Recent analyses, performed using multihadronic Z 0 decays collected with the OPAL detector between 1991 and 2000 at the Large Electron Positron (LEP) collider, are presented. The first one is a study of correlations of particles with restricted momenta, indicating some difficulties with the Local Parton-Hadron Duality (LPHD) hypothesis when applied to many-particle inclusive observables of soft hadrons. In the second analysis, unbiased gluon jet properties are studied using a new technique called jet boost algorithm

Giunta, Marina

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Study of hadronic final states at LEPMarina Giunta  University of California Riverside, USA / CERN, SwitzerlandE-mail: marina.giunta@cern.chRecent analyses, performed using multihadronic Z0 decays collected with the OPAL detectorbetween 1991 and 2000 at the Large Electron Positron (LEP) collider, are presented. The first oneis a study of correlations of particles with restricted momenta, indicating some difficulties withthe Local Parton-Hadron Duality (LPHD) hypothesis when applied to many-particle inclusiveobservables of soft hadrons. In the second analysis, unbiased gluon jet properties are studiedusing a new technique called jet boost algorithm.International Europhysics Conference on High Energy PhysicsJuly 21st - 27th 2005Lisboa, PortugalSpeaker.PoS(HEP2005)049cCopyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/Study of hadronic final states at LEP Marina Giunta1. IntroductionThe analyses described in the following use multihadronic Z0 decay events observed with theOPAL detector [1] at the LEP e   e  collider at CERN. In the second section, preliminary resultsof a study [2] of correlations of particles with restricted momenta are summarized; section threepresents a study [3] of unbiased gluon jet properties.2. Correlations of particles with restricted momentaIn order to extract the non-Poissonian nature of multiparticle fluctuations, it is convenient tocharacterize multiplicity distributions by normalized factorial moments:FqΩ  nn  1 					n  q  1  n Ω ρqp1  pq  ∏qi  1 d piΩ d pρ1p  q q  1(2.1)here n is the multiplicity in the phase space region Ω and ρq is the inclusive q-particle density ofparticles with momenta pi. Normalised factorial cumulants KqΩ  are a measure of genuine multi-particle correlations: being related to the FqΩ  , the KqΩ  become equal to zero for uncorrelatedparticle production.It is well known that multiplicity distributions of hadrons produced in high energy collisionsare wider than the Poisson distribution, but it was shown [2], within Double Leading LogarithmicApproximation (DLLA) calculations, that it is possible to select restricted regions of phase spacewhere partons are less correlated and the multiplicity distribution is narrower. Two types of factorialmoments are defined:11.11.210 -1 1 10F q q = 2OPAL (prelim.)y > 0y > 1.52 jets, y > 0ZEUS11.251.51.75210 -1 1 10q = 311.522.5310 -1 1 10q = 41234510 -1 1 10q = 5pT   cut   [GeV]F qFigure 1: Factorial moments of charged particles with pT  pcutT as a function of pcutT , compared to those of2-jet events, in the rapidity window y>1.5 and data from ZEUS. Only statistical uncertainties are shown.PoS(HEP2005)049049 / 2Study of hadronic final states at LEP Marina Giunta  cylindrically cut factorial moments, FqpT  i pcutT  : due to angular ordering (effect of QCDcoherence) low transverse momentum (pT ) gluons are produced independently, so the mul-tiplicity distribution becomes Poissonian: Fq  1  Kq  0, for pcutT Q0 (Q0, the virtualitycut-off, is few hundred MeV).  spherically cut factorial moments, Fqpi pcut  : the multiplicity distribution remains non-Poissonian for absolute momentum pcut  0: Fq  const  1 and Kq  const  0.If LPHD is valid, the same behaviour should be observed in experimental hadronic data.The OPAL data on cylindrically cut factorial moments (full dots in Fig. 1) do not show thePoisson limit and the Monte Carlo (Pythia 6.2, Ariadne 4.1 and Herwig 6.3) predictions follow thedata trend: with decreasing pcutT the moments decrease towards a minimum at pcutT 1 GeV thenthey rise strongly, in disagreement with the perturbative QCD result for partons. The minimumcould indicate a border-line between a region where perturbative approach is valid and a regionwhere it is not, because of strong hadronisation effects that mask the expected partonic behaviour.This result confirms a previous one by the ZEUS Collaboration, except for the minimum atpcutT 1 GeV, absent in the ZEUS measurements. OPAL repeated the analysis in rapidity intervalsand using only two-jet events, in order to mimic the ZEUS experimental conditions (Fig. 1); theminimum disappears, suggesting