I review all the new features of the HERWIG event generator which are
relevant to Linear Collider (LC) physics starting from version 6.1Comment: 9 pages, latex, 4 figures; talk given at the International Workshop
  on Linear Colliders (LCWS2002), Jeju Island, Korea, 26-30 August 200

Moretti, S.

English

arXiv

I review all the new features of the HERWIG event generator which are relevant to Linear Collider (LC) physics starting from version 6.1I review all the new features of the HERWIG event generator which are relevant to Linear Collider (LC) physics starting from version 6.

Moretti, Stefano

CERN Document Server

CERN-TH/2002-237IPPP/02/53DCPT/02/106September 2002HERWIG: AN EVENT GENERATORFOR e+e− LINEAR COLLIDERSStefano MorettiCERN Theory Division, CH-1211 Geneva 23, Switzerland andInstitute for Particle Physics Phenomenology, Durham DH1 3LE, UKI review all the new features of the HERWIG event generator which are relevantto Linear Collider (LC) physics starting from version 6.1.1 IntroductionHERWIG is a general-purpose Monte Carlo (MC) event generator for high-energy processes 1, providing a full simulation of hard lepton-lepton, lepton-hadron and hadron-hadron scattering and soft hadron-hadron collisions in asingle package, comprising the following features.1. Initial- and nal-state QCD jet evolution with soft gluon interference takeninto account via angular ordering.2. Colour coherence of (initial and nal) partons in all subprocesses, includingthe production and decay of Supersymmetric (SUSY) particles.3. Matrix Element (ME) corrections to the parton shower (PS) algorithm.4. Spin correlations in the decay of heavy fermions.5. Lepton beam polarisation in selected processes.6. Azimuthal correlations within and between jets due to gluon interferenceand polarisation.7. A cluster model for jet hadronisation (with optional colour rearrangement)based on non-perturbative gluon splitting and a similar model for soft andunderlying hadronic events.The HERWIG source codes and related information can be found in 2.2 Linear Collider physics with HERWIGOf particular relevance to the programme of future electron-positron LCs areitems 3.{5. We will describe these in the following, after mentioning the Min-imal Supersymmetric Standard Model (MSSM) processes currently availablein lepton-antilepton annihilations.12.1 MSSM processes in `+`− collisions (` = e, µ)Starting from version 6.1, HERWIG includes the production and decay of(s)particles, as given by the MSSM. The conventions used and a detailed de-scription of the implementation can be found in 3.HERWIG does not contain any built-in models for SUSY-breaking scenar-ios. In all cases the general MSSM particle spectrum and decay tables mustbe provided just like those for any other object, with the following caveats: (i)SUSY particles do not radiate (which is reasonable if their decay lifetimes aremuch shorter than the QCD connement scale); (ii) CP-violating SUSY phasesare not included. A package, ISAWIG, has been created to work with ISAJET4 to produce a le containing the SUSY particle masses, lifetimes, couplingsand mixing parameters. This package takes the outputs of the ISAJET MSSMprograms and produces a data le in a format that can be read into HERWIGfor the subsequent process generation. The user can produce her/his own leprovided that the correct format is used. To this end, we invite the consulta-tion of the ISAWIG webpage 5, where some example of input les can be found(the SUSY benchmark points recommended in Ref. 6 are also available).Table 1. The MSSM processes via ‘+‘− annihilations implemented in HERWIG.IPROC MSSM processes700-99 R-parity conserving SUSY processes700 ‘+‘− → 2-sparticle processes (sum of 710{760)710 ‘+‘− → neutralino pairs (all neutralinos)706+4IN1+IN2 ‘+‘− → ˜0IN1˜0IN2 (IN1,2=neutralino mass eigenstate)730 ‘+‘− → chargino pairs (all charginos)728+2IC1+IC2 ‘+‘− → ˜+IC1˜−IC2 (IC1,2=chargino mass eigenstate)740 ‘+‘− → slepton pairs (all flavours)736+5IL ‘+‘− → ‘˜L,R‘˜L,R (IL = 1; 2; 3 for ‘˜ = ~e; ~; ~)737+5IL ‘+‘− → ‘˜L‘˜L (IL as above)738+5IL ‘+‘− → ‘˜L‘˜R (IL as above)739+5IL ‘+‘− → ‘˜R‘˜R (IL as above)740+5IL ‘+‘− → ˜L˜L (IL = 1; 2; 3 for ˜e; ˜µ; ˜τ )760 ‘+‘− → squark pairs (all flavours)757+4IQ ‘+‘− → q˜L,Rq˜L,R (IQ = 1:::6 for q˜ = ~d:::~t)758+4IQ ‘+‘− → q˜Lq˜L (IQ as above)759+4IQ ‘+‘− → q˜Lq˜R (IQ as above)760+4IQ ‘+‘− → q˜Rq˜R (IQ as above)In addition to the decay modes implemented in the ISAJET package,ISAWIG also includes the calculation of all 2-body squark/slepton and 3-bodygaugino/gluino R-parity violating (RPV) decay modes (alas, RPV lepton-2gaugino and slepton-Higgs mixing is not considered). Moreover, the emulationof RPV processes is also a feature of the production stage. Tab. 1 illustratesall MSSM modes initiated by `+`− scattering that are available at present(version 6.4). Here, IPROC is the input label selecting the hard process.Table 1. Continues.800-99 R-parity violating SUSY processes800 Single sparticle production, sum of 810{840810 ‘+‘− → ˜0i, (all neutralinos)810+IN ‘+‘− → ˜0INi, (IN=neutralino mass state)820 ‘+‘− → ˜−e+i (all charginos)820+IC ‘+‘− → ˜−ICe+i , (IC=chargino mass state)830 ‘+‘− → ˜iZ0 and ‘+‘− → ‘˜+i W−840 ‘+‘− → ˜ih0=H0=A0 and ‘+‘− → ‘˜+i H−850 ‘+‘− → ˜iγ860 Sum of 870 and 880870 ‘+‘− → ‘+‘−, via LLE only867+3IL1+IL2 ‘+‘− → ‘+IL1‘−IL2 (IL1,2=1,2,3 for e; ;  )880 ‘+‘− → dd, via LLE and LQD877+3IQ1+IQ2 ‘+‘− → dIL1 dIL2 (IQ1,2=1,2,3 for d; s; b)Table 1. Continues.910-975 Higgs processes910 ‘+‘− → ``h0 + ‘+‘−h0920 ‘+‘− → ``H0 + ‘+‘−H0960 ‘+‘− → Z0h0970 ‘+‘− → Z0H0955 ‘+‘− → H+H−965 ‘+‘− → A0h0975 ‘+‘− → A0H02.2 ME corrections to the standard PSThe study of top quark production and decay will be one of the main areasof research activity at future LCs. Hence, it is of paramount importance theavailability of a MC event generator describing the dynamics of the e+e− !tt ! bbW+W− process (mt = 175 GeV) with great accuracy. As the topquark is a coloured particle, of particular concern are higher order eects fromQCD. Whereas those induced by the exchange of virtual gluons do not changethe lowest order kinematics, real radiation of the latter does so. This is ofrelevance when it comes to reconstruct the top quark parameters (primarily,mt and Γt) from some exclusive jet observables, in presence of W ! jet-jetdecays.According to the HERWIG standard algorithm for PS, gluon radiation is3treated in the soft and collinear approximation and no emission is permittedin the ‘dead zones’ (hard and large-angle parton radiation) seen in the left-hand side of Figs. 1{2. The current version of the HERWIG event generatorhas been improved by applying so-called ME corrections: i.e., the dead zone ispopulated by the use of the exact rst-order matrix element (‘hard correction’)and the O(αS) result is used in the already-lled region any time an emissionis the ‘hardest so far’ (‘soft correction’) 7.Figure 1: The t → W+1 b2g3 decay: (Left) The total (solid) and HERWIG (dashed)phase space in terms of the x1 and x3 momentum fractions; (Right) The y3 dis-tributions according to the new HERWIG implementation (dashed), to the old one(dotted) and to the exact O(S) calculation (solid). Rates are at 360 GeV.Figure 2: The e+e− → t1t2g3 production: (Left) Total (solid) and HERWIG (dotted)phase space limits for tt production in terms of the x1 and x3 momentum fractions;(Right) The y3 distributions according to the old HERWIG implementation (dashed),to the new one, the latter populating either the small- and large-angle (solid) or onlythe large-angle region (dotted) of the dead zone. Rates are at 500 GeV.4A good measure to gauge the improvement due to the new algorithm canbe gained from investigating the threshold value of the Durham jet-clusteringalgorithm 8 for all events to yield three jets, each with ET > 10 GeV andrelative R > 0.7 in transverse energy and cone separation. Just aboveps =2mt, where real radiation from the production stage is negligible, the benecialeects of the new HERWIG implementation can be appreciated in the right-hand side of Fig. 1, in the case of top quark decays. At large y3 values, wherethe xed order O(αS) description 9 is expected to well describe the underlying(hard gluon) dynamics, we see the latter to coincide with the current HERWIGdescription, which in turns retains sizable dierences from the O(αS) result asy3 ! 