(Submitted to Physics Letters B) OCR Output

of the Lund Monte Carlo tuned to data at PEP / PETRA energies.decays. The measured production cross sections are compared with predictionsin 1990. Kg, K **, A(K) and E‘(§+) have been identified by their characteristic90,000 selected hadronic Z0 decays collected by the DELPHI detector at LEPson into multihadronic final states is presented. The analysis is based on aboutAn analysis of the production of strange particles from the decays of the ZU bo

Abstract

DELPHI Collaboration

O the Hadromc Decays of the ZO O I Production of Strange Part icles in

1B_ B332· 8 19 November 1991

CERN—PPE/91-207

AR RESEARCHEUR

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Cem - PPG Bvio?

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°Fachbereich Physik, University of \Vuppertal, Postfach 100 127, D-5600 \rVuppertal l, FRG OCR Outputllnst. Nuclear Studies and, University of Warsaw, Ul. Hoza 69, PL-00681 Warsaw, Polandalnstitut Iiir Hochenergiephysik, Oesterreich Akad. d. Wissensch., Nikolsdorfergasse 18, A-1050 Vienna, AustriaMolecular y Nuclear, Univ. de Valencia, Avda. Dr. Moliner 50, E—46100 Burjassot (Valencia), Spain2[nst. de Fisica Corpuscular [FIC, Centro Mixto Univ. de Valencia-CSIC, and Departamento de Fisica Atomica'Department ol` Radiation Sciences, University of Uppsala, P.O. Box 535, S-751 21 Uppsala, Swedenand Istituto di Fisica, Unjversita di Udine, I-33100 Udine, Italy°Dipartimento di Fisica, Université di Trieste and INFN, Via A. Valerio 2, [-34127 Trieste, Italy°Dipartimento di Fisica Sperimentale, Universita di Torino and INFN, Via P. Giuria 1, [-10125 Turin, Italy° Institute of Physics, University of Stockholm, Vanadisviigen 9, $-113 46 Stockholm, SwedenIlnst. for High Energy Physics, Serpukow P.O. Box 35, Protvino, (Moscow Region), USSR.°Faculta»d de Ciencias, Universidad de Santander, av. de los Castros, E · 39005 Santander, Spain5 Istituto Superiore di Sanita, [st. Naz. di Fisica Nucl. (INFN), Viale Regina Elena 299, [-00161 Rome, Italy*CEN-Saclay, DPhPE, F—91191 Gif-sur-Yvette Cedex, France°Dipartimento di Fisica, Universita di Roma II and INFN, Tor Vergata, I-00173 Rome, Italy2Rutherl`ord Appleton Laboratory, Chilton, GB - Didcot OXII OQX, UKIDipartimento di Fisica, Universita di Padova and INFN, Via Marzolo 8, I-35131 Padua, Italy°Nuclear Physics Laboratory, University of Oxford, Keble Road, GB — Oxford OX1 3RH, UK° Physics Department, University ol` Oslo, Blindern, N-1000 Oslo 3, Norway° National Technical University, Physics Department, Zografou Campus, GR·l5773 Athens, GreeceINIKHEF·H, Postbus 41882, NL-1009 DB Amsterdam, The Netherlands6Niels Bohr Institute, Blegdamsvej 17, DK-2100 Copenhagen 0, Denmark°Dipartimento di Fisica, Universita di Milano and INFN, Via Celoria 16, [-20133 Milan, Italy*Univ. d’Aix - Marseille II — Case 907 - 70, route Léon Lachamp, F-13288 Marseille Cedex 09, France°Universidad Complutense, Avda. Complutense s/n, E-28040 Madrid, Spain2Université Claude Bernard de Lyon, 43 Bd du 11 Novembre 1918, F-69622 Villeurbanne Cedex, France[Department of Physics, University of Lund, Siilvegatan 14, S-22363 Lund, SwedenULPNHE, Universités Paris VI et VII, Tour 33 (RdC), 4 place Jussieu, F-75230 Paris Cedex 05, France°Department of Physics, University of Liverpool, P.O. Box 147, GB - Liverpool L69 3BX, UK°L[P, Av. Elias Garcia 14 - le, P-1000 Lisbon Codex, PortugalISchool ol` Physics and Materials, University of Lancaster - Lancaster LA1 4YB, UK° Université de Paris-Sud, Lab. de l'Accélérateur Linéaire, Bat 200, F-91405 Orsay, France°High Energy Physics Laboratory, Institute of Nuclear Physics, Ul. Kawiory 26 a, PL-30055 Krakow 30, Poland* Institut Iiir Experimentelle Kernphysik, Universitat Karlsruhe, Postfach 6980, D-7500 Karlsruhe 1, FRG°.Ioint Institute for Nuclear Research, Dubna, Head Post Office, P.O. Box 79, 101 000 Moscow, USSR.Finland

