Three recent OPAL studies are presented in which the fragmentation process in
quark and gluon jets and in identified up, down and strange flavour jets is
studied. The first is a measurement of charged particle, pi0, eta and K0
multiplicities in quark and gluon jets. No evidence is found for a
particle-species dependent multiplicity enhancement in gluon jets. In another
study leading pion, kaon, K0, proton and lambda rates have been measured in up,
down and strange flavour jets. The results confirm the leading particle effect
in the fragmentation of light flavour jets. In addition, a direct determination
of the strangeness suppression factor has been performed, yielding
gamma_s=0.422 +/- 0.049(stat.) +/- 0.059(syst.). In a third study, mean charged
particle multiplicities were measured for up, down and strange flavoured Z0
decays and found to be identical within the uncertainties of the measurement,
as expected from the flavour independence of the strong interaction.Comment: 6 pages, LaTeX, to appear in the proceedings of the 30th
  International Symposium on Multiparticle Dynamics, October 9 - 15, 2000,
  Tihany, Hungar

Vossebeld, J. H.

English

arXiv

Three recent OPAL studies are presented in which the fragmentation process in quark and gluon jets and in identified up, down and strange flavour jets is studied. The first is a measurement is of charged particle, pi0, eta and K0 multiplicities in quark and gluon jets. No evidence is found for a particle species dependent multiplicity enhancement in gluon jets. In another study leading pion, kaon, K0, proton and lambda rates have been measured in up, down and strange flavour jets. The results confirm the leading particle effect in the fragmentation of light flavour jets. In addition a direct determination of the strangeness suppression factor has been performed, yielding gamma_s=0.422 +/- 0.049(stat.) +/- 0.059(syst.). In a third study mean charged particle multiplicities were measured for up, down and strange flavoured Z0 decays and found to be identical within the uncertainties of the measurement, as expected from the flavour independence of the strong interaction.Three recent OPAL studies are presented in which the fragmentation process in quark and gluon jets and in identified up, down and strange flavour jets is studied. The first is a measurement is of charged particle, pi0, eta and K0 multiplicities in quark and gluon jets. No evidence is found for a particle species dependent multiplicity enhancement in gluon jets. In another study leading pion, kaon, K0, proton and lambda rates have been measured in up, down and strange flavour jets. The results confirm the leading particle effect in the fragmentation of light flavour jets. In addition a direct determination of the strangeness suppression factor has been performed, yielding gamma_s=0.422 +/- 0.049(stat.) +/- 0.059(syst.). In a third study mean charged particle multiplicities were measured for up, down and strange flavoured Z0 decays and found to be identical within the uncertainties of the measurement, as expected from the flavour independence of the strong interaction

Vossebeld, J.H.

