The analyses of recent LEP data provide us with a rich ground for pQCD and
LPHD tests. A number of non-trivial effects predicted in the framework of MLLA
are experimentally verified. Recent measurements also enrich our knowledge
about the hadronization process.Comment: 3 pages. Presented at the International Europhysics Conference High
  Energy Physics 99, Tampere, Finland, July, 199

Nomokonov, Vasilii

English

arXiv

The analyses of recent LEP data provide us with a rich ground for pQCD and LPHD tests. A number of non-trivial effects predicted in the framework of MLLA are experimentally verified. Recent measurements also enrich our knowledge about the hadronization process.The analyses of recent LEP data provide us with a rich ground for pQCD and LPHD tests. A number of non-trivial effects predicted in the framework of MLLA are experimentally verified. Recent measurements also enrich our knowledge about the hadronization process

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arXiv:hep-ex/9910059v1  27 Oct 1999New Results on Multiplicity, Identified Particles andSpin Measurements in e+e− collisionsVasilii Nomokonov (Helsinki Institute of Physics)on behalf of the DELPHI collaborationAbstractThe analyses of recent LEP data provide us with a rich ground for pQCD and LPHD tests. Anumber of non-trivial effects predicted in the framework of MLLA are experimentally verified.Recent measurements also enrich our knowledge about the hadronization process.1 IntroductionRecent LEP results on multiparticle production in hadronic jets enable further important testsof QCD. One of the most interesting and intriguing outcomes of the pQCD calculations is theprediction of so-called colour coherence effects [1].A number of analyses presented here demonstrate that there is an agreement of the datawith the QCD MLLA predictions and provide further support for the concept of local partonhadron duality (LPHD) [2].There are also new results in the physics of large distances, where the processes not calcu-lable within the pQCD provide us with abundant information needed for tuning hadronizationmodel parameters.2 Testing analytical predictions of Perturbative QCDInclusive charged hadron distributions measured by DELPHI at 189 GeV [3] were presentedas a function of the variables: rapidity, ξp = ln(1/xp), momentum and transverse momentum.The ξp distribution demonstrates the so-called “hump backed” behaviour predicted for partonsin the framework of the Modified Leading Logarithmic Approximation (MLLA) [2]. The si-multaneous fit to the ξp distribution with a Fong-Webber distorted Gaussian [4] at differentenergies including the present measurement at 189 GeV show very good agreement of the dataand the prediction (χ2/dof = 99.6/97) giving support for the LPHD hypothesis.MLLA also provides a definite prediction for the energy evolution of the maximum ofthe ξ distribution, ξ∗. As hadronization and resonance decays are expected to act similarly atdifferent centre-of-mass energies, the energy evolution of ξ∗ is expected to be less sensitive tononperturbative effects. The ξ∗ values entering in the analysis were determined by fitting adistorted Gaussian with the parameters given by the Fong-Webber parametrisation. For the189 GeV data one obtains ξ∗ = 4.157±0.030. A fit of the MLLA prediction again demonstratesgood agreement while ruling out a phase space expectation and the DLA prediction.The energy evolution of the momentum distribution is well described by the fragmenta-tion model. An interesting observed feature is the approximate ECM independence of hadronproduction at very small momenta p < 1 GeV. This has been explained in [5],[2] to be due to thecoherent emission of long wavelength gluons by the total colour current which is independentof the internal jet structure and is conserved under parton splittings. Therefore, low-energygluon emission is expected to be almost independent on the number of hard gluons radiated1and hence of the centre-of-mass energy. Provided the LPHD hypothesis is correct, the numberof produced hadrons at small momenta is approximately constant.A sample of 2.2 million hadronic Z decays, selected from the data recorded by the Delphidetector at LEP during 1994-1995 was used for a precise measurement of inclusive distributionsof pi+, K+ and p and their anti-particles in gluon and quark jets [6]. As observed for inclusivecharged particles, the production spectra of the individual identified particles were found tobe softer in gluon jets compared to the quark jets, with a higher multiplicity in gluon jets. Asignificant