zResearch Institute for High Energy Physics, University ol` Helsinki, Siltavuorenpenger 20 C, SF-00170 Helsinki 17,l Institut des Sciences Nucléaires, Université de Grenoble 1, F~38026 Grenoble, France°Dipartimento di Fisica, Université di Genova and INFN, Via Dodecaneso 33, [-16146 Genova, Italyglnstitute ol` Nuclear Physics, N.C.S.R. Demokritos, P.O. Box 60228, GR-15310 Athens, Greece8Divisi0n des Hautes Energies, CRN — Groupe DELPHI and LEPSI, B.P.20 CRO, F-67037 Strasbourg Cedex, FranceICERN, Cl·I·1211 Geneva 23, Switzerland°College de France, Lab. de Physique Corpusculaire, 11 pl. M. Berthelot, F-75231 Paris Cedex 05, France°Dipartimento di Fisica, Universita di Bologna and INFN, Via Irnerio 46, I-40126 Bologna, Italy* Department of Physics, University ol` Bergen, Allégaten 55, N—5007 Bergen, Norway°Physics Laboratory, University of Athens, Solonos Str. 104, GR·10680 Athens, GreeceMons, Belgiumand Service de Phys. des Part. Elém., Faculté des Sciences, Université de l'Etat Mons, Av. Maistriau 19, B-7000and IIHE, ULB-VUB, Pleinlaan 2, B-1050 Brussels, Belgium2Physics Department, Univ. Instelling Antwerpen, Universiteitsplein 1, B-2610 I/Vilrijk, Belgium*Ames Laboratory and Department of Physics, Iowa State University, Ames IA 50011, USA

R.zuks¤¤v1¤1¤ Funchal°, G.Zumerle3l, J.Z1migs‘P.Za1ewskil°, D.ZavrtanikI, E.Zevg0latakos°, G.Zhang'"’, N.I.Ziminl°, M.zat0°°, R,zn¤u¤*°,D.Wor:nald2°, G.Wormserl°, K.W0scl1.nagg°l , N.Yamdagni", P.YepesI, A.Zaitsev°I, A.Zalewska"',J.Werner4° , A.M.Wetl1e1:ellI, .I.H.Wickens2, J.Wikne2°, G.R.Wi]ldnson°°, W.S.C.Williams°°, M.Wir1ter°,

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(a) momentum p larger than 0.1 GeV/c; OCR OutputCharged particles were used in the analysis if they had:

momentum resolution Ap/ p varies from 0.005 to 0.01p (p in GeV/ c), depending on 0.age for 9 between 25° and 155°, with reconstruction efficiency near to 1. The average

Measurements from the TPC, ID and OD, taken together, provide a complete coverthe liquid radiator was operational during 1990, and for only one tenth of the data taking.uses two different Cherenkov radiators; one liquid (CGFI.,) and one gaseous (CSFN). OnlyIn order to cover a large momentum range (1 to 20 GeV/ c), the DELPHI Barrel RICHextracted by using the velocity information combined with the momentum measurement.sion of Cherenkov light, and thus the velocity. The mass of the charged particle is thenbetween 40° and 140°. It identifies the charged particles by measuring the angle of emisRICH (Ring Imaging Cherenkov) detector. The Barrel RICH [1] covers the polar angleGeV/c has been obtained from the measurement of the 1r* / K * ratio using the Barrel