CERN Document Server

arXiv:hep-ex/0101009 v2   2 Feb 2001RECENTRESULTSONPARTICLEPRODUCTIONFROMOPALJ.H.VOSSEBELDCERN,CH-1211Geneva23,SwitzerlandE-mail:Joost.Vossebeld@cern.chOPALCR-459hep-ex/0101009ThreerecentOPALstudiesarepresentedinwhichthefragmentationprocessinquarkandgluonjetsandinidentiedup,downandstrangeavourjetsisstudied.Therstisameasurementofchargedparticle,0,andK0multiplicitiesinquarkandgluonjets.Noevidenceisfoundforaparticle-speciesdependentmultiplicityenhancementingluonjets.Inanotherstudy,leading,K,K0S,p(p)and()rateshavebeenmeasuredinup,downandstrangeavourjets.Theresultsconrmtheleadingparticleeectinthefragmentationoflightavourjets.Inaddition,adirectdeterminationofthestrangenesssuppressionfactorhasbeenperformed,yieldings=0:4220:049(stat:)0:059(syst:):Inathirdstudy,meanchargedparticlemultiplicitiesweremeasuredforup,downandstrangeavouredZ0decaysandfoundtobeidenticalwithintheuncertaintiesofthemeasurement,asexpectedfromtheavourindependenceofthestronginteraction.1IntroductionThelargenumberofhadronicZ0decayscollectedbytheLEPexperimentsatps=MZ0allowfordetailedstudiesofthedynamicsofthestronginteraction.InthreerecentOPALstudiespresentedinthispaper,productionratesofvarioushadronspeciesinthefragmentationofquarkandgluonjetsandofidentiedup,downorstrangeavourjetshavebeenstudied.Theresultsconstituteadetailedtestofmodelsdescribingthefragmentationofpartonsintohadronsinthenalstate.20,,K0andchargedmultiplicitiesinquarkandgluonjetsBecauseofthecolourenhancementofthegluon-gluoncouplingwithrespecttothequark-gluoncoupling,theparticlemultiplicityingluonjetsishigherthaninquarkjets.Asthismultiplicityenhancementappearsatthelevelofthecouplings,itisexpectedtobelargelyindependentoftheparticlespeciesobservedinthenalstate,withsmallcorrectionsonlyduetoe.g.thedecaypropertiesofheavyhadrons.Inmostfragmentationmodels,bothforquarkandgluonjetfragmentation,theneutralisationofcoloureldsoccursviathecreationofquark-antiquarkpairs.Ithashoweverbeensuggested1thatinthefragmentationofagluonjet,colourneutralisationcouldalsooccurviaISMD2000:submittedtoWorldScienticonFebruary2,200116810128 10 12 14 16 18 20 22 24 26OPALQjet(GeV)<Nch><Nchg><Nchq>a)22.533.544.555.58 10 12 14 16 18 20 22 24OPALQjet(GeV)<Npi><Npig><Npiq>b)0.20.30.40.58 10 12 14 16 18 20 22 24OPALQjet(GeV)<Nη ><Nηg><Nηq>c)0.60.811.210 15 20 25OPALQjet(GeV)<NK><NKg><NKq>d)Figure 1. Mean charged particle, 0,  and K0multiplicity in quark and gluon jets, as afunction ofQjet. The curves are the result of a t to the charged particle multiplicity, scaledby a normalisation factor in the gures b,c and d.the creation of gluon pairs. In this case, an enhanced production of isoscalarmesons such as the  would be expected in the fragmentation of gluon jetsand, if they exist, also of glueballs.OPAL has measured 0, , K0and charged particle multiplicities in quarkand gluon jets2as a function of the hardness scale of the jet3dened asQjet Ejetsin2. From hadronic Z0decays with a 3-jet topology two samplesof jets are selected consisting of the second and the third highest energy jetfrom each event, respectively. In the region 8 GeV < Qjet< 26 GeV, thesesamples contain dierent fractions of quark and gluon jets and the mean 0, ,K0and charged particle multiplicities measured for the jets in these samplescan be used to unfold the multiplicities for pure quark and gluon jets.In Fig. 1 we show the mean charged particle, 0,  and K0multiplicitiesin quark and gluon jets as a function of Qjet. Also shown is the result of a tto the charged particle multiplicities, which has been scaled by an appropriatenormalisation factor in gures b,c and d.Fig. 2 shows the ratio of the mean multiplicity in gluon and in quark jetsfor charged particles, 0,  and K0. All cases can be described by the t resultobtained for charged particles. The multiplicity enhancement in gluon jets isthus found to be independent of the studied particle species. These results donot conrm an enhanced  production in three-jet events reported by the L3collaboration.4ISMD2000: submitted to World Scientic on February 2, 2001 20.811.21.410 15 20 25OPALQjet (GeV)<Nch g>/<Nch q>a)0.80.911.11.21.31.41.51.610 15 20 25OPALQjet(GeV)<Npi g>/<Npi q> b)0.811.21.41.610 15 20 25OPALQjet(GeV)<Nη g>/<Nη q>c)0.811.21.41.610 20OPALQjet(GeV)<NK g>/<NK q>d)Figure 2. Ratio of the mean charged particle, 0,  and K0multiplicity in gluon and inquark jets as a function of Qjet. The curves are the result of a t to the charged particlemultiplicities.3 Leading particle production in light avour jetsOPAL has measured leading , K, K0S, p(p) and () rates in up, downand strange avour jets.5As suggested in6, these highest energy particles ina jet often carry the avour of the quark from which the jet originated. Sofar few experimental results7have conrmed this leading particle eect in thefragmentation of light avour jets. The method used by OPAL to determineleading particle rates in light avour jets was proposed in8and is based oncounting all events in which a leading hadron is tagged in one of the eventhemispheres and all events in which both hemispheres have a leading hadrontag. Using this information, our knowledge of the avour composition ofhadronic Z0decays and imposing some constraints based on isospin symmetryand avour independence, the production rates for the dierent hadron speciesin up, down and strange avour jets can be unfolded.In Fig. 3, the leading and Krates in up, down and strange avourjets are shown as a function of xcut, the lower threshold applied on the scaledmomentum of the particle. The rates are compared to the correspondingJETSET9and HERWIG10predictions. In accordance with the expected lead-ing particle pattern, the highest rates are those of pions in up and down avourjets and kaons in strange avour jets. The kaon rate in up avour jets is lowerthan would navely be expected from the leading particle eect. This is due tothe suppressed production of strange