Hard Processes withCoherent QCD Radiation, Nucl. Phys. B310 (1988) 461.[20] K. Hamilton and P. Richardson, A simulation of QCD radiation in top quark decays, JHEP 02(2007) 069, [hep-ph/0612236].[21] S. Frixione and B. R. Webber, Matching NLO QCD computations and parton shower simulations,JHEP 06 (2002) 029, [hep-ph/0204244].[22] S. Frixione, P. Nason, and B. R. Webber, Matching NLO QCD and parton showers in heavyflavour production, JHEP 08 (2003) 007, [hep-ph/0305252].[23] S. Frixione and B. R. Webber, The MC@NLO 3.3 event generator, hep-ph/0612272.[24] P. Nason, A new method for combining NLO QCD with shower Monte Carlo algorithms, JHEP 11(2004) 040, [hep-ph/0409146].[25] S. Frixione, P. Nason, and C. Oleari, Matching NLO QCD computations with Parton Showersimulations: the POWHEG method, JHEP 11 (2007) 070, [arXiv:0709.2092].[26] O. Latunde-Dada, Herwig++ Monte Carlo At Next-To-Leading Order for e+e- annihilation andlepton pair production, JHEP 11 (2007) 040, [arXiv:0708.4390].[27] O. Latunde-Dada, S. Gieseke, and B. Webber, A positive-weight next-to-leading-order MonteCarlo for e+ e- annihilation to hadrons, JHEP 02 (2007) 051, [hep-ph/0612281].[28] O. Latunde-Dada, Applying the POWHEG method to top pair production and decays at the ILC,arXiv:0806.4560.[29] K. Hamilton, P. Richardson, and J. Tully, A Positive-Weight Next-to-Leading Order Monte CarloSimulation of Drell-Yan Vector Boson Production, arXiv:0806.0290.[30] S. Catani, F. Krauss, R. Kuhn, and B. R. Webber, QCD matrix elements + parton showers, JHEP11 (2001) 063, [hep-ph/0109231].[31] S. Pla¨tzer, Diploma Thesis , . Universita¨t Karlsruhe, 2006.[32] D. Grellscheid and P. Richardson, Simulation of Tau Decays in the Herwig++ Event Generator,arXiv:0710.1951.[33] M. Ba¨hr et. al., Herwig++ Physics and Manual, arXiv:0803.0883.[34] Particle Data Group Collaboration, C. Amsler et. al., Review of particle physics, Phys. Lett.B667 (2008) 1.[35] J. M. Butterworth, J. R. Forshaw, and M. H. Seymour, Multiparton interactions inphotoproduction at HERA, Z. Phys. C72 (1996) 637–646, [hep-ph/9601371].[36] M. Ba¨hr, S. Gieseke, and M. H. Seymour, Simulation of multiple partonic interactions inHerwig++, JHEP 07 (2008) 076, [arXiv:0803.3633].[37] M. Ba¨hr, J. M. Butterworth, and M. H. Seymour, The Underlying Event and the Total CrossSection from Tevatron to the LHC, arXiv:0806.2949.[38] UA5 Collaboration, G. J. Alner et. al., The UA5 High-Energy anti-p p Simulation Program, Nucl.Phys. B291 (1987) 445.