In addition, a determination of the K * cross section for momenta between 1 and 2

primary interaction region.not fully implemented in this analysis and was used only to find the position of theat radii 9 and 11 cm covering polar angles between 45° and 135". This detector wasThe Microvertex Detector (VD): two cylindrical layers of silicon microstrip detectorscm, covering polar angles between 50° and 130°The Outer Detector (OD): 5 layers of drift cells at a radius between 192 and 208141° up to 16 space points can be used.space points are available for track reconstruction, while for angles between 39° andradius and a length of 2.7 m. For a polar angle 0 between 22° and 158° at least 4The Time Projection Chamber (TPC): a cylinder with 28 cm inner and 122 cm outerradius 22 cm) covering polar angles between 29° and 151°The Inner Detector (ID): a cylindrical drift chamber (inner radius 12 cm and outer

detectors:

The present analysis relies mainly on the information provided by the central trackingstates (with emphasis on the detection of charged particles) are outlined in referenceelsewhere Features of the apparatus relevant for the analysis of multihadronic finalthe LEP e"'e' collider during 1990. A description of the DELPHI detector can be found

The sample of events used in the analysis was collected by the DELPHI detector at

t1on

2 Expemmental procedure and event selec

parameters tuned to data at PEP / PETRA energies7.2 Monte Carlo model (using parton shower generation and string fragmentation) with

The cross section measurements are compared with the predictions of the JETSETThe data were taken around the Z0 peak in 1990.using data collected by the DELPHI detector [1] at the e+e' storage ring LEP at CERN.

The production ofthe strange particles Kg, K"*, A(K), and E"(§+) has been analysedcoupling of the Z0 to si pairs.can be used to examine the underlying dynamics of jet fragmentation, as well as the

OCR OutputThe study of the production of strange particles with different quark flavors and spins

sum was greater than 0.1 GeV/ c. This also removes a small fraction of the Kg signal. OCR Outputby requiring that the transverse momentum of the decay products relative to their vectorseparate secondary vertices. Photon conversions and A decays were effectively removedrandom associations of badly measured primary charged particles and particles fromfrom other decays, secondary interactions or photon conversions, and that coming from

The remaining secondary vertices include two types of background to Kg: that coming

than 1.5 cm.

The separation of the primary and secondary vertex in the xy plane was greatertwo intersections in the xy plane, the solution with smaller distance in z was chosen.The separation in z (beam axis) of the two tracks was less than 0.5 cm. In case ofvertex had to be greater than 0.15 cm and at least one greater than 0.5 cm.The impact parameters in the xy plane of both tracks projected to the primarysecondary vertex was less than 0.04.vector sum of the charged particle momenta and the line joining the primary to theIn the xy plane (perpendicular to the beam axis), the sine of the angle between the

rejected by ensuring the following:minimised. Most candidate vertices are not formed by strange particle decay and weresuch that the X2 obtained from the distance of the vertex to the extrapolated tracks waspairs of oppositely charged particles. The vertex defined by each such pair was determinedZ0 decay. Candidate secondary vertices, VK, were found by considering the tracks of all

Kg —+ 1r+1r“ decays are detected at the secondary vertex separated from the primary

3.1 Kg production

3 Analysis and Results

analysis programmes as the real data.tizations in each detector. These data were processed with the same reconstruction andparticles were followed through the detailed geometry of DELPHI giving simulated digiMonte Carlo programme [2] (J ETSET PS in the following) with default parameters. Thegramme DELSIM Events were generated using the JETSET 7.2 Parton Shower

The influence of the detector on the analysis was studied with the simulation pro0.05 cm when the VD data are not used.cm in the plane perpendicular to the beam axis when using their VD data and aboutthe VD data The typical resolution on the impact parameters of tracks is about 0.015

The mean position of the primary Z0 decay vertex has been measured for each fill usingfrom 1·+·r°` events was calculated to be less than 0.2%.to 77 interactions have been estimated to be less than 0.1% of the sample; background

A total of 86,948 events satisfied these cuts. Events due to beam-gas scattering andpion mass.

