Inclusive J, ψ′ and χ_c production in hadronic Z decays

Article

Reference

Inclusive J, ψ′ and χ

production in hadronic Z decays

L3 Collaboration

AMBROSI, Giovanni (Collab.), et al.

Abstract

We report on measurements of the inclusive production of J, Ψ′ and χc mesons in hadronic Z decays, based on 3.2 million hadronic events collected using the L3 detector at LEP. The J and Ψ′ mesons are reconstructed through their decays into lepton pairs, while the χc mesons are reconstructed via the decay mode χc → J + γ. The measured branching fractions are: Br(Z

→ J + X) = (3.40 ± 0.23 (stat.) ± 0.27 (sys.)) × 10⁻³, Br(Z → Ψ′ + X) = (1.6 ± 0.5 (stat.) ± 0.3 (sys.)) × 10⁻³, Br(Z → χc1 + X) = (2.7 ± 0.6 (stat.) ± 0.5 (sys.)) × 10⁻³. In the absence of a clear χc2 signal, the upper limit at 90% C.L. is set: Br(Z → χc2 + X) < 3.2 × 10⁻³.

L3 Collaboration, AMBROSI, Giovanni (Collab.), et al . Inclusive J, ψ′ and χ

production in hadronic Z decays. Physics Letters. B , 1997, vol. 407, p. 351-360

DOI : 10.1016/S0370-2693(97)00790-9

Available at:

http://archive-ouverte.unige.ch/unige:113179

Disclaimer: layout of this document may differ from the published version.

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4 September 1997

Physics Letters B 407 (1997) 35 l-360

PHYSICS LETTERS B

Inclusive J, @’ and xc production in hadronic Z decays

L3 Collaboration

M. Acciarriab, 0. Adrianiq, M. Aguilar-Benitez=, S. Ahlen k, J. Alcaraz=, G. Alemanni w, J. Allaby ‘, A. Aloisioad, G. Alverson[, M.G. Alviggi ad, G. Ambrosi t, H. Anderhub”y,

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0370-2693/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved.

PII SO370-2693(97)00790-9

352 i.3 ~oll~ration /Physics Letters B 407 (1997) 351-360

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W ~oi~~oration f Physics Letters B 407 /1997f 35f-360 353

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z 1. Phys~~f~sches ins&t, RWTH. D-52056 Aachen, FRG ’ fir. ~~~s~~~is&~s lnstitut, RWTf& D-52056 AacRen, FRC t

b ~~~i~~~ fnsr&& for E&h Energy ~~~~~~~~ NLKHEE and Universiry of~terd~ N~-~~P DB ~~~fe~~~~~ The Nether~n~

c Unjv~rsj~ of Micbig~, Ann Arbor; ML $8109, USA

Laboratoire d’Annecy-le-Keux de Physique des Parttcules, ~AP~~N2P3-~NRS, BP 110, F-74941 Annecy-le-Vieux CEDEX, France e Johns Hopkins University, Baltimore, MD 21218, USA

f Institute of Physics, University of Basel, CH-4056 Basel, Switzerland z Institute of High Energy Physics, IHEP 100039 Beijing, China 6

h Humboldt University, D-10099 Berlin, FRG '

i University of Bologna and INFN-Sezione di Bologna, I-40126 Bologna, Iialy j Tata Institute of Fundamental Research, Bombay 400 005, India

ii Boston University, Boston, MA 022i5, USA

’ h~ortheastern Universitx Bostara, MA 02135, USA

m [email protected] 0~’ Atomic Physics and Universes of Bucharest, R-769&3 Bucharest, Ro?~~~a

R Central Research Institute for Physics of the ~nn~ar~~a Academy of Sciences, H-l 525 Budapest 114, Hungary’

0 Harvard University, Cambridge, MA 02139, USA v Massachusetts Institute of Technology, Cambridge, MA 02139, USA q INFN Sezione di Firenze and University of Florence, I-50125 Florence, Italy r European Laboratory for Particle Physics, CERN, CH-1211 Geneva 23, Switzerland

s World Laboratory FBLJA Project, CH-1211 Geneva 23, Switzerland

’ University of Geneva, CH-1211 Geneva 4, Switzerland

u Chinese University of Science and Techalogy, USTC, Hefei, Anhui 230 029, China 6 v SEFT, Research Institute for High Energy Physics, PO. Box 9, SF-00014 Helsinki, Finland

w Um’versi~ of tausanne, CH-1015 ~nsanne, Switzerland

x INFN-Sez~one di Lecce and Universitd Degh Studi di Lecce, f-73100 Lecce, Italy r Los Alamos Nature Laboratory, Los Alamos, NM 87544, USA

