Recent measurements of the properties of W+W- events produced in e+e-
collisions at Ecm=183 GeV at LEP are reviewed. The data are used to investigate
the predicted effects of final state interactions, specifically "colour
reconnection".Comment: 6 pages, 7 figures. To appear in the "Proceedings of the XXIX
  International Conference on High Energy Physics", Vancouver, CA, 23-29 July
  199

Watson, N. K.

English

arXiv

Recent measurements of the properties of W+W- events produced in e+e- collisions at Ecm=183 GeV at LEP are reviewed. The data are used to investigate the predicted effects of final state interactions, specifically "colour reconnection"

Watson, N K

CERN Document Server

BHAM-HEP/98-0128th September 1998W+W− HADRONIC DECAY PROPERTIESN. K. WATSONSchool of Physics and Astronomy, University of Birmingham, P.O. Box 363, Birmingham B15 2TT, United KingdomInvited talk at ICHEP’98, 22− 30th July 1998, Vancouver, Canada.Recent measurements of the properties of W+W− events produced in e+e− collisions atps  183 GeV at LEP are reviewed. Thedata are used to investigate the predicted eects of nal state interactions, specically \colour reconnection".1 MotivationHadronic data in e+e− collisions can be characterised byevent shape distributions and inclusive observables suchas charged particle multiplicities and momentum spectra.In addition to tests of Monte Carlo models, measure-ment of the properties of the hadronic sector of W+W−decays allows the question of ‘colour reconnection’ 1 tobe addressed experimentally. The decay products of thetwo W decays may have a signicant space-time overlapas the separation of their decay vertices at LEP2 ener-gies is small compared to characteristic hadronic distancescales. In the fully hadronic channel this may lead to -nal state interactions. Colour reconnection is the generalname applied to the case where such nal state interac-tions lead to a rearrangement of the colour flow betweenthe two W bosons. At present there is general consensusthat observable eects of interactions between the coloursinglets during the perturbative phase are expected to besmall. In contrast, signicant interference in the hadroni-sation process is considered to be a real possibility. Withthe current knowledge of non-perturbative QCD, such in-terference can be estimated only in the context of specicmodels. 1;2;3;4 One should be aware that other nal stateeects such as Bose-Einstein correlations between iden-tical bosons may also influence the observed event prop-erties. The combined action of these two eects may beeither to reduce or enhance possible characteristic signa-tures of their presence.2 Event PropertiesThe results shown are based on  55 pb−1 data perLEP collaboration. 5 Simple observables such as the in-clusive charged multiplicity are obvious candidates forstudy. There have been predictions 4 that the eects ofcolour reconnection may be  10% on hn4qchi, the meancharged multiplicity in W+W−!qqqq events. The ref-erence sample against which such changes are gauged istypically taken to be twice the multiplicity of the hadron-ically decaying W in W+W−!qq‘‘ events, hnqq‘ch i.0102030405010 20 30 40 50 60 700102030405010 20 30 40 50 60 70nch for qq_qq_No. of eventsOPAL dataPYTHIASK ISK IISK II/Backgroundnch for qq_qq_No. of eventsARIADNEAR 2AR 3(a)(b)OPAL PreliminaryFigure 1: Observed track multiplicity, W+W−!qqqq events.Charged particles associated with the leptonic compo-nent of such events are excluded. The dierence is de-ned hnchi = hn4qchi−2hnqq‘ch i. This reduces the depen-dence on the modelling of single W decays at the expenseof a lower statistical signicance of the test performed.The observed (uncorrected) track multiplicities for se-lected W+W−!qqqq candidates are shown in Figure 1by OPAL, together with the predictions from a variety ofcolour reconnection models. The mean charged particlemultiplicities, hn4qchi and hnqq‘ch i, may be extracted fromsuch data, after subtraction of the predicted background,by employing a matrix-based unfolding procedure. Thisuses the event-by-event correlation, taken from MonteCarlo, between the charged multiplicity at hadron leveland that observed in the detector after all analysis cutshave been applied. A subsequent correction for the eectsof initial state photon radiation and to full acceptance isapplied. Alternative methods for measuring hn4qchi and1Table 1: Mean charged particle multiplicities, errors