In the calculation of the energies, all charged particles have been assumed to have the(7) there were at least 5 charged particles with momenta above 0.2 GeV/ c.(B) the total energy of the charged particles exceeded 15 GeV;

90°) exceeded 3 GeV;(a) the total energy of the charged particles in each hemisphere (9 above and belowHadrouic events were then selected by requiring that:(c) 9 between 25° and 155°(b) measured track length in the TPC above 30 cm;

to represent the background, and changing the bin size and the mass range. OCR Outpute systematic error includes contributions from varying the fit. by choosing: A third-order polynomial or an exponential

is consistent with measurements in e+e’ interactions at lower energiesevents in JETSET PS to the total number of hadronic events from 0.1 to 0.5. This resultJ ETSET PS. The result of the fit was found to be stable when varying the fraction of ssalso assumes that the production of K ° from B and D decays is correctly modelled ina function of 2:,, only, and was not dependent on the origin of the Kg. The measurementhas been obtained. In this analysis it was assumed that the reconstruction efficiency was

(2)0.30 zh 0.02 (stat)ih ·

determined by tuning this parameter in JETSET PS to best fit the data. A valueexciting a si pair from the vacuum to the probability of exciting a ui (or dd) has been

Finally, from the K ° differential cross section, the ratio 7,/7,, of the probability ofin agreement with the OPAL result[8].

< Ngo > + < NRG >= 2.12 :h 0.05(stat) zh 0.04(sys) ,gave

unmeasured regions are represented by the normalised JETSET PS prediction. Thisthe distribution as a function of :1:,, for :1:,, between 0.005 and 0.30, and assuming the

The mean K°(K°) multiplicity, < NKO > + < NRO >, was obtained by integratingFigure 2. The prediction from J ETSET PS reproduces the measurement reasonably well.ficiency and unseen K 0 decay modes. The results are given in Table 1, and plotted insubtraction procedure in each momentum bin and correcting for the reconstruction efclusive K U and KU production at the Z0 peak was measured by repeating the backgroundsection (1/a,,)(da/dx,) (where 0;, is the cross section for e+e' ——> 20 —+ hadrons) for intional momentum ::2,, = 2p/(/E in the region up to x,, = 0.4. The differential cross

The reconstruction efficiency is tabulated in Table 1 as a function of the particle fracconsistent with the world average of 89.2 j; 0.2 ps[7].function Ae2:p(—t/TKQ). This gave ·rK<; = 89.6 :h 1.9 ps (the error is statistical only),reconstructing simulated Kg, and then fitted by least squares to an exponential decayof the proper decay times t of the reconstructed K 2 was corrected by the efficiencies forAs a check of these procedures, the Kg lifetime, TKUS, was measured. The distributionsis. The efficiency for K 2 —+ 7l'+7\’_ reconstruction was evaluated from the simulated data.function of the kinematical variables and was subtracted throughout the following analy

In order to calculate differential cross sections, the background was evaluated as aof momentum and cannot be adequately described by a single Gaussian.world average of 497.67i 0.03 MeV/c2[7]. The resolution of about 8 MeV/cz is a functionchoice of the parametrization in the fit'. The measured K 0 mass is consistent with theon the energy loss of the pions while crossing the material in the detector, and to the0.2 (stat) ;h 2.1 (sys) MeV/cl. The systematic error on the mass is due to the uncertainty0.65 GeV/cz. The fit gives a total of 9411 ;h 94 (stat) :h 34 (sys) Kg, and mKg : 496.6 ifunction and a sum of two Gaussians for the signal in the range from 0.35 GeV/c2 to

The spectrum has been fitted using the MINUIT [6] package to a linear backgrounda fraction of the two decay particles in the 1990 data.secondary vertices because many K g decay beyond the VD and it was efficient for onlyhits in the VD. However this analysis does not use the VD information for reconstructingto about 1% in the part of the sample where the measurement precision is improved usingbackground due to random associations of badly measured primary particles is reducedFigure 1; a clear Kg signal is seen over a small background, below 10% at the peak. The

The 7l'+7l’_ invariant mass spectrum from the accepted VK candidates is shown in

· 2 2 . 2 *1 (m -m K-) +(m¤<·I`¤<·) OCR OutputBWK· md’§"‘BG (m -— m,,4 - mK¤ )°° . e:cp(a.1m + asmz + 4161113)dm

al . BG -1- az . BWK· wheredN

has been fitted between 0.65 and 1.5 GeV/cz with the expressionFigure 4 shows the K§1r* mass distribution; a clear K " signal is seen. The distribution

nominal K ° mass and then combined with all remaining charged particles in the event.mass distribution. These Kg candidates are fitted by constraining the pions to make theK fg are selected to lie within three standard deviation of the peak of the 1r+1r" invariant