enhancement of protons in gluon jets is observed. The ratio of the average multi-plicity in g jets with respect to q jets was found for all identified particles to be consistent withthe ratio measured for all charged particles. The normalised ratio for protons in Y events wasmeasured to be:Rp = 1.205± 0.041,which differs significantly from unity.The maxima, ξ∗, of the ξ-distributions for kaons in gluon and quark jets are observed tobe different.A particularly nice illustration of the phenomenon of QCD coherence and a test of LPHDwas obtained by DELPHI using symmetric 3-Jet Events [7]. It is known that soft radiation issensitive to the total colour flow in the underlying hard partonic structure. Let’s consider, forexample, two extreme two-jet topologies of a qq¯g event. If the gluon is collinear to one of thequarks, the colour flow in the event will be identical to the qq¯ case, whereas if the gluon exactlyrecoils with respect to the two quarks, the colour flow will correspond to that of a gg event.In the latter case the soft radiation at large angle is expected to be increased by the colourfactor ratio CA/CF as compared to the qq¯ case. The evolution between those extreme caseshas been calculated as a function of the opening angles between the jets [5]. Thus, the chargedhadron multiplicity in a cone perpendicular to the event plane of symmetric three-jet eventswas determined as a function of an inter-jet angle for the data collected at the Z resonance. Aclear dependence of the multiplicity on the opening angle was observed and appears to be inagreement with QCD predictions [8],[5].An interesting example of the intra-jet QCD coherence is the restriction of forward gluonemission for heavy quarks. A calculation in the framework of MLLA predicts the followingangular distribution of gluon emission [1]:dndθ2∼ θ2/(θ2 + θ2min)2, θmin ≡mQEwhere mQ and E are a quark mass and energy, respectively. Provided LPHD holds, the effectshould be seen, for example, in a comparison of the primary-particle angular distribution forbb¯ and qq¯ (where q denotes u,d or s quark) Z decays. Delphi has presented preliminary resultsthat show a difference in this behaviour. Hadrons containing the original quark or originatingfrom the decay of such particle are carefully excluded from consideration.The phenomenon of colour coherence is not only a subject for tests, but also may be usedas a tool for reconstruction of the event colour structure on an event-by-event basis. The ideapresented in [9] is based on the fact that soft particles do not originate from a particular partonbut rather their production depends on the whole colour topology of the event. Therefore,it is possible to define a way to estimate the colour connection strength between the partonsby analysing the behaviour of the soft particles. The method can then be used for partonidentification. The proposed algorithm is described below: first, fast particles are used toreconstruct cluster directions and then to define a weight wij that a particle i may be connected2to a cluster j with:wij =Cik2ijwhere k2ij = 2E2i (1−cosΘij), with normalisation Σjwij = 1. Then each particle with wij < 0.95is assigned to the cluster pair kl for which the sum wikl = wik + wil is maximal. Then partonpair connectedness is defined asWkl = CklΣig(Ei)(wik + wil)where i runs through all the particles assigned to the clusters kl.The method was tested by the Delphi collaboration by using double b-tagged 3-jet Mer-cedes events collected at the Z pole. The gluon jet is known in these kind of events as one whichis not b-tagged. On the other hand, the gluon should have two colour connections and thusthe colour connectedness Wkl of the bb¯ pair must have the smallest value in any given event.Matching the two gives the purity of the method which is found to be above 60% and could beimproved by requiring the smallest colour connection coefficient to be below a predeterminedthreshold value.The method with various modifications could be used for identifying colour connectionsin numerous applications (pairing, background rejection).One of the approaches to study a parton shower cascade is to employ the multiplicity mo-ments technique. The oscillations in the ratio of the cumulant factorial to the factorial chargedparticle multiplicity moments in Z Decays is known to show a quasi-oscillatory behaviour whenplotted versus the order of the moment, as was observed by the SLD collaboration some timeago [10]. This peculiarity is also predicted by the NNLLA of perturbative QCD within theLPHD framework [11].However, using