Several recent results of the ALEPH Collaboration covering different aspects of baryon production on the Z resonance are presented. In particular production rates of hyperons, the full kinematical reconstruction of the Lambda_b, observation of Xi_b in its semileptonic decay, and the measurements of the polarization of Lambda and Lambda_b baryons are discussed

Becker, U.

arXiv

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arXiv:hep-ex/9608004v1  6 Aug 1996IHEP–HD/96 – 6Baryon Productionin ALEPHPresented on behalf of the ALEPH Collaboration byU. Becker 1Institut fu¨r Hochenergiephysik der Universita¨t HeidelbergHeidelberg, GermanyAbstractSeveral recent results of the ALEPH Collaboration covering different aspects ofbaryon production on the Z resonance are presented. In particular productionrates of hyperons, the full kinematical reconstruction of the Λb, observation ofΞb in its semileptonic decay, and the measurements of the polarization of Λ andΛb baryons are discussed.1Talk given at the HADRONS 96 conference 9.6. – 16.6. 1996 in NOVY SVET, Ukraine11 IntroductionBaryon production in quark fragmentation is still poorly understood. The existing modelsreproduce the available data quite well. To distinguish between these models and to come to amore fundamental understanding of the production mechanism of baryons more experimentalinput is needed. Results of the ALEPH Collaboration on hyperon and b – baryon productionin Z decays are reported here. Whenever possible, not only the overall production rates butalso the momentum spectra of baryons have been investigated and compared to Monte Carlopredictions.While the number of produced hyperons is quite large the production rate of b – baryonsis low. Only few of them have been observed by LEP experiments so far. The full recon-struction and mass determination of the Λb and the observation of the Ξb through Ξ– leptoncorrelations will be described.Z decays provide many possibilities to test the Standard Model. Quarks produced in Zdecays are expected to obtain a strong longitudinal polarization due to parity violation inthe electroweak interaction. While mesons always cascade down to spin zero pseudoscalarstates retaining no polarization information, baryons might preserve a large fraction of theinitial quark polarization. Recent measurements of the Λ and Λb polarizations are presentedhere.The data sample used for these studies has been taken with the detector ALEPH atthe LEP storage ring at a center of mass energy of about 91GeV. The detector and itsperformance are described in detail elsewhere [1].2 Hyperon productionALEPH has studied recently the inclusive production rates of several octet and decuplethyperons: Σ±(1385), Ξ−, Ξ0(1530) and Ω−[2]. Except for the Ω− also the energy spectrahave been measured. The four hyperons have been reconstructed in their decays:Σ±(1385) → Λpi±Ξ− → Λpi−Ξ0(1530) → Ξ−pi+Ω− → ΛK−where the Λ candidates are tagged in the decay channel Λ → ppi−. The resulting massdistributions are shown in Fig. 1a - d.Except for the Ω− (due to low statistics) the energy spectra have been determined andcompared with Monte Carlo predictions from JETSET 7.3[6] and UCLA[7] (Fig.2). Theagreement is quite reasonable with slight preferences for JETSET.The mean multiplicities per Z decay have then been determined using JETSET to extrapolateinto the inaccessible low energy region.2Figure 1: (a)The signal for Ξ− → Λpi−. (b) The signal for Ω→ ΛK−. (c) The mass spectrumfor Σ±(1385) → Λpi±, fit to a background shape, a Ξ− contribution, plus a Breit –Wignerresonance. Also shown is the background – subtracted spectrum. (d) The mass – differencespectrum for Ξ0(1530)→ Ξ−pi+ candidates, fit to the background shape plus a Breit –Wignerresonance convolved with a gaussian.The preliminary results are:〈NΞ−〉+ 〈NΞ¯+〉 = 0.0285± 0.0007± 0.0020〈NΞ0(1530)〉+ 〈NΞ¯0(1530)0〉 = 0.0072± 0.0004± 0.0006〈NΣ±(1385)〉+ 〈NΣ¯(1385)±〉 = 0.065± 0.004± 0.009〈NΩ−〉+ 〈NΩ¯+〉 = 0.0010± 0.0002± 0.0002While our measurement of the Σ±(1385) rate is in good agreement with the Monte Carloprediction it is nevertheless about 1.7 times higher than the corresponding measurementsfrom DELPHI and OPAL[3],[4],[5]. Fig.3 gives an overview about the hyperon productionrates at LEP[8].3Figure 2: The measured xE distribution for Σ±(1385), Ξ−, and Ξ0(1530) (with antiparticlesincluded), compared with predictions from the UCLA and JETSET 7.3 models.ALEPHOPALDELPHIL3Hyperon Mass (GeV/c2)(NBaryon /(2I+1)) per eventJETSET 7.4 PSHERWIG 5.8UCLA 7.44ΛΣΞΣ✳Ξ✳ΩFigure 3: Production rates of hyperons at LEP together with predictions of JETSET, HER-WIG and UCLA models. Lines are drawn to guide the eye.43 b – baryonsThe situation in the sector of heavy baryons containing a b – quark is quite different. Due totheir low production rates combined with small branching ratios (in the order of a few %)and long decay chains only very few b – baryons have been observed at LEP so far, namelythe Λb, Ξb (which will be presented here) and Σb, Σ∗b by the DELPHI Collaboration [9].3.1 The ΛbThe Λb has been observed indirectly since several years in Λl− and Λ+c l− correlations (wherel = muon or electron). Its production rate has been determined by e.g. ALEPH to beBr(b → Λb) · Br(Λb → Λ+c lν¯X) = (0.76 ± 0.15± 0.13)10−3[10] but its mass remained onlypoorly known. Now the Λb has been fully reconstructed in its decay channel Λb → Λ+c pi−with Λ+c → pKpi, pK¯0 or Λpipipi. Several cuts have been applied to reduce the background.The result is shown in Fig.4The background has been estimated to be 0.38 ± 0.05 entries leaving us with a statisti-cal significance of the observed peak at the 3.3σ level, not taking into account the massclustering of the four events. The Λb mass has been found to be (5614 ± 21(stat.) ±4(sys.))MeV/c2[11]. This is in good agreement with the recent measurements of DEL-PHI (5668± 16± 8)MeV/c2[12] and CDF (5623± 5± 4)MeV/c2[13] and the PDG94 value(5641± 50)MeV/c2[14].Figure 4: (a) Λcpi invariant mass distributions for the right – sign combination and (b) forthe wrong – sign combinations. (c) Λb invariant masses for the four selected candidates. Thedotted lines indicate the ±1σ values around the ALEPH average measurement.53.2 The ΞbA search for Ξ−l− correlations (again l stands for muon or electron) in the same hemisphereof the detector has been made to find evidence for Ξb production. After correcting for otherpossible sources of Ξ−l− pairs as e.g. Λb B or accidentals an excess of 22.5 ± 5.7 like – signcombinations with respect to opposite – sign combinations has been observed. It is thereforeinterpreted as from the semileptonic decay of the Ξb and can be translated into the followingproduct of branching fractions:Br(b→ Ξb) ·Br(Ξb → Ξ−l−ν¯X) = (5.3± 1.3± 0.7)10−4per lepton species[15]. This value is in good agreement with the DELPHI result[16].4 PolarizationPolarization of the primary quarks coming from Z decays is due to the parity violatinginequality of the Z coupling to right and left handed fermions. Ignoring mass effects andgluon radiation (which may depolarize the quarks by about 3% [17]) the expected polarizationcan be written asP qL =−2gvgag2V + g2Awhere gv and ga are the vector and axial vector coupling constants of the Z to the quark.With sin2 ϑW = 0.23 a polarization of −94% for downtype quarks is expected according tothe Standard Model, but may be diluted substantially during the hadronization phase dueto spin – spin forces.Mesons always cascade down to spin zero pseudoscalar particles which do not retain anypolarization information. In contrast baryons are expected to preserve a sizeable fractionof the initial quark polarization which, however, gets smaller if the weakly decaying groundstate baryon is produced in the decay of a higher resonance[18],[19]. Therefore the meanmeasureable polarization will depend on the production rate of these resonances and willthus give additional information about the hadronization phase.4.1 The Λ polarizationThe polarization of Λ hyperons can be measured by investigating the decay angle ϑ∗ intheir decay Λ → ppi−. Here ϑ∗ is defined as the angle between the proton momentumin the Λ restframe and the Λ boost direction. The ϑ∗ distribution is expected to followR(ϑ∗) = 1 + αPΛL cosϑ∗ with the asymmetry parameter α = 0.642 ± 0.013 [14] and the Λpolarization PΛL . The observed Λ polarization is diluted with respect to the primary s –quark polarization by those Λ baryons which do not contain a primary quark. To derivea Monte Carlo prediction of PΛL , first the fraction of Λ’s containing a primary quark has6been determined in the data by measuring the Λ forward – backward asymmetry and thecorrelation of back – to – back ΛΛ¯ pairs. The differences between data and MC have beenused as correction factors for all possible sources of polarized Λ’s (see [21] for details).Then the fraction of Λ baryons coming from e.g. Λb,Σ etc. has been corrected, and theexpected polarizations [20] have been added up. The ALEPH measurement is shown inFig.5 together with the expectation. For Λ’s with z = p(Λ)/PBeam > 0.3 the polarizationhas been determined to be PΛL = −0.32 ± 0.07 [21], in good agreement with the JETSETprediction of −0.39± 0.08.00.250.50.7511.25-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.800.10.20.30.40.50.60 0.1 0.2 0.3 0.4 0.5 0.6Figure 5: Right: Fit of the longitudinal Λ polarization to the decay angle distribution forz > 0.3 .Left: The measured longitudinal Λ polarization is shown as as function of z together withthe JETSET prediction after multiplying by the correction factor (shown as solid line). Thedashed lines indicate the estimated uncertainty of the JETSET prediction. The HERWIGprediction after multiplication by the correction factor is shown as dotted line.4.2 The Λb polarizationThere is no need to estimate the ‘primary’ Λb fraction since b – quarks nearly always originatedirectly from Z decays. It has been suggested for some time to measure the Λb polarization(which is equal to the b – quark polarization in the heavy quark limit) in its semileptonicdecay[22].Apart of the interest in the polarization itself, weak decays of polarized b – quarks provideanother interesting aspect. It has been pointed out by Gronau and Wakaizumi [23] that7the left handedness of the b – quark coupling to the charged weak current has never beenproven. They have shown that, assuming the same handedness for the leptons involved inthe decay, the chirality could not be determined in b –meson decay . Amundsen et. al. [24]suggested to resolve this ambiguity through an investigation of the semileptonic decay ofpolarized b – quarks which are available in Λb’s from the Z resonance. When deriving thepolarization from the lepton spectra, a V+A coupling would result in a change of sign ofthe measured with respect to the true polarization. This is in principle analogous to thedetermination of the Michel parameter ξ in τ decays.The ALEPH analysis of the Λb polarization [25] has followed a suggestion of Bonviciniand Randall [26] who proposed to study the ratio y of the mean energies of charged leptonsand neutrinos from semileptonic Λb decays. This procedure offers several advantages: In-dependence from the Λb fragmentation function, greater sensitivity than the charged leptonalone and more statistics than any exclusive decay.For this purpose Λpi+l− pairs have been selected. The neutrino energy is calculated fromthe visible amount of energy in the detector: Eν = Etot−Evis, with Etot =√s/2+ (M2same−M2oppo)/2√s. Here Msame (Moppo) is the invariant mass in the same (opposite) hemisphere.Biases due to selection cuts and neutrino energy reconstruction have been corrected withMonte Carlo simulations.00.511.52PΛbRyRyexp = 1.09 ± 0.10PΛb = −0.23ALEPH−0.20+0.240. −0.25 −0.50 −0.75 −1.Figure 6: The method to extract the Λb polarization value: Comparison between the measuredRexpy value and the theoretical prediction Ry8The polarization in the data is extracted by comparing Rexpy = ydata/yMC(P = 0) (whereyMC(P = 0) is the expected ratio between lepton and neutrino mean energy without anypolarization) with the expected Ry curve and correcting for differences between Data andMonte Carlo (Fig.6):PΛb = −0.23+0.24−0.20(stat.)+0.08−0.07(syst.)This result can be compared with an expectation of about -0.73 estimated from [19] togetherwith the Σ(∗)b production rates measured by DELPHI [9]. It is quite low (but consistent withthe expectation), indicating additional depolarization effects during hadronization.References[1] D. Decamp et al., ALEPH Collaboration, Nucl. Instrum. Meth. A 294 (1990) 121D. Buskulic et al., ALEPH Collaboration, Nucl. Insttrum. Meth. A 360 (1995) 99[2] D. Buskulic et al., ALEPH Collaboration, Hyperon production in Z decays, contributedpaper to EPS–HEP 95, Brussel,Belgium[3] P.D. Acton et al., OPAL Collaboration, Phys. Lett. B 291 (1992) 503[4] P. Abreu et al., DELPHI Collaboration, CERN -PPE/96 - 12, Submitted to Z. Phys. C[5] P.D. Acton et al., OPAL Collaboration, Proceedings of the August 1994 meeting of theAPS Divison of Particles and Fields, Albuquerque, N.M.; OPAL Physics Note PN138[6] M. Bengtson and T. Sjo¨strand, Phys. Lett. B 185 (1987) 435; T. Sjo¨strand, CERN–TH.6488/92 (1992)[7] S.B. Chun and C.D. Buchanan, Phys. Lett. B 308 (1993) 153. Curves obtained fromC.D. Buchanan by private communication.[8] Numbers taken from P.V. 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Baryon production in ALEPH

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Baryon production in ALEPH

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