Herwig++ Status Report

Durham Research Online

arXiv:0809.2002v1  [hep-ph]  11 Sep 2008Herwig++ Status Report∗CP3-08-42DCTP/08/134IPPP/08/67KA-TP-20-2008MCnet/08/10M. Bähra, S. Giesekea, M.A. Giggb, D. Grellscheidb, K. Hamiltonc, O. Latunde-Dadad, S. Pl̈atzera,P. Richardsonb,e, M.H. Seymoure,f , A. Sherstnevd , J. Tullyb, B.R. Webberda Institut für Theoretische Physik, Universität Karlsruhe,b IPPP, Department of Physics, Durham University,c Centre for Particle Physics and Phenomenology, Universit´e Catholique de Louvain,d Cavendish Laboratory, University of Cambridge,e Physics Department, CERN,f School of Physics and Astronomy, University of Manchester.†AbstractHerwig++ is the successor of the event generatorHERWIG. In its present ver-sion 2.2.1 it provides a program for full LHC event generation which is su-perior to the previous program in many respects. We briefly summarize itsfeatures and describe present work and some future plans.1 IntroductionWith the advent of the LHC era it was decided to completely rewrit the general purpose event generatorHERWIG [1, 2] in C++ under the name Herwig++, based on the package ThePEG [3, 4]. The goal is notonly to provide a simple replacement ofHERWIG but to incorporate physics improvements as well [5].From 2001 until now Herwig++ has been continuously developed and extended [6–10]. The currentversion is 2.2.1, cf. [11]. The physics simulation of the current version is more sophisticated than the oneof FortranHERWIG in many respects. In this report we will briefly summarize thestatus of the differentaspects of the simulation. These are the hard matrix elements available, initial and final state partonshowers, the hadronization, hadronic decays and the underlying event. We conclude with an outlook toplanned future improvements.2 Physics simulation steps2.1 Matrix elementsThe event generation begins with the hard scattering of incoming particles or partons in the case ofhadronic collisions. We have included a relatively small number of hard matrix elements. These includee+e− annihilation toqq̄ pairs or simply toZ0 bosons and deep inelastic scattering. In addition thereis the Higgsstrahlung processe+e− → h0Z0. For hadron–hadron collisions we have the QCD2 → 2processes including heavy quark production. For colourless final states we have the following matrixelements,hh → (γ, Z0) → ℓ+ℓ− , hh → W± → ℓ±νℓ(ν̄ℓ) , hh → h0 , hh → h0Z0 , hh → γγ .We also provide matrix elements for processes with additional jets in the final state, likehh → (γ, Z0,W±) + jet , hh → h0 + jet .∗to appear in the proceedings of the HERA and the LHC workshop.†herwig@projects.hepforge.orgIn addition, there are matrix elements for perturbative decays of the top quark, which will be simu-lated including spin correlations (see below). There will be some more matrix elements added in futureversions, e.g. forhh → qqh0. Despite the rather small number of matrix elements, there is no real limita-tion to the processes that may be simulated with Herwig++. Inpractice, one may use any matrix elementgenerator to generate a standard event file [12] which in turncan be read and processed by Herwig++.For processes with many legs in the final state we follow a different strategy. When the numberof legs becomes large — typically larger than 6–8 particles in the final state — it will be increasinglydifficult to achieve an efficient event generation of the fullmatrix element. For these situations wehave a generic framework to build up matrix elements for production and decays of particles in orderto approximate any tree level matrix element as a simple production process with subsequent two orthree body decays. This is a good approximation whenever thewidths of the intermediate particles aresmall. The spin correlations among these particles can be restored with the algorithm described in [13].Also finite width effects are taken into account [14]. The full simulation of several processes of manymodels for physics beyond the standard model (MSSM, UED, Randall–Sunrum model) is thus possible inHerwig++ [15]. Here, all necessary matrix elements for production and decay processes are constructedautomatically from a model file.2.2 Parton Showers and matching with matrix elementsAfter the hard process has been generated, typically at a large scale∼ 100 GeV–1 TeV, the coloured par-ticles in the process radiate a large number of additional partons, predominantly gluons. As long as theseare resolved by a hard scale of∼ 1 GeV this is simulated with a coherent branching algorithm, as out-lined in [16] which generalizes the original algorithm [17–19] used inHERWIG. The main improvementswith respect to the old algorithm are boost invariance alongthe jet axis, due to a covariant formulation,and the improved treatment of radiation off heavy quarks. Weare using mass–dependent splitting func-tions and a description of the kinematics that allows us to dynamically