that the often assumed equivalence between one hemisphere ine e  collisions and the current region of the Breit Reference Frame in e p collisions, used in theZEUS analysis, can be misleading for soft particle production.The OPAL data show a raise also in the spherically cut factorial moments for small value ofpcut , contrary to the theoretical predictions for partons.The cumulants show the same behaviour as the factorial moments, proving that the effect isdue to genuine correlations of hadrons.The results obtained by OPAL, showing how the expected limit values are not reached forsmall values of pcutT or pcut , indicates difficulties with the LPHD hypothesis when applied to many-particle inclusive observables of soft hadrons.3. Unbiased gluon jets’ properties using the jet boost algorithmThe jet boost algorithm [3] (BA) is a method to determine properties of unbiased gluon jetsproposed by the Lund theory group. After testing the method, OPAL used it to study unbiasedgluon jets at different energies and then compared the results with theoretical predictions.The BA is based on the color dipole model of QCD and can be summarized in the followingway: in three-jet events, symmetric with respect to the gluon direction, the two independent dipoles(one connecting the q and the g and the other going from the q¯ and the g) can be boosted andcombined to yield the dipole structure of a gg event.Experimentally, three jets are reconstructed in each multihadronic event and the gluon jet isidentified, obtaining a final sample of 25396 events with a gluon jet purity of 85%. Then each eventwas boosted to a frame where the angle between the q and the g was the same as the angle betweenthe q¯ and the g (θqg  θ q¯g  θ ), yielding a symmetric event. The multiplicity of the gluon jet isgiven by the number of particles lying inside the cone-like region defined by the bisectors of θqgPoS(HEP2005)049049 / 3Study of hadronic final states at LEP Marina Giunta11.21.41.61.822.25 6 7 8 9 10 20 30 40Eg (GeV)*(a)r g/q=<nquark><ngluon>ch.ch.3NLO fitDipole ModelFixed αS Numerical SolutionHerwig (hadrons)Herwig (partons)OPALGluon (boost) / quark dataGluon (gincl) / quark dataGluon (CLEO) / quark dataFigure 2: The ratio between the mean charged particle multiplicities of unbiased gluon and uds flavoredquark jets, rq   g, as a function of jet energy E g .and θ q¯gand the corresponding energy scale of the unbiased gluon jet, E  g , is equal to the transversemomentum of the gluon jet, defined by:p  gluon 12sqgs q¯gs(3.1)where si j (i  j  q  q¯  g) is the invariant mass squared of the i j pair and s  E 2c m  . The events aredivided in 7 subsamples according to the gluon jet energy E  g .Using Herwig 6.2, OPAL compare results obtained with the BA method on simulated three-jetevents with results for unbiased gluon jets from simulated color singlet gg events. They comparethe multiplicity distributions (in the 7 energy intervals) and found a good agreement for E  g  5GeV; they also measured the fragmentation functions which agreed for E g  14 GeV.After establishing the validity of the method, many measurements of unbiased gluon jet prop-erties are performed: the mean multiplicity (in the 7 energy bins), actually the most precise resultfor 5 	 25  E g 20 GeV; the first measurement of the factorial moments F2 and F3 of unbiasedgluon jets multiplicity distributions over an energy range; the ratio between the mean charged par-ticle multiplicities of unbiased gluon and uds flavored quark jets, rq  g, as a function of jet energy(see Fig. 2). The measurements are compared to many different theoretical predictions and anoverall global agreement was found. Finally, the fragmentation function was measured for jetswith E g  14 GeV and was fitted using the DGLAP evolution equation. The fit gives a good de-scription of the measurements and yields a result for the strong coupling constant consistent withthe world average:αsmZ  0 	 128  0 	 008stat 	  0 	 015syst 	  (3.2)which provides a unique consistency check of QCD.References[1] OPAL Collaboration, The OPAL detector at LEP, Nucl. Instr. and Meth. A305 (1991) 275.OPAL Collaboration, The OPAL silicon strip microvertex detector with two coordinate readout, Nucl.Instr. and Meth. A346 (1994) 476.[2] OPAL Collaboration, OPAL Physics Note OPAL-PN528, QCD coherence and correlations ofparticles with restricted momenta, July 2005, and references therein.[3] OPAL Collaboration, Experimental Studies of unbiased gluon jets from e+e- annihiltions using the jetboost algorithm, Phys. Rev. D69 (2004) 032002, and references therein.PoS(HEP2005)049049 / 4

Study of hadronic final states at LEP

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