0 (soft and collinear gluon), where the all-order PS algorithm is known tobe reliable. While similar improvements can be seen in the case of top quarkproduction, it is in this instance more instructive to compare the same y3distribution as obtained from the old HERWIG description (incorporating MEcorrections but not the complete  m2t/s mass eects) to those given by twomore advanced options, currently being implemented: the default one llingboth the small- and large-angle dead zones and the intermediate one occupyingonly the large-angle region, both accounting for the above mass corrections (seethe right-hand side of Fig. 2). The sizable dierences between the three clearlymakes the point in favour of the more complete HERWIG implementation inview of future LC studies.Another example of ME corrections available in HERWIG is the case ofmulti-jet production in electron-positron annihilations via exact xed-ordere+e− ! n parton MEs, limitedly to the case n = 4. Here, the event generationcan be initiated by the e+e− ! qqgg and e+e− ! qqQ Q MEs, with the fourparton ‘interfaced’ to the subsequent PS 10 (i.e., a hard cut-o is enforced inorder to separate the partons thus preventing the MEs from diverging and thePS is attached to each of the latter only over the reduced phase space). Thisis an alternative to the standard description based on the e+e− ! qq(g) hardscattering process followed by the traditional PS algorithm. While the newimplementation is only reliable for n  4 and for jet separations larger thanthe corresponding partonic cut-o, it remedies the drawback of the old onein describing the angular orientation of four-jet events, as seen by, e.g., theALEPH Collaboration 11 (see also Ref. 12).The ability of a MC event generator to describe correctly a four-jet nalstate is particularly relevant in the context of LC searches for a light Higgsstate, predominantly produced via e+e− ! ZH ! 4 jets events, as BR[Z !jet-jet]=0.7 and BR[H ! bb]  1. Take for example an hadronic samplecomposed of two b-jets and two untagged jets at a 500 GeV LC, selected byusing the Durham jet-clustering algorithm with resolution y4 = 0.001 and with5Figure 3: Distributions for the polar angle of the reconstructed Z for the Higgs signaland the two description of the QCD four-jet background mentioned in the text.one pair of jets reproducing the Z mass within 10 GeV, i.e. jMjj −MZ j < 10GeV. Then compare the polar angle distribution of the di-jet system emulatinga Z decay, for the QCD four-jet background and the above Higgs signal (forMH = 150 GeV): see Fig. 3. It is clear that, in the region populated by thelatter, the discrepancy between the old (HERWIG v5.9) and new (HERWIGv6.1) description of the former is rather large, similar in size to the signalexcess. Needless to say, under these circumstances, a clear understanding ofthe behaviour of the QCD background is crucial in order to extract the Higgsresonance and measure its relevant parameters (MH , ΓH , etc.) a.A more sophisticated implementation ‘matching’ MEs and PS, meaning theprocedure of exploiting a combined approach which uses MEs for large phasespace separations, PS in the infrared (i.e., soft and collinear) limit and modied(by Sudakov form factors) MEs in the overlapping region 15, is available inAPACIC++, for n  6 16 b.2.3 Spin correlations and lepton beam polarisationSince v6.4, spin correlations are available in processes where SUSY particles,top quarks and τ -leptons are produced 18. Whenever these particles decay,2-, 3- and 4-body MEs are used to describe their dynamics (with or withoutthe spin correlations). In the case of τ ’s, an interface to TAUOLA 19 is alsoavailable. Polarisation has been implemented for incoming leptonic beams inaOptions