generate the dead–cone effect.In addition to initial and final state parton showers there are also parton showers in the decay of heavyparticles, the top quark in our case.When extrapolating to hard, wide–angle emissions, the parton shower description is not suffi-ciently accurate in situations where observables depend onlarge transverse momenta in the process. Inthese cases we supply so–called hard matrix element correcti ns that describe the hardest parton emis-sion, usually a hard gluon, with the full matrix element for the process that includes that extra parton. Inorder to consistently describe the whole phase space one hasto apply soft matrix element corrections.Matrix element corrections are available for Drell–Yan type rocesses, Higgs production ingg fusionande+e− annihilation toqq̄–pairs. In addition, we apply a matrix element correction intop–quark de-cays [20].From the point of view of perturbation theory, the hard matrix element correction is only one partof the next–to–leading order (NLO) correction to the Born matrix element. The full NLO calculation alsoincludes the virtual part with the same final state as the Bornapproximation. When trying to match NLOcalculations and parton shower algorithms systematically, we have to avoid double counting of the realemission contributions. Two systematic approaches are being successfully discussed and applied in eventgenerators: MC@NLO [21–23] and the POWHEG approach [24, 25]. In Herwig++ we have includedworking examples of matching in both approaches. The MC@NLOmethod, adopted to Herwig++ isdescribed in [26]. Whereas the POWHEG method has already been applied for several processes ine+e− annihilation [27, 28] and also for Drell–Yan production [29]. Parts of these implementations willbecome available in future releases.Another viable possibility to improve the description of QCD radiation in the event generation isthe matching to multiple tree–level matrix elements, that describe the radiation ofn additional jets withrespect to the Born level. Theoretically most consistent isthe CKKW approach [30] which has beenstudied in the context of an angular ordered parton shower in[31].2.3 Hadronization and decaysThe hadronization model in Herwig++ is the cluster hadronization model which has not been changedmuch from its predecessor inHERWIG. After the parton shower, all gluons are split nonperturbativelyinto qq̄ pairs. Then, following the colour history of the parton cascade, all colour triplet–antitriplet pairsare paired up in colourless clusters which still carry all flavour and momentum information of the originalpartons. While these are heavier than some threshold mass they will fission into lighter clusters until allclusters are sufficiently light. These light clusters will then decay into pairs of hadrons.The hadrons thus obtained are often heavy resonances that will eventually decay on timescales thatare still irrelevant for the experiment. These hadronic decays have been largely rewritten and are modeledin much greater detail in Herwig++. While inHERWIG they were often simply decayed according to theavailable phase space only, we now take into account more experimental information, like form factors,that allow for a realistic modeling of decay matrix elements[32,33]. In a major effort, a large fraction ofthe decay channels described in the particle data book [34] have been included into Herwig++.2.4 Underlying eventThe underlying event model of Herwig++ is a model for multiple hard partonic interactions, based on aneikonal model, similar toJIMMY [35]. In addition to the signal process there are a number of additionalQCD scatters, including full parton showers, that contribue to the overall hadronic activity in the finalstate and eventually also give rise to a (relatively soft) jet substructure in the underlying event. Themodel has two important parameters, one parameterµ, describing the spatial density of partonic matterin the colliding protons. Secondly, there is one cut off parameterp⊥,min that gives a lower bound on thedifferential cross section for QCD2 → 2 jet production. The model has been carefully tuned to Tevatrondata [36]. Further possible bounds on the model parameters have been studied in [37]. An alternativemodeling of the underlying event on the basis of the UA5 model[38] is also available for historic reasons.Currently, the multiple partonic interaction model is limited to hard scattering while a soft com-ponent is simply not present. For a realistic simulation of mini um bias events a soft component is,however, very important. An extension into the soft region,allowing us the simulation of minimum biasevents is currently being studied and is likely to be included in the next release of Herwig++.3 AvailabilityThe latest version of Herwig++ is always available from hepforge:http://projects.hepforge.org/herwigThere one can also find wiki pages to help with