are sum ofstatistical and systematic components. y: only partially unfolded.Expt. hn4qchi hnqq‘ch i hnchiALEPHy 35:33 0:73 17:01 0:37 +1:31 0:83DELPHI 37:36 1:00 19:48 0:73 −1:6 1:5L3 36:3 0:9 18:6 0:6 −1:0 0:9OPAL 39:4 1:0 19:3 0:4 +0:7 1:0hnqq‘ch i are the integration of any charged particle eventshape, such as the distributions of momentum, rapidityor transverse momentum. Each collaboration uses morethan one method to control possible systematic eects.Changes in hn4qchi predicted vary with the colour re-connection model used, and the extent to which it mayhave been retuned. Representative shifts, dened ashn4qchireconnection− hn4qchinormalfor each model, are:Sjo¨strand-Khoze (SK) I  −0:3SK II; SK II0  −0:2Ariadne 2  −0:1Ariadne 3 (AR 3)  −1:0As unfolding the data may introduce a bias towards themodel used, the procedure is repeated using a varietyof models to estimate any associated systematic eect.L3 and OPAL also unfold their data using several colourreconnection models as part of such studies. ALEPHcorrect their data for experimental eects, such as nitedetector resolution, but not for losses due to tracking in-eciency in the low momentum region (pT < 200 MeV),or phase space and topological biases invariably presentin experimental event selections. By using only a partialunfolding, their data should be less aected by model bi-ases but cannot be compared directly with other exper-imental results, or to model predictions without passingthrough their detector simulation and analysis cuts.The mean multiplicities, hn4qchi, hnqq‘ch i, and hnchimeasured by the LEP collaborations are shown in Ta-ble 1. Although the ALEPH mean multiplicities shouldnot be compared directly with other data, the associateddierence may be more meaningfully compared. TheLEP average multiplicity dierence obtained is:LEP hnchi = +0:20 0:50(stat:+ syst:);This is consistent with there being no change in thehn4qchi due to colour reconnection eects. As the sys-tematics considered and the dominant source varied be-tween the collaborations, this average assumes all sys-tematics were uncorrelated. If, instead, the smallest over-Table 2: Dispersions of charged particle multiplicities, errors aresum of statistical and systematic componentsExpt. hn4qchi hnqq‘ch i hnchiDELPHI 8:24 0:51 5:76 0:49 +0:09 0:84OPAL 8:8 0:7 6:1 0:5 +0:2 0:6all systematic estimated by any collaboration is consid-ered as fully correlated, the average value obtained ishnchi = +0:150:80(stat:+ syst:). Performing the av-erage while excluding the ALEPH results leads to thesame conclusions.All of the Sjo¨strand-Khoze, Herwig and Ariadnemodels are consistent with data, although some more ex-treme models, such as the instantaneous reconnectionscenarios in the Sjo¨strand-Khoze model, and also theAriadne model AR 3 3 in which gluons having energiesgreater than ΓW are allowed to interact, are disfavoured.DELPHI and OPAL also measure the dispersions ofthe charged multiplicity distributions, D4q and Dqq‘‘ ,and the dierence dened as D = D4q −p2Dqq‘‘ .Their corrected results, shown in Table 2, are consistentwith Monte Carlo expectations and there is no indicationfor dierences in shape based on the rst moments of themultiplicity distributions.2.1 Ellis-Geiger ModelThe Ellis-Geiger model, 4 implemented in the MonteCarlo program Vni, has been studied by the LEP col-laborations. ALEPH and OPAL illustrate this by quot-ing some predictions from Vni. It is noted that theEllis-Geiger model as implemented within Vni has notbeen tuned recently to describe Z0 data. ALEPH com-pared the predictions of Vni with other W+W− eventgenerators, including detector simulation. Within theacceptance of their charged tracking and event selectioncriteria, ALEPH nd Vni to give a charged track distri-bution that has hn4qchi  2 units higher than data andan r.m.s.  1:7 units broader than in data. They alsoobserve no dependence 4 of hn4qchi on the minimum an-gle between jets assigned to dierent W bosons. OPALnd from hadron level studies without detector simula-tion that Vni gives very high charged particle multiplic-ities, hn4qchi > 50, and also does not reproduce the largepredicted shifts in hn4qchi in any of the colour blind, coloursinglet or colourful scenarios.4 These studies were carriedout atps = 183 GeV, but the same conclusions hold atps = 166 GeV, the lowest centre-of-mass energy allowedby Vni for W+W− production. For these reasons, thecollaborations