The K ** is detected by its decay to Kg1r* with the subsequent decay Kg —» 1r+1r‘

3.3 K ** production

the average (K0 + K 0) multiplicity.(K + + K ") multiplicity in the range of momentum used for the measurement agrees withthe uncertainty on possible difl`e1·ences in K and 1r identification emciencies. The averagepoint includes a relative systematic error of 20% added in quadrature, to account forThe corresponding measurement of the K1: cross section is shown in Figure 2. Thisare clearly visible in the mass plot in Figure 3b for momenta between 1 and 2 GeV/ c.3a. against the particle momentum. The peaks corresponding to pions, kaons and protons

The measured Cherenkov angle distribution from the Barrel RICH is plotted in Figure

3.2 K* production

a function of the fractional momentum 1:,,.Table 1: Efciency for Kg —+ 1r+1r" and diferential cross sections for K 0 production, as

0.305 0.405 3.2d:1.0 0.810.5

4.55:1.8 0.810.60.285 0.305

0.265 3.8:%:1.5 1.511.10.285

0.245 7.8;+:1.7 1.610.50.265

2.210.50.225 0.245 9.2:1:1.6

2.510.58.5:k1.40.205 0.225

2.7:1:0.40.185 0.205 11.0:b1.4

3.41:0.412.8:1:1.30.165 0.185

4.5:1:0.513.51:1.20.1650.145

4.4i0.40.145 16.7:b1.10.125

6.4i0.518.2:1:1.10.105 — 0.125

7.5:1:0.520.8:t1.00.085 0.105

10.1t0.522.7i0.90.065 0.085

13.1:1:0.622.9i0.80.0650.045 —

18.1:h0.818.0:1:0.60.025 0.045

19.5:b1.46.7 i0.30.005 0.025

(iii)mp = Q EHiciency (%)

Figure 5; a clear A(A) signal is seen, with resolution of about 2.5 MeV / cz. The spectrum OCR OutputThe p1r"(§1r"') invariant mass spectrum from the accepted VA candidates is shown in

than 0.02 GeV/ c with respect to their vector sum.the e+e‘ hypothesis and by requiring both particles to have transverse momentum largerto e+e" pairs were excluded by requiring their mass to be greater than 0.1 GeV/cz fortaken as mr, was less than 3 standard deviations from the Kg mass. Photon conversionpairs. The Kg background was reduced by rejecting pvr candidates whose mass, when

Ambiguities occur with K 0 decays into 7'l’+7\’— and with conversions of photons to e+e'

greater than 3 cm.• The radial separation of the primary and secondary vertex in the xy plane was

vertex had to be greater than 0.05 cm and at least one greater than 0.5 cm.• The impact parameters in the xy plane of both tracks projected to the primary

vertex was less than 1°of the charged particle momenta and the line joining the primary to the secondary

• In the xy plane (perpendicular to the beam axis), the angle between the vector sum

candidates were chosen to satisfy the following requirements:The A(A) baryon can be detected by its decay in flight into pir`. The A decay vertex

3.4 A production

level of accuracy is not adequate to distinguish between the various hypotheses.to 3 /4, but a value 2 0.5 seems experimentally favoured at lower energies [10]. The presentcorrectly modelled in JETSET PS. According to simple spin-statistics, R should be equalThe measurement assumes also that the production of K “* from B and D decays is

R = 0.70i0.18. (4)

most of the fragmentation models. The result ishas been measured by a fit using JETSET PS. This probability is a free parameter in

Finally, the relative probability R of producing a prompt strange meson with spin 1

used for the fit.

approximately equal contributions, the choice of the background shape and the intervaland to correct for the unobserved decay modes. The systematic error comes from twoand acceptance, to extrapolate the cross section in the unobserved momentum region,by following the same procedure discussed in section 3.1 for Kg to evaluate efficiency