similar to the one available in HERWIG for n = 4 can also be found inPYTHIA 13 and 4JPHACT 14.bA similar standalone implementation for n ≤ 4 is available in HERWIG too 17.6SUSY processes. These eects are included both in the production of SUSYparticles and via the spin correlation algorithm in their decays.As an example of the impact of such spin eects, consider pair productionof the two lightest neutralinos at an (un)polarised LC, via the following graphs:e−e+~χ02~χ01e−e+~eL,R~χ02~χ01e−e+~eL,R~χ01~χ02with ~χ01 the Lightest Supersymmetric Particle (LSP) and ~χ02 decaying via:~χ02~χ01ffZ0~χ02f~χ01f~fL,R~χ02~χ01ffh0, H0, A0(f = e, µ)~χ02ff~χ01~fL,R7Figure 4: Angle between the lepton produced in e+e− → ~02 ~01 → f f ~01 ~01 (f = e; )and the incoming electron beam in the laboratory frame for√s = 500 GeV with: (a)no polarization, (b) negatively polarized electrons and positively polarized positronsand (c) positively polarized electrons and negatively polarized positrons. The solidline shows the default result from HERWIG which treats the production and decayas independent, the dashed line gives the full result from the four-body ME and thedotted line the result of the spin correlation algorithm. (The SUSY parameters are:M0 = 210 GeV, A0 = 0, tan  = 10, M1 = 450 GeV, M2,3 = 350 GeV.)From Ref. 18, we show Fig. 4, illustrating the angular behaviour of theabove SUSY signals, as given by a typical MC implementation (HW), whichdoes not include spin eects in production and decay and/or ME correctionsin the latter, to the output of HERWIG v6.4 (HW+Spin). Once again, theimprovement is clear if one confronts the latter with the distributions obtainedfrom a complete calculation (ME), wherein the event generation is based on theexact 2 ! 6 ME description.I thank the LCWS2002 organisers for the excellent atmosphere and stimu-lating environment that they have created during the workshop and The RoyalSociety of London, UK, for partial nancial support in the form of a ConferenceGrant.1. G. Marchesini, B.R. Webber, G. Abbiendi, I.G. Knowles, M.H. Seymourand L. Stanco, Comput. Phys. Commun. 67, 465 (1992); G. Corcella,I.G. Knowles, G. Marchesini, S. Moretti, K. Odagiri, P. Richardson, M.H.Seymour and B.R. Webber, hep-ph/9912396; JHEP 01, 010 (2001);hep-ph/0107071; hep-ph/0201201.82. See: http://hepwww.rl.ac.uk/theory/seymour/herwig/.3. S. Moretti, K. Odagiri, P. Richardson, M.H. Seymour and B.R. Webber,JHEP 04, 028 (2002); S. Moretti, hep-ph/0205105.4. F.E. Paige, S.D. Protopopescu, H. Baer and X. Tata, hep-ph/9804321;hep-ph/ 9810440.5. See: http://www.hep.phy.cam.ac.uk/~richardn/HERWIG/ISAWIG/.6. B.C. Allanach et al., Eur. Phys. J. C 25, 113 (2002).7. G. Corcella and M.H. Seymour, Phys. Lett. B 442, 417 (1998); hep-ph/9911335; G. Corcella, E.K. Irish and M.H. Seymour, hep-ph/0012319.8. Yu.L. Dokshitzer, contribution cited in the ‘Report of the Hard QCDWorking Group’, in Proceedings of the Workshop ‘Jet Studies at LEPand HERA’, Durham, December 1990, J. Phys G 17, 1537 (1991);S. Catani, Yu.L. Dokshitzer, M. Olsson, G. Turnock and B.R. Webber,Phys. Lett. B 269, 432 (1991).9. L.H. Orr, T. Stelzer and W.J. Stirling, Phys. Lett. B 354, 442 (1995);Phys. Rev. D 56, 446 (1997).10. S. Moretti, hep-ph/000819.11. ALEPH Collaboration, Z. Phys. C 76, 1 (1997).12. A. Ballestrero et al., J. Phys. G 24, 365 (1998); S. Moretti and W.J.Stirling, Eur. Phys. J. C 9, 81 (1999).13. T. Sjo¨strand, L. Lo¨nnblad and S. Mrenna, hep-ph/0108264.14. A. Ballestrero, in preparation.15. S. Catani, F. Krauss, R. Kuhn and B.R. Webber, JHEP 11, 063 (2001).16. F. Krauss, R. Kuhn and G. So, J. Phys. G 26, L11 (2000); Acta Phys.Polon. B 30, 3875 (1999).17. See: http://webber.home.cern.ch/webber/.18. P. Richardson, JHEP 11, 029 (2001).19. S. Jadach, Z. Was, R. Decker and J.H. Kuhn, Comput. Phys. Commun.76, 361 (1993); M. Jezabek, Z. Was, S. Jadach and J.H. Kuhn, Comput.Phys. Commun. 70, 69 (1992); S. Jadach, J.H. Kuhn and Z. Was,Comput. Phys. Commun. 64, 275 (1990).9

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