questions concerning installation, changing particularparameters and other frequently asked questions. The installation process is straightforward on anymodern variant of linux. The physics details of the program are now documented in great detail in ourmanual [33]. The pdf version of the manual contains addionalli ks to the online documentation of thecode. All important parameters have been carefully tuned toa wealth of available data and the code isshipped with default paramters that give the best overall description of the data that we have tuned to.Details of the tune can also be found in the manual [33].AcknowledgmentsThis work has been supported in part by the EU Marie Curie Research and Training Network MCnetunder contract MRTN-CT-2006-035606.References[1] G. Corcellaet. al., HERWIG 6: An event generator for hadron emission reactions with interferinggluons (including supersymmetric processes), JHEP01 (2001) 010, [hep-ph/0011363].[2] G. Corcellaet. al., HERWIG 6.5 release note, hep-ph/0210213.[3] L. Lönnblad,ThePEG, Pythia7, Herwig++ and Ariadne, Nucl. Instrum. Meth.A559 (2006)246–248.[4] M. Bertini, L. Lönnblad, and T. Sjöstrand,Pythia version 7-0.0: A proof-of-concept version,Comput. Phys. Commun.134 (2001) 365–391, [hep-ph/0006152].[5] S. Gieseke,Event generators: New developments, hep-ph/0210294.[6] S. Gieseke, A. Ribon, M. H. Seymour, P. Stephens, and B. Webber,Herwig++ 1.0: An eventgenerator for e+ e- annihilation, JHEP02 (2004) 005, [hep-ph/0311208].[7] S. Giesekeet. al., Herwig++ 2.0 beta release note, hep-ph/0602069.[8] S. Giesekeet. al., Herwig++ 2.0 release note, hep-ph/0609306.[9] M. Bähr et. al., Herwig++ 2.1 Release Note, arXiv:0711.3137.[10] M. Bähret. al., Herwig++ 2.2 Release Note, arXiv:0804.3053.[11] http://projects.hepforge.org/herwig.[12] J. Alwall et. al., A standard format for Les Houches event files, Comput. Phys. Commun.176(2007) 300–304, [hep-ph/0609017].[13] P. Richardson,Spin correlations in Monte Carlo simulations, JHEP11 (2001) 029,[hep-ph/0110108].[14] M. A. Gigg and P. Richardson,Simulation of Finite Width Effects in Physics Beyond the StandardModel, arXiv:0805.3037.[15] M. Gigg and P. Richardson,Simulation of beyond standard model physics in Herwig++, Eur.Phys. J.C51 (2007) 989–1008, [hep-ph/0703199].[16] S. Gieseke, P. Stephens, and B. Webber,N w formalism for QCD parton showers, JHEP12(2003) 045, [hep-ph/0310083].[17] G. Marchesini and B. R. Webber,Simulation of QCD Jets Including Soft Gluon Interference, Nucl.Phys.B238 (1984) 1.[18] B. R. Webber,A QCD Model for Jet Fragmentation Including Soft Gluon Interference, Nucl. Phys.B238 (1984) 492.[19] G. Marchesini and B. R. Webber,Monte Carlo Simulation of General Hard Processes withCoherent QCD Radiation, Nucl. Phys.B310 (1988) 461.[20] K. Hamilton and P. Richardson,A simulation of QCD radiation in top quark decays, JHEP02(2007) 069, [hep-ph/0612236].[21] S. Frixione and B. R. Webber,Matching NLO QCD computations and parton shower simulations,JHEP06 (2002) 029, [hep-ph/0204244].[22] S. Frixione, P. Nason, and B. R. Webber,Matching NLO QCD and parton showers in heavyflavour production, JHEP08 (2003) 007, [hep-ph/0305252].[23] S. Frixione and B. R. Webber,The MC@NLO 3.3 event generator, hep-ph/0612272.[24] P. Nason,A new method for combining NLO QCD with shower Monte Carlo algrithms, JHEP11(2004) 040, [hep-ph/0409146].[25] S. Frixione, P. Nason, and C. Oleari,Matching NLO QCD computations with Parton Showersimulations: the POWHEG method, JHEP11 (2007) 070, [arXiv:0709.2092].[26] O. Latunde-Dada,Herwig++ Monte Carlo At Next-To-Leading Order for e+e- annihilation andlepton pair production, JHEP11 (2007) 040, [arXiv:0708.4390].[27] O. Latunde-Dada, S. Gieseke, and B. Webber,A positive-weight next-to-leading-order MonteCarlo for e+ e- annihilation to hadrons, JHEP02 (2007) 051, [hep-ph/0612281].[28] O. Latunde-Dada,Applying the POWHEG method to top pair production and decaysat the ILC,arXiv:0806.4560.[29] K. Hamilton, P. Richardson, and J. Tully,A Positive-Weight Next-to-Leading Order Monte CarloSimulation of Drell-Yan Vector Boson Production, arXiv:0806.0290.[30] S. Catani, F. Krauss, R. Kuhn, and B. R. Webber,QCD matrix elements + parton showers, JHEP11 (2001) 063, [hep-ph/0109231].[31] S. 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Herwig++ is the successor of the event generator HERWIG. In its present version 2.2.1 it provides a program for full LHC event generation which is superior to the previous program in many respects. We briefly summarize its features and describe present work and some future plans.Herwig++ is the successor of the event generator HERWIG. In its present version 2.2.1 it provides a program for full LHC event generation which is superior to the previous program in many respects. We briefly summarize its features and describe present work and some future plans

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