(3)< NK·+ > + < NK·- >= 1.33 zb 0.11(sta,t) i 0.24 (sys) ,

The average multiplicity for K *1* production has been measured to besome 10 MeV/cz due to the mass resolution.the world average of 891.8 zi: 0.2 MeV/c2[7], while the width includes a contribution offit are mK· = 892 :l; 4MeV/cz and FK· = 43 :l: 12 MeV/cz. The mass is consistent withpion momentum in the K vr rest frame and the 0.; are free parameters. The results of thecentral mass value of the K * and I`K· the width. m is the mass of the K w system, q theWigner function with mass dependent width corresponding to a p-wave. mK· is theBG represents the chosen background parametrisation and BWK· the relativistic Breit

¢11<· mI\’K· i FK' • I

was kept fixed to 0.3. OCR Output(5 = 0.4), tuned on experiments at PEP / PETRA energies. In the fit, the value of 7,/7,,was obtained. This result agrees within errors with the default value in JETSET PS

5 = 0.34 i 0.07 (stat) (6)

the same procedure described for the tuning of 7,/7,,, a resultJETSET PS, to the experimental data on the differential A(A) cross section. By using

The parameter 5 was estimated by fitting the theoretical cross section, predicted bysuppression parameter 5 , defined as: 5 = (P(us)/P(ud))/(P(s)/P(d)).tation model, this probability is related to the value assigned to the strange di-quarkproducing a strange di-quark in the fragmentation process. In the Lund string fragmen

The cross section for inclusive A or A production is sensitive to the probability ofA production by E0 decay is included in this total multiplicity.tance, the choice of the background parametrization, and the JETSET PS extrapolation.The systematic error reflects the uncertainties due to the calculated efficiency and accep

(5)< NA > + < NA >= 0.36 zh 0.03 (stat) :1: 0.06 (sys) .

been determined to be

regions and correcting for unseen A decay modes, the total average A(A) multiplicity hasUsing JETSET PS to extrapolate the cross section to the unobserved momentum

the measured cross section is compared with the prediction from J ETSET PS.the parametrization of the background in the fit of the signal in Figure 5. In Figure 2the statistical and the systematic contributions; the systematic error comes mainly fromTable 2. The errors on the efficiency and on the differential cross section include both

ferential cross section for inclusive A and A production at the ZU is shown ineach :2:,, region and the overall efficiency was calculated as in the K 0 analysis. The difceptance effects, has been calculated by simulation. The background was subtracted for

The momentum dependent efficiency for A(A) reconstruction, including detector aconly), consistent with the world average of 263 :k 2 ps[7].bution to an exponential decay function gives ·rA = 268 :1: 10 ps (the error is statisticalcalculated from the simulation. A least-square fit of the corrected experimental distrimined from the selected sample. The correction factors for each bin of proper time arecontributions as that for the K 3 mass measurement. The A lifetime, ·rA, has been detera total of 1915 zt 69 (stat) :1: 227 (sys) A. The systematic uncertainty includes the same0.9 (sys) MeV/cz, (consistent with the world average of 1115.63 :t 0.05 MeV/c2[7]), andground in the range from 1.1 to 1.18 GeV/c2. The fit gives mA = 1115.0 i 0.5 (stat) ihas been fitted by a sum of two Gaussian functions for the signal and a linear back

the fractional momentum 2:,,.Table 2: Eficiency and diferential cross section for A(A) production, as a function of

0.46:}:0.130.15 - 0.20I 7.7:1:1.3

0.841:0.110.12 - 0.15I 8.6;+:1.01.09:b0.120.09 - 0.12I 10.9:t0.91.60:}:0.150.06 - 0.09I 10.6:}:0.8

2.75t0.270.04 - 0.06I 8.5:i;0.6

3.30:b0.440.01 - 0.04I 3.8:t0.4

(g£)mp = f | Efficiency (%)

K0 [8] spectra. OCR Outputtion [11,12] (MLLA). Such analyses have already been performed at LEP on 1r° [13] anddifferent particles is given by QCD calculations in the Modified Leading Log Approxima

A useful framework for comparing the differential cross sections for the production of

3.6 Comparison of the cross sections for strange particles

production at the Z0 peak is tabulated in Table 3 and plotted in Figure 2.20(sys) E`(E+), the measured differential cross section (1/o·;,)(d0/dan;) for inclusive Econsistent with JETSET PS of approximately 0.025. From a total of 132 i 20 (stat) zi:

< NE- > + < Ny >= 0.020 zi: 0.004 (stat) ;l; 0.003 (sys), (7)

multiplicity has been determined to be:modes of the A, and the detector eifects estimated from the simulation. The average E

The overall efficiency for E'(E+) reconstruction includes corrections for unseen decays4.2 i 0.6 MeV/cz representing the mass resolution.(consistent with the world average of 1321.3 zi; 0.1 MeV/c2[7]) and a Gaussian width ofsignal with a linear background. The {it gives mg; = 1319.0:b0.7 (sta.t)d: 1.2 (sys) MeV/c2,responds to E production. The spectrum has been fitted to a Gaussian function for thefigure 6b the wrong-sign (A1r+,An·") combinations. The narrow peak in Figure 6a corFigure 6a shows the masses of the resulting right-sign Avr combinations (A1r‘, A1r+) and

respect to their sum.• the transverse momentum of the A and vr must be larger than 0.05 GeV/c with

ucts) to the production vertex must be less than 6mm in the xy plane.• The impact parameter of the E‘ trajectory (determined by summing its decay prodplane must exceed 30 mm.

• The separation between the 20 decay and the candidate E` decay vertex in the xyjections in the xy plane must be less than 8 mm.

• The distance in the z direction between the tracks at the intersection of their pro

decay vertex candidate must satisfy the following requirements:line representing the A flight path and the circle representing the 1r trajectory. This E'make a E' candidate. The intersection in the xy plane was found between the straightnominal A mass. Each A candidate was paired with every negatively charged particle tofor the further analysis, and all the combinations from this region were assigned the

Only the A candidates with invariant mass between 1105 and 1125 MeV / cl were kept

and 105 MeV / c with respect to their vector sum.• The transverse momentum of the A decay product candidates must be between 40

must exceed 40 mm.• The separation between the ZU decay and the candidate VA vertex in the xy planethan 7mm.

plane must be less than 7mm, and at this point, their z separation must also be less• The distance of closest approach between the proton and pion trajectories in the xy

requiring:

In order to enrich the A sample with E decay products, the A —» pvr decay is reconstructedThe E" (or E+) baryon can be detected by its cascade decay chain E -—> Avr, A —> p1r.

the sensitivity in the low·:1.·, region. OCR OutputThe bin sizes chosen for the differential A cross section are different from those tabulated in Table 2, in order to increase

where statistical and systematic errors have been summed in quadrature.

0.020 t 0.005<N_;->+<N;_+>

0.36 i 0.07<NA>+<N;\>

1.33 zh 0.26<NK.+>+<NK.->

2.12 zl; 0.07<NK¤>+<N,;¤>

The following average multiplicities for strange particles have been measured:90,000 hadronic Z0 decays collected by the DELPHI detector at LEP.

The inclusive production of strange particles has been analysed from a sample of :1

4 Summary and Conclusions

0.35 :l: 0.10 GeV.Q0(A)

0.43 zb 0.04 GeVQ¤(K°)

0.15 1 0.02 GeVQ0(aZl charged)

culations in reference [12] toThe positions of the peaks, assuming ACH = 0.15 GeV, correspond through the cal

between the maxima of kaons and lambdas.for all charged particles; no difference is seen within the accuracy of the measurement

The position of the maximum is observed to be lower for kaons and lambdas thanwhere the errors allow for the choice of different ranges for the fit]

{R = 2.82;b0.25

{Qu = 2.62 zh 0.11

Edllchargcd 2

of the maximum, to a distorted Gaussian [14], the following results are obtained:decrease as the mass of the produced particle increases. When fitting %, for fp within 1particles produced. The position f" of the maxima of the distribution is expected toa cut0H` of the parton cascade QD which is expected to grow with growing masses of the

The results of the MLLA calculation depend on an effective QCD scale ACH and onplotted as a function of the variable fp = ln 1/xp.