do not use the Ellis-Geiger model as cur-rently implemented in Vni to estimate possible system-atic eects on the W boson mass.202040600.5 0.6 0.7 0.8 0.9 1ThrustNo. of events/0.02L3qqqqpreliminaryKORALWPYTHIA SC IPYTHIA SC IIPYTHIA im. rec.Figure 2: Observed thrust, W+W−!qqqq events.2.2 ThrustThe thrust, T , distribution shown in Figure 2 from L3illustrates how W+W−!qqqq events are more sphericalthan the Z0=γ ! qq background, which is dominated bytwo-jet events. Qualitatively, colour reconnection eectsare expected to be enhanced in W+W− events where thehadronic cascades from two W bosons overlap, but this isprecisely the background dominated, two-jet like regiongenerally excluded by the experimental event selections.It is interesting to compare the hadron level pre-dictions of a standard W+W− event generator such asKoralw with the Vni generator in its colour blind sce-KORALWVNI1-TEvents00.020.040.060 0.1 0.2 0.3 0.4 0.5 0.6Figure 3: Hadron level predictions, W+W−!qqqq events.ALEPH preliminary1818.51919.5200 0.2 0.4 0.6 0.8 1  2.534    /     6P1   18.88  0.7559E-02PRECOmean multiplicity1616.51717.5180 0.2 0.4 0.6 0.8 1  5.857    /     6P1   16.76  0.7559E-02PRECOmean multiplicity333435360 0.2 0.4 0.6 0.8 1  5.651    /     6P1   33.52  0.1134E-01PRECOmean multiplicityFigure 4: Tuning of Herwig reconnection model.nario. The distribution of 1 − T is shown in Figure 3for these two models. It can be seen that Vni predictsW+W− events to be more spherical than other models.While this might influence the eciency with which suchevents are selected, they are topologically more distinctfrom the background than predicted by most models, soshould fall well within the acceptance of the detectors.2.3 Model TuningIn general each model must be retuned to data after x-ing reconnection related strength parameters or probabil-ities. The Sjo¨strand-Khoze models do not require suchretuning once the Jetset hadronisation model has beentuned. Herwig, Ariadne and the Ellis-Geiger model allrequire retuning. OPAL use a tuning of Ariadne sum-marised in,6 while ALEPH describe in detail their tuningof Herwig. The mean charged multiplicities in Herwigfor Z0=γ, W+W−!qq‘‘ and W+W−!qqqq events asa function of the reconnection probability, PRECO, areshown in Figure 4 by ALEPH. The eect of retuning ateach PRECO, and its necessity, is clear. A good descrip-tion of the W+W− events may be achieved after suitable3KORALWHERWIGAR 3xp(1/σ dσchqqqq /dx p) / (2/σ dσchqqlν /dx p)OPAL Preliminary0.60.811.21.40 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2Figure 5: Momentum distribution ratio, W+W−!qqqq andW+W−!qq‘‘.retuning to Z0 data.2.4 Alternative ObservablesThe eects of colour reconnection are generally predictedto be enhanced in the soft particle region, therefore stud-ies of fragmentation functions are interesting. Figure 5shows the ratio of the corrected, scaled momentum dis-tributions for W+W−!qqqq and W+W−!qq‘‘ events,together with predictions of W+W− event generators andthe colour reconnection model in which they found thelargest eects, AR 3. Although this latter predicts dier-ences which are as large as 5% in the lowest momentuminterval, it is still consistent with data. OPAL also mea-sure a variety of mean event shape variables, all of whichare consistent with model predictions (neglecting Vni):hx4qp i = (3:16 0:05 0:04) 10−2hxqq‘‘p i = (3:25 0:07 0:04) 10−2hxpi = (−0:09 0:09 0:06) 10−2h(1− T )4qi = 0:240 0:015 0:009hjy4qji = 1:017 0:016 0:016DELPHI show in Figure 6 the transverse momentum dis-tributions relative to the thrust axes for W+W−!qqqqand W+W−!qq‘‘ events, and their ratio. In the semi-leptonic events the thrust axis is calculated taking themomentum imbalance in each event to be the neutrinomomentum.There is no signicant dierence found be-tween the data in the two channels.3 Heavy HadronsIt was recently predicted 7 that the eects of colour re-connection may be further enhanced by restricting anal-yses to heavier hadrons such as charged kaons and pro-tons in the low momentum region, 0:2 < p < 1:2 GeV.D E L P H ID E L P H I183 GeV183 GeVb)a)PYTHIAPYTHIAFigure 6: Transverse momentum distribution, W+W−!qqqq andW+W−!qq‘‘.W DataZ DataW SimulationZ SimulationpTracks0510152025303540450.2 0.4 0.6 0.8 1 1.2 1.4Figure 7: Comparison of W+W− and Z0 pair data.4Numerically this increases the relative size of the eect,but taking into account the branching fractions to suchspecies