’ In&at de Physique NuclJaire de Lyon, ~N2P3-~NRS,Um’vers~t~ Claude Bernard, F-69622 Vilieurbanne, France aa Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas, CIEMAI; E-28040 Madrid, Spain 3

ab INFN-Sezione di Milann, l-20133 Milan, ltaty

ac Institute of Theoretical and Experbnsntal Physics, ITEP Moscow, Russia ad INFN-Sezione di Napoli and University of Naples, I-80125 Naples, Italy ae Department of Natural Sciences, Unr’versity of Cyprus, Nicosia, Cyprus aF Universily of Ntjmegen and NlKiA?X NL-6525 ED Ntjmegen, The Netherlands

~ Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA sh ~li~orn~a Ins&m of Technology. P~ade~, CA 9il25, USA

ai INFN-Sezione di Perugia and Unniversitd Degfi Studi di Peru& i-06100 Peru& Italy 4 Carnegie MeRon University, Pit~~~~ PA 15213, USA

ak Princeton Uaivers~~, Princeton, NJ O&%4, USA

a& TNFN-Sezione di &ma and University of Rome, “La Sapienza ‘I, l-00185 Rome, Italy am Nuclear Physics It&ate, St. Petersburg, Russia

an University and INFN, Salerno, l-84100 Salerno, Italy aa University of California, San Diego, CA 92093, USA

ap Dept. de Fisicu de Particutas Elementales, Univ. de Santiago, E-15706 Santiago de Compostela, Spain q Bulgarian Academy of Sciences, Central Lab. of Mechatronics and Instrumentation, BU-1 I13 Sofa. Bulgaria

ar Center for High Energy Physics, Korea Adv. Inst. of Sciences and Technology. 305-701 Taejon. South Korea as University of Alabama, Tasca~oosa~ AL 35486, USA

St Utrecht Universes and NIRHEE NL -3584 CB Utrecht, The Nether~a~s

” Purdue University, West Laj&pffes IN 47907p USA av Pa& Scherrer ~~titu%. PSI, CH-5232 ~~l~g~n~ Sw~~er~nd aw DESY-insfifut or Hoche~erg~epbysi~ D-15738 Zeuihen, FRG

aY Eidgeniissische Tech&the Hochschule, ETH Ziirich. CH-8093 arich, Switzerland

354 13 Collaboration/Physics Letters B 407 (1997) 351-360 a’ University of Hamburg, D-22761 Hamburg, FRG

ba High Energy Physics Group, Taiwan, ROC Received 25 April 1997

Editor: K. Winter

Abstract

We report on measurements of the inclusive production of J, @’ and xc mesons in hadronic Z decays, based on 3.2 million hadmn~c events collected using the L3 detector at LER The J and $’ mesons are reconstmcted through their decays into lepton pairs, while the xc mesons are reconstructed via the decay mode xc --+ J + y. The measured branching fractions are:

Br(Z + J f X) = (3.40f 0.23 (stat.) i 0.27 (sys.)) x lo-“, Br(Z -+~‘+X)=(1.6rt:0.5(stat.)~0.3(sys.))x10~7,

Br( Z -+ xc1 + X) = (2.7 & 0.6 (stat.) k 0.5 (sys.) ) x lo-‘. In the absence of a clear xc2 signal, the upper limit at 90%

CL. is set: Br(Z -+ xc2 f X) < 3.2 x lo-“. @ 1997 Elsevier Science B.V.

1. Introduction

The production of J, cc/’ and xc charmonium states in Z decays is predicted to proceed mainly via the decay of b hadrons. Only a minor fraction is expected to be prompt, deriving from the fragmentation of a gluon or a c quark in Z +qq decays. The calculation of Br(b -3 J + X) suffers from large QCD uncertainties; the results range from 0.2% to 2% [l--3]. Estimates for the rate of prompt J pr~uction Br( Z --+ J + X), vary from 1 x low4 to 4 x 10v4 [4-71.