In Figure 7, the production cross sections for A, K 0 and all charged particles are

ofthe fractional momentum mp.Table 3: Efcicncy and dijcrential cross sections for E`(E+) production, as a function

0.30 — 0.50] 3.7;]:2.1]0.009 i0.0070.20 - 0.30] 5.5:j:2.4]0.035 i0.01Q·0.10 - 0.20] 10.5:]:2.1]0.0400.00 · 0.10] 6.6d:1.0I0.110;;f;0.025

mp : is lEfHcicncy (%)|

members of the CERN -SL Division for the excellent performance of the LEP collider. OCR Outputagencies for their support in building and operating the DELPHI detector, and to thediscussions. We are greatly indebted to our technical collaborators and to the funding

We want to thank V. Khoze, T. Sjiistrand and especially Yuri Dokshitzer for clarifying

Acknowledgements

found to be consistent with measurements in e+e‘ interactions at lower energies.ability of exciting a u`1I (or dd), and the strange di-quark suppression factor 5, have been

The ratio 7, /7,, of the probability of exciting a s`5 pair from the vacuum to the proband differential cross sections.

tuned from the data at PEP / PETRA energies, are consistent with these results on totalThe predictions of the JETSET 7.2 version of the Lund Monte Carlo program, as

[14] C.P. Fong and B.R. Webber, Phys. Lett. B229 (1989) 289. OCR Output[13] B.Adeva et al. (L3 Collaboration), Phys. Lett. B259 (1991) 119.[12] Yu.L.Dokshitzer et al. LU TP 91-12 (1991), to appear in Int. J. Mod. Phys. A.

Frontieres, Paris, 1991.Yu.L.Dokshitzer et al., in "Basics of Perturbative QCD”, ed.A.Muel1er, Editions

[11] Ya.I. Azimov et al., Z. Phys. C27 (1985) 213 and C31 (1986) 213.[10} P.V. Chliapnikov and V.A. Uvarov, Phys. Lett. B240 (1990) 519.

A. Wroblewski, Acta Physica Polonica B16 (1985) 379.[9] P.K. Malhotra and R. Orava, Z. Phys. C17 (1983) 85.{8] G.Alexander et al. (OPAL Collaboration), Phys. Lett. B264 (1991) 219.[7] Review of Particle Properties, Particle Data Group, Phys. Lett. B239 (1990).

MINUIT - Function Minimization and Error Analysis.F .J ames and M.Roos, CERN Program Library D506.[6]DELSIM Reference Manual, DELPHI 87-98 PROG-100, CERN, July 1989.DELSIM User Manual, DELPHI 87-96 PROG-99, CERN, July 1989.[5}Zeit. Phys. C.P. Abreu et al. (DELPHI Collaboration), CERN-PPE/91-131, to be published in[4}

[3] P. Aarnio et al. (DELPHI Collaboration), Phys. Lett. B240 (1990) 271.T. Sjiistrand and M. Bengtsson, Comp. Phys. Comm. 43 (1987) 367.T. Sjostrand, Comp. Phys. Comm. 27 (1982) 243, ibid. 28 (1983) 229.T. Sjiistrand et al., "The Lund Monte Carlo Programs”, CERN, 1 November 1989.[2}233.P. Aarnio et al. (DELPHI Collaboration), Nucl. Instr. and Meth. A303 (1991)[1]

References

10

function (see text). OCR Outputof fp. The curves correspond to fits near the maxima using a distorted GaussianComparison of the cross sections for K 0,A and all charged particles as a functioncombinations. The curve on figure 6a is the result of the fit.A1r invariant mass for secondary decay candidates with (a) right and (b) wrong sign

. par invariant mass spectrum for secondary decay candidates.the dashed curve the fitted background.Kgvri invariant mass spectrum. The dotted curve shows the fitted K * signal and1 and 2 GeV/ c.(b) Distribution of the calculated mass for charged particles with momenta betweenparticle (GeV/c).(a) Measured Cherenkov angle in Barrel RICH versus the momentum of the incidentfractional momentum xp. Curves are for JETSET PS with default parameters.circles, (K0 + KU) - triangles, (A + A) - squares, (E" + El`) - diamonds, versus theDifferential cross section (1/0·;,)(da·/d:z:,,) for production of: all charged particles·rr+1r" invariant mass spectrum for secondary decay candidates.

Figure captions

11

Momentum fraction xp OCR Output

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

10

10

10

JETSET 7.2 PS

0 E

mzl 0 I] /\

K° (A K*' from BRICI-I)O All charged particles

OCR OutputFigurc2