the jet multiplicity distributions obtained from the Cambridge jet algo-rithm, in order to vary the dependence on the LPHD hypothesis, the L3 collaboration found[12] that the oscillations appear only for non-perturbative energy scales, namely ≤ 100 MeV.From this conclusion it follows that the observed oscillations are unrelated with the behaviourpredicted by the NNLLA perturbative QCD calculations.Another challenging way to study the cascade is to measure multiplicity fluctuations inrings around the jet axis and in off-axis cones. The DELPHI collaboration performed themeasurement [13] and compared them with analytical perturbative QCD calculations for thecorresponding multiparton system, using the concept of LPHD. Some qualitative features wereconfirmed by the data but substantial quantitative deviations are observed.3 Fragmentation physicsOur knowledge about the hadronization process has been significantly enriched by recent mea-surements at LEP.Thus, results on the production of the Λ(1520) are presented, as obtained from hadronic Zdecays recorded by DELPHI [14]. The Λ(1520) scaled momentum (xp) spectrum is determined.The relative importance of Λ(1520) production increases with xp similarly to that of orbitallyexcited mesons. It is shown that the Λ(1520) primarily originates from fragmentation and notfrom heavy particle (b, c) decays. The large Λ(1520) production rate NΛ(1520)/NZ = 0.030 ±0.004± 0.005 suggest that many stable baryons descend from orbitally excited baryonic states.The OPAL collaboration measured the helicity density matrix elements ρ00 of ρ(770)±and ω(782) mesons produced in Z0 decays [15]. Over the measured meson energy range, thevalues are compatible with 1/3, corresponding to a statistical mix of helicity −1, 0 and 1 states.3For the highest accessible scaled energy range 0.3 < xE < 0.6, the measured ρ00 values of theρ± and the ω are 0.373 ± 0.052 and 0.142 ± 0.114, respectively.The ALEPH collaboration performed an extensive study of the production rates andthe inclusive cross sections of the isovector meson pi0, the isoscalar mesons η and η′(958), thestrange meson K0S and the Λ baryon. This was done as function of scaled energy (momentum)in hadronic events, two-jet events and each jet of three-jet events from hadronic Z decaysand compared the results to Monte Carlo models [16]. The JETSET modelling of the gluonfragmentation into isoscalar mesons is found to be in agreement with the experimental resultsfor the measured region. HERWIG fails to describe the K0S spectra in gluon-enriched jets andthe Λ spectra in quark jets.An interesting idea which helped to understand the production rates of light-flavourhadrons was proposed by P.Chliapnikov [17]: the difference between the production rates ofhadrons composed of the same quarks and belonging to the different SU(3) multiplets but thesame SU(6) multiplet is essentially determined by the hyperfine mass splitting. This trend showsup when the direct production rates are plotted versus the sum of the constituent quark massesΣi(mq)i. In this case the vector-to-pseudoscalar and decuplet-to-octet suppressions are found tobe the same. In the proposed scenario the strangeness suppression factor, λ = 0.295± 0.006, isthe same for mesons and baryons and related to the difference in the constituent quark masses,λ = e−(ms−mˆ)/T , where mˆ = mu = md, and the temperature T = 142.4± 1.8 MeV/c2.References[1] For a recent review see Khoze V A, Ochs W 1997, Int. J. of Mod. Phys. A 12 N17[2] Azimov Ya et al. 1985, Z.Phys. C 27 65; Azimov Ya et al. 1986, Z.Phys. C 31 213[3] DELPHI collaboration, EPS HEP99, contributed paper #1 225[4] Fong C P and Webber B R 1989, Phys. Lett. B 229 289[5] Khoze V, Lupia S and Ochs W 1997, Phys. Lett. B 394 179[6] DELPHI collaboration, EPS HEP99, contributed paper #3 146[7] DELPHI collaboration, EPS HEP99, contributed paper #1 145[8] Azimov Ya et al. 1985 Phys. Lett. B 165 147[9] Orava R et al. 1998, Eur. Phys. J. C 5 471[10] Abe K et al. 1996, Phys. Lett. B 371 149[11] Dremin I M and Nechitailo V A 1993, JETP Lett. 58 881[12] L3 collaboration, EPS HEP99, contributed paper #1 276[13] DELPHI collaboration, EPS HEP99, contributed paper #1 222[14] DELPHI collaboration, EPS HEP99, contributed paper #3 147[15] OPAL collaboration, EPS HEP99, contributed paper #3 63[16] ALEPHI collaboration, EPS HEP99, contributed paper #1 394[17] DELPHI collaboration, EPS HEP99, contributed paper #3 5614

New results on multiplicity, identified particles and spin measurements in e+e- collisions

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