The interest on quarkonium production has been re- cently renewed by measurements at the TEVATRON collider [ 81, which give prompt qu~konium cross sec- tions much higher than predicted by first order color singlet model [ 91 calculations. It has been shown [ lo]

that the color octet mode1 [ 11,121 is able to reproduce both the rate and the observed pt spectrum of cE bound states. At LEP present measurements of prompt J pro- duction are consistent with both the color octet and the color singlet predictions [ 13- 151. Features distinctive of the color octet model could show up in the decays

’ Supported by the Getman Bundesministerium fur Bildung, Wis- senschaft, Forschung und Technoiogie.

2 Supported by the Hungarian OTKA fund under contract num- bers T14459 and T2401 i.

z Supported also by the ComisiiSn Interministerial de Ciencia y Technolog~a.

4 Also supported by CONICET and Unive~idad National de Ia Plats, CC 67, 1950 La Plata, Argentina.

5 Also supported by Panjab University, Chandigarh-160014, India.

f~ Supported by the National Natural Science Foundation of China.

of b hadrons into xc states, where both color-singlet and color-octet matrix elements contribute at first or- der [ 11 I. The production of xc1 states could be sig- nificantly enhanced, as would be revealed by the value of: Ric, = Br(b -+ ,yct i- X)/Br(b -+ J + X), where most of QCD unce~ainties cancel out, which in the color singlet model is computed to be R:<, N 0.2’ . The production of ,ycn and xc2 states, which is strongly suppressed in the color singlet model framework [ 11, would proceed at first order through the color-octet matrix elements. Since the xc2 has a non-negligible branching fraction into J y, its production could result in a visibIe signal close to the xc1 peak. Moreover, the obse~ation of a large xc2 signal would validate another model of charmonium production, the color evaporation model [ 161.

We have previously measured the inclusive J and ,yc production in Z decays [ 13,171. The present analysis is performed on the data collected by the L3 exper- iment [ 181 from 1991 through 1994, corresponding to 3.2 million hadronie Z decays, with a tripled statis- tics compared to the previous measurements. An im- proved resolution for both the J and the xc signal is obtained by adopting tighter lepton selection criteria.

A direct measurement of the xc to J production ratio is perfo~ed and the contribution of xc2 states to the signal is investigated.

7 as can be derived from [ 11 after accounting for the feed-down from xc and I@ decays into J.

2. The branching fmctiom Br(Z + J + X) and Br(Z -+ +’ + X)

Hadronic Z decays are selected requiring high mul- tiplicity and high and wet1 balanced visible energy [19]. The selection efficiency is (98.9 f O.l)%, as determined using Z --+qq Monte Carlo events [ 201.

f and $’ candidates are reconstructed via their de- cays into muon or electron pairs. Muons are identi- fied and measured in the muon chamber system. Muon tracks are required to consist of track segments in at least two of the three layers of the muon chambers, and co point to the interaction region. Electrons are identi- fied as electromagnetic clusters in the electromagnetic calorimeter associated to a track in the central tracking chamber. Both the barrel and the end cap regions of the calorimeer are used. Tight r~uirements are made on the shape of the electromagnetic shower and on the quality of the angular and momentum matching with the track. The energy of the electrons is measured us- ing the electromagnetic calorimeter.

In the laboratory system, the J --f !+e- decay typi- cally results in one high and one iow momentum lep- ton. We therefore select muon (electron) candidates with a momentum in excess of 2.5 (1.5) GeV for the feast energetic one and larger than 4 GeV for the most energetic one. In order to reduce the combina- torial background, we require the opening angle be- tween two oppositely charged lepton candidates to be smaller than 80”.

Further details on the adopted selection criteria can be found in [ 21,221. All data distributions are in good agreement with the Monte Carlo simulation of 6 mil- lion Z + q?j decays. In Fig. 1 the lepton spectra in the J invariant mass region are shown.

The invariant mass spectra of the selected ~+,,6 and e+e- pairs are shown in Fig. 2. We perform an un- binned maximum-likelihood fit to the two distributions in the mass region 2.0 ~rl Mt*e- < 4.5 GeV. In this fit the J -+ e+L’- and I,# + e+e- decays are described by two Gaussian functions and the background is mod- elled using a fourth order polynomial. The two Gaus- sian functions are constrained to have the same width and a shift of 589 MeV between rhe central values, co~~~nding to the known difference between the $;’

and J masses [ 23 1. From the fit we obtain 241 i 25 J 4 ,uc+pu- and 200 i 18 J -+ efe- decays, together with 27f 10 $I’ -+ p”&- and 12f7 $J’ + e’e- can-

0 Data b)

Fig. 1. Spectrum of (a) the most energetic lepton and (b) the least energetic lepton for tit?- pairs in the mass region 2.8 < Met- < 3.4 OeV. All selection cuts are applied but the ones on the lepton momenta, The awws indicate the position of the cuts.

didates. The resulting mass values and resolutions for theJsignalare: A$;+‘-= (3118112) MeV,gti+,- = (123 f 12) MeV and Mfe- = (3093 & 8) MeV, rr,+,- = (85 f 9) MeV, in agreement with the expec- tations from the Monte Carlo simulation.

The signal is simuiated by I6OCKl Monte Carlo 2 --+

b6 events [ 201, imposing the decay chain b --+ J + X followed by J ---f @e- for one of the b hadrons. The Peterson fragmentation function for b quarks is used with (xb) = 0,702 in accordance with the LEP mea- surements [24 1. The generated events are reweighted according to the J momentum in the B reference sys- tem, in order to repraluce measurements performed at the Y (4s) [ 251 I From the Monte Carlo simula- tion we estimate the selection efficiencies B;‘~-. = 0.259 f 0.005 and .+zFS”- = 0,212 f 0.005, where the quoted errors are statistical only.

The summary of the relative systematic errors on Br(Z -+ J +X> is given in Table 1. ~nc~~ties due to the adopted lepton iden~~ca~ion criteria are evaluated by varying the lepton identification pa- rameters. The cut on the least energetic lepton mo-

356 W Collaboration/Physics Letters B 407 ~19971351-360

66

0 d

Dilepton Invariant Mass {GisV;

b) We%

Fig. 2. (a) Invariant mass spectrum of pLfpCL- and (b) e+e- selected pairs. The line represents the result of the unbinned maximum-likelihood fit described in the text.

mentum is varied in the (Z-4) GeVrange for muons and (l-3) GeVrange for electrons; the cut on the most energetic lepton momentum is varied in the (Z- 6) GeVrange. We also vary the angular cut between the two leptons, in the 60”-100” range. The total vari- ation on Br(Z -+ J + X) observed by varying kine- matical cuts is 3.9%, to be compared to the 2% un- certainty estimated by varying < xb > between 0.711 and 0.695 in the Monte Carlo simulation of the signal.

Detector inefficiencies are evaluated using the online database information, and checked with an alternative method, based on the analysis of Z --+ =!+!- decays.

Relative to a fully efficient detector, acceptance fac- tors of 0.8 1 f 0.06 for J --+ e+e- and 0.85 f 0.05 for J --+ pL+pu- decays are estimated, where the quoted errors are conservatively derived from the differ- ence of the two evaluations. The fitting procedure is checked by counting the number of events inside the invariant mass window Mr f 3~~+~-- and subtracting

the background found in simulated Z + qq decays.

Table 1

Relative systematic errors on the measurement of Br(Z --+J+ X).

The connations from J -+ c+e-, J -P p+,u- and the combined channel are detailed.

ABr(Z + J + X)/Br(Z -+ J +X) 3 -+ e+e- J -* p+,~- J --+ !+!?- Lepton identification 3.5 8 2.2 8 2.2 %

Kinematical cuts 3.7 % 4.1% 3.9%

Detector inefficiencies 7.4% 6.2 % 5.0%

MC. statistics 1.8% 1.8% 1.3%

Fitting procedure 3.5 % 3.9% 2.6 %

Br(J -+ e+e-) _{3.2 % 3.2 8 3.2 %}

Total IO.1 % 9.4% 7.9 %

Using Br(J -+ !?+e-) = 0.0601 f 0.0019 [23], we measure:

Br(Z + J(e+e-) +X)

= (3.42 & 0.31 III 0.35) x 10-3;

Br(Z + J(p+p-) + X)

= (3.38 f 0.35 f 0.32) x 10-3.

Combining the two m~urements and taking into ac- count common systematic errors, we obtain the fol- lowing branching fraction:

Br(Z --t J + X) = (3.40 i 0.23 f 0.27) x lo-“.

From the number of reconstructed (jl’ --f P’C- de- cays we measure:

Br(Z -+ rc/’ +X) = (1.6 f 0.5 f 0.3) x 10m3, where the major systematic uncertainty arises from the evaluation of the b~kground under the II/’ peak and from the knowledge of Br(@’ + lft-) [ 231.

The branching fractions Br(b -+ J f X) and Br(b -+ @’ + X) can be evaluated, taking into ac- count the fraction of J and $’ not produced in b decays.

The estimates of prompt J production yield values of Br(Z + J f X), from 1 x 10e4 to 4 x 10d4, depend- ing on the model adopted and with at least a factor two uncert~nty in the calculations [ 4-77. Present exper- imental results on prompt J production [ I3- 151 are consistent with these predictions. We therefore use:

Br(Z -+ J + X), = (21t2) x IOe4. For prompt 9’ pro- duction we use Br(Z --+ $’ + X), = (I f 1) x lop4

[ 71. Assuming the Standard Model value &, = 0.2156 1261, we derive:

Br( b --+ J + X) = ( 1,06 f 0.08 f 0.11) x lo-*, Br(b J E$’ + X) = (O-46 j, 0.16 f 0.08) x 1W2,

3. The branching fraction Br(Z --+ xet + X) Inclusive xc1 meson production in Z decays is mea- sured via the xc1 ---f J+ y decay. In order to increase the statistics in the J --+ e+e- channel, some of the electromagnetic identification criteria are relaxed. A total number of 440 f 32 (background subtracted) J + C+e- events are selected in the mass window be- tween 2.8 and 3.4 GeV.

Photon candidates are identified as isolated clusters in the electromagnetic calorimeter having a shape con- sistent with an electromagnetic shower, and with no charged track pointing to the cluster within four times the expected angular resolution in the azimuthal plane, In order to reduce the combinatorial background, only photons inside a 40” cone around the reconstructed J direction and with energy larger than 1.1 GeV are se- lected. Further details on the selection variables can be found in [22].

The xc1 candidates are selected in the M(e”&-y) - M(lf<-) = (4t4 f 56) MeV mass-difference win- dow, which is centred around the M(,yct ) - M(J) expected value [23], and about three times wider than the average Monte Carlo resolution, UMC = 18.7 MeV, Fig. 3 shows the energy spectrum of photons selected in the signal mass region and Fig. 4 the momentum of the selected xc1 candidates. In Fig. 5 the measured

~(~+~-~) - ~(~+~-) spectrum is shown. A total of 64 events is selected in the chosen signal region.

The background can be divided in two components:

events where a fake J is selected and events where a true J is combined with an uncorrelated photon.

The background due to fake J is studied using data taken from the sidebands of the J peak and from e&pi pairs in the J mass region. The background due to random 3 - y combinations is studied &rough Z -+ J + X Monte Carlo events, excluding xc produc- tion. The obtained distributions are shown in Fig. 6.

Their shapes are similar, and no residual structure is seen in J - y combinations. The total expected back- ground shape is obtained by summing the two con-

Ey (GW

Fig. 3. Spectrum of photois in the mass region JW++e-T9 - M(&+P) = {414f 56) MeV, after all sekction cxxts 8re applied but the one on the photon energy. The arrow indicates the position of the cut.

> ⁰ Data Q) 15

0 0 wt+qq

u! MCX~+JY

c# t * I

3

ai

‘ii

&

a 5 E

I

Oo 10 20 30 40 50

PU+ I’ r) (GM

Rg. 4, Mamcntum of @e-y combinations in the we+e-7) - M(e+e-) = (414f 56) rev mass =gion.

tributions, weighted according to the signal to back- ground ratio found under the J mass peak. A function oftheform: fno(~) = A.exp(ax+b/x2), wherex = M( W-y) - M( !+e- ) , is used to describe the back- ground shape. The a and b parameters are determined through a maximum-likelihood fit. The normalisation

358 W Collubtwaiian /Physics L&ten B 407 llY97) 351-360

M(I+ I- r) - M(I+ I-) (GeV)

fig. 5. Measund M(PCPy) - M(e+C) spectrum. llre firrews

delimit tbe signal region. In the insert the background subtracted distribution is shown.

01 0.5 1

M(I* I’ y) - M(I+ I’) (GeV)

Fig. 6. M(@+e-r) - M(E+C- f background. The points are data taken from the sideband of the J peak and from e*$ pairs in the J mass region (fake J background) associated with inclusive photon candidates. The histogram is the prediction from the Monte Carlo of inclusive J production without xc states (background due to random J - y combinations). The curve is the background shape fitted frum the sum of the two types of background. The no~~sation is to the expected fake f background.

T&le 2

Relative Systematic errors on the measurement of Rzc, and Br(Z --+ xc1 f XI.