as well as experimental eects such as nite par-ticle detection eciency, background and modelling, itis not clear this will lead to an improved signicance inthe analyses. DELPHI use their TPC, RICH and ver-tex detector for particle identication, attaining taggingeciencies for kaons and protons of order 90{50%, withpurities in the range 90{75%. They observe 181 heavyhadrons in W+W−!qqqq events, 88 in W+W−!qq‘‘.After unfolding for tagging eects this gives:(qqqq)=(2qq‘‘) = 1:03 0:15(stat:):The statistical uncertainty on this measurement seemsunlikely to be less than 5% even with the nominal LEP2integrated luminosity of 500 pb−1. To reduce the statisti-cal uncertainty on these studies, they also use Z0 calibra-tion data as a reference sample. Pairs of Z0 events, afterboosting, are used to emulate W+W−!qqqq events, anda direct comparison is made with genuine W+W−!qqqqdata. The two data samples and the correspondingMonte Carlo predictions are shown in Figure 7. Thequalitative agreement is reasonable, although there maybe dierences between W+W− data and predictions forp > 0:6 GeV. The nal result of their analysis is the ratio:hnheavyi;W+W−Z0Z0= 0:870 0:090(stat:) 0:044(syst:):4 W Mass BiasesUpdated bias estimates for the eects of colour recon-nection on the W mass measurement were presented byALEPH, DELPHI and OPAL, as summarised in Table 3.It is noted that ALEPH and OPAL estimates are usingtheir respective default MW analyses, while the DELPHIresults are a variant on their main MW analysis. Thebias estimates from Herwig and Ariadne are made af-ter these models have been retuned to describe Z0 dataatps = 91 GeV. As a cautionary note, there are non-negligible statistical uncertainties on all of these bias es-timates which must be reduced for future analyses. Nobias estimates are presented for the Ellis-Geiger modelfor reasons discussed earlier.5 SummaryStudies of colour reconnection are maturing, in particu-lar the area of Monte Carlo tuning is being addressed.There is no signicant dierence observed in the chargedparticle multiplicity of a single W boson produced in ei-ther a W+W−!qqqq or a W+W−!qq‘‘ event. Mostmodels are consistent with data based on an integratedTable 3: Model dependent, colour reconnection bias estimates forMW determination in W+W−!qqqq channel (statistical uncer-tainties). y: fast detector simulation, z: modied version of model.Expt. ALEPH DELPHIy OPALSK I +25 21z +40 12 +50 17SK II +5 15 − +17 17SK II0 +17 15 +3 10 +18 17GH − +13 18 −HW −31 25 −AR 2 − − +73 18AR 3 − − +146 18luminosity of  55 pb−1 per collaboration. Identiedparticle studies in progress pose an interesting experi-mental challenge. The Ellis-Geiger model, as currentlyimplemented and tuned, is not used for MW bias esti-mates as it does not describe the data. Finally, the 1998data should allow some models to be excluded leading tobetter control of the colour reconnection systematic onMW measurements.AcknowledgementsWork supported by PPARC grant GR/L04207. I wouldlike to thank colleagues in the LEP collaborations, par-ticularly R. Jones, B. Pietrzyk., P. de Jong, J. Mnich,C. Matteuzzi, N. Neufeld and M.F. Watson. It is withdeepest sorrow that I note the recent, tragic death ofKlaus Geiger, who will be missed by many.References1. G. Gustafson, U. Pettersson and P.M. Zerwas,Phys. Lett. B 209 (90) 1988.2. T. Sjo¨strand, Comp. Phys. Comm. 82 (1994) 74;T. Sjo¨strand and V.A. Khoze, Z. Phys. C 62(281) 1993. G. Gustafson, J. Ha¨kkinen, Z. Phys.C 64 (659) 1994; C. Friberg, G. Gustafson andJ. Ha¨kkinen, Nucl. Phys. B 490 (289) 1997; G.Marchesini et al., Comp. Phys. Comm. 67 (465)1992.3. L. Lo¨nnblad, Z. Phys. C 70 (107) 1996; Comp.Phys. Comm. 71 (15) 1992.4. J. Ellis and K. Geiger, Phys. Rev. D54 (1967) 1996;K. Geiger, Comp. Phys. Comm. 104 (70) 1997; J.Ellis and K. Geiger, Phys. Lett. B 404 (230) 1997.5. \Preliminary W+W− event properties and colourreconnection contributions", ALEPH 98-066CONF 98-036 abstract #909, DELPHI 98-101CONF 169 abstract #289, DELPHI 98-16 CONF117 (22/6/1998 version) abstract #287, L3 Note2278 abstract # 507, OPAL PN365 abstract #395.56. \Experimental properties of gluon and quark jetsproduced from a point source", OPAL PN344 ab-stract #296.7. T. Sjo¨strand and V.A. Khoze, \Soft particle spec-tra as a probe of interconnection eects in hadronicW+W− events’", hep-ph/98042026

W+W- Hadronic Decay Properties

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