quark-antiquark pairs from the hadronicsea. The strangeness suppression factor, associated with this eect, can bedetermined directly from the ratio of leading Kproduction in up and strangeavour jets and the ratio of leading K0production in down and strange avourjets, shown in Fig. 4a and 4b. The strangeness suppression factor obtained iss= 0:422  0:049 (stat:)  0:059 (syst:);ISMD2000: submitted to World Scientic on February 2, 2001 310-210-110.1 0.2 0.3 0.4 0.5 0.6 0.7xcutTagging probability - mesonsOPALdata JETSETu/d → pions → pions → kaonu → kaond → kaon10-210-110.1 0.2 0.3 0.4 0.5 0.6 0.7xcutTagging probability - mesonsOPALdata HERWIGu/d → pions → pions → kaonu → kaond → kaonFigure 3. Tagging probabilities for pions and kaons in up, down and strange avour jets,compared to JETSET (left) and HERWIG (right).which is about one standard deviation high in comparison with othermeasurements,11in most of which swas determined rather indirectly, how-ever.We have also studied baryon production in the nal state. In the MonteCarlo models to which we compare our data, baryon production is governedby the creation of diquark pairs when breaking up strings (in JETSET) orwhen decaying colourless clusters (in HERWIG). In Fig. 5, the ratio of protonover pion production in up avour jets and of lambda over kaon production instrange avour jets are compared to JETSET and HERWIG. The HERWIG0.10.20.30.40.50.60.70.80.910.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6xcutη uK/ηsKOPAL dataJETSETHERWIGJETSET s/u =  γs= 0.31OPAL (a)0.10.20.30.40.50.60.70.80.910.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6xcutη dK0 /ηsK0(b)Figure 4. Ratio of tagging probabilities of Kin up and strange avour jets (a) and of K0in down and strange avour jets (b), compared to HERWIG and JETSET. The dash-dottedcurves indicate the strangeness suppression factor (s= 0:31) used in JETSET.ISMD2000: submitted to World Scientic on February 2, 2001 40.10.20.30.40.50.60.70.80.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6xcutη up /η upiOPAL dataJETSETHERWIGJETSET qq/q = 0.085OPAL(a)0.10.20.30.40.50.60.70.80.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6xcutη sΛ/ηsK(b)Figure 5. Ratio of tagging probabilities of protons and pions in up avour jets (a) andof lambdas and kaons in down avour jets (b), compared to HERWIG and JETSET. Thedash-dottedcurves indicate the diquark suppression factor (qq=q = 0:085) used in JETSET.Monte Carlo signicantly overestimates the fraction of baryons produced.4 Charged particle multiplicity in up, down and strangeavoured Z0decaysOPAL has studied mean charged particle multiplicities in up, down andstrange avoured Z0decays.12Because of the avour independence of thestrong interaction, these multiplicities are expected to be identical, with smallcorrections only due to the decay properties of heavy hadrons.For the purpose of the measurement, the charged particle multiplicityhas been determined for three event samples, dened by the observation of aleading Kwith fractional energy xE> 0:5, of a leading K0Swith xE> 0:4,or of a leading charged hadron with xE> 0:7, respectively. These samplesdier in the relative fractions of up, down and strange avoured events theycontain and have been used to unfold the mean charged particle multiplicityfor purely up, down or strange avoured Z0decays. These are found to behnui = 17:77  0:51 (stat:)+0:86 1:20(syst:);hndi = 21:44  0:63 (stat:)+1:46 1:17(syst:);hnsi = 20:02  0:13 (stat:)+0:39 0:37(syst:);where we point out that in particular the results for up and down avouredevents are strongly anti-correlated. Within the uncertainties, the measuredmean multiplicities are consistent with being identical, as expected from theavour independence of the strong interaction.ISMD2000: submitted to World Scientic on February 2, 2001 55 ConclusionsMean 0, , K0and charged particle multiplicities have been determined forquark and gluon jets. The multiplicity enhancement in gluon jets is found tobe independent of the studied particle species.We have measured leading hadron production rates for various hadronspecies in up, down and strange avour jets. The results conrm the lead-ing particle eect in the fragmentation of light avour jets and have beenused for a direct determination of the strangeness suppression factor, yieldings= 0:422  0:049 (stat:)  0:059 (syst:). In addition the results constitutea detailed test of fragmentation models. In particular the HERWIG MonteCarlo model is found to strongly overestimate baryon production in the nalstate.The mean charged particle multiplicity has been determined for up, downand strange avoured Z0decays. The results are identical within the uncer-tainties of the measurement, as expected from the avour independence of thestrong interaction.References1. C. Peterson and T.F. Walsh, Phys. Lett. B 91, 455 (1980).2. OPAL Collaboration, G. Abbiendi et al, Eur. Phys. J. C 17, 373 (2000).3. Y.L. Dokshitzer et al, Basics of perturbative QCD, Editions Frontieres(1991).4. L3 Collaboration, M. Acciarri et al, Phys. Lett. B 371, 126 (1996).5. OPAL Collaboration, G. Abbiendi et al, Eur. Phys. J. C 16, 407 (2000).6. R.D. Field and R.P. Feynman, Nucl. Phys. B 136, 1 (1978).7. SLD Collaboration, K. Abe et al, Phys. Rev. Lett. 78, 3442 (1997).8. J. Letts and P. Mattig, Z. Phys. C 73, 217 (1997).9. T. Sjostrand and M. Bengtsson, Comp. Phys. Comm. 43, 367 (1987).10. G. Marchesini and B.R. Webber, Nucl. Phys. B 310, 461 (1988);G. Marchesini et al, Comp. Phys. Comm. 67, 465 (1992).11. I.G. Knowles and T. Sjostrand (convs.) et al, in G. Altarelli, T. Sjostrandand F. Zwirner (eds.), 'Physics at LEP2', CERN 96-01, vol.2, p112.12. OPAL Collaboration, G. Abbiendi et al, CERN-EP-2000-128, submittedto Eur. Phys. J. C.ISMD2000: submitted to World Scientic on February 2, 2001 6

Recent Results on Particle Production from OPAL

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