M.C. statistics y setection J purity ACJ / EJ Bkg. subtraction xc counting hBr/BrQcz -+I+?+

~Br/Br++e+ e- ) Total

ARSct NC, AWBr(z+x,, +x)

2% 5%

10% 10%

4%

7%

7% 7%

8% 8%

6% 6%

3%

17% 18%

fac&x A is derived from the data events counted in the side bands of the xc mass window. After background subtraction, the number of observed xc1 candidates is N( xc1 ) = 32.8 f 8.0.

The xc1 signal is simulated using Z -+ bb decays [ZOJ _ The acceptance for finding the xct, once the 3 has been selected, is 0.342 i 0.007. The total ef- ficiency, including the J, selection, is found to be 0.08 1 f 0.004, where the errors are statistical.

The evaluation of systematic uncertainties on both the Br(Z --+ xc1 +X) and RzxC$ = Br(Z -+ xc1 +X)/

Br(Z -+ J + X) m~uremen~ is summa&d in Table 2. We vary the cut on the energy of the photon between 0.5 and 1.5 GeV. The acceptance angle between the photon and the J direction is varied between 20” and 60“. Varying also the shower shape and isolation re- quirements, we estimate the total systematic error in- duced by the photon selection to be 10%. The system- atic errors induced by the J selection efficiency cancel out in the R:C, ratio. This ratio is however affected by the purity of J selected in the dilepton sample.

The d~te~nation of the a, b parameters and of the normalisation of the function fao gives a statistical error on the background under the xc1 signal region of 7.4%. We assume that all the selected candidates are from xc1 decays. We do not expect any cont~nation due to xd mesons, since their etching fraction into J y is negligible. The contamination due to xc2 -+ J y decays has, however, to be investigated. A xc2 sig- nal would be located at M( !?PY) - M( &+l-) = 459.29 MeV. Shrinking the width of the acceptance window around the expected xcr signal position by flMo, we obtain a 7% increase of the measured branch-

L3 Collaboration/Physics Letrers B 407 ~1997~351-360 359

0 0.2 0.4 0.6 0.8 1 !

M(l*

M(I+

Fig. 7. Maximum likelihood fit to the M(t?l-y) - M(l+.!-) mass diffet%erence spectrum. In the insert the background-subtracted dist~bution of the signal region is shown.

ing fraction; enlarging the acceptance window by PMC results in a 3% decrease. As a further check, an alter- native counting procedure is adopted. We perform an unbinn~ maximum-likelihood~t to the ~(~+~-~) - M( e+l-) distribution. The same function ~BG (x) is used to describe the background, but a and b are left as free parameters in the fit. Two Gaussian shapes are used for the xc1 and the xc2 signal, constr~n~ to have the same width and a shift between central values con- sistent with the known M,,, - M,, mass difference of 45.6 MeV [ 231. The result of this fit is shown in Fig. 7. We obtain a mass difference M(xcl) - M(J) of 413.2:$ MeV, consistent with the expected value of 413.65 i 0.13 MeV [23]. The fitted resolution is lO.8:;,2 MeV. The numbers of xc1 and xc2 candi- dates found by the fit are fig, = 30.1~~~ and flkz = 8.8?54:46. NE, is in good agreement with the N(xcl) value obtained previously in this paper. Including the variation of Br(Z --) xc1 -t- X) observed by changing the mass-difference acceptance window, we assign to the counting procedure a relative systematic error of fS% .

Using 13r(Xcl 4 J y) = 0.273 i 0.016 [23] we derive:

Br(Z ---t xci +X) = (2.7iO.6iO.5) x 10p3,

RZxcc, =

Using the results from the two-~aussi~ fit, the 90%

confidence level of the likelih~function corresponds to an upper limit of 16 observed xc2 events. Using Br( xc2 --+ J y) = 0.135 + 0.011 [ 233 and accounting for systematic errors, we set an upper limit:

Br(Z -+ xc2 I- X) < 3.2 x low3 at the 90% confidence level.

The xc1 production rate from fragm~utation processes is expected to be of the same order of magnitude as the ‘5. The color singlet predic- tion is Br(Z -+ xc1 +X), 21 0.5 x 10m4 [4]

whereas a higher value, around 1.4 x 10e4, is ex- pected in the color octet framework [7]. Assuming Br(Z --+ xc1 + X), = (1.4 i 1.4) x lop4 we derive:

Br(b -+ xc1 + X) = (0.84 f 0.20 f 0.16) x lo-*.

Even ~suming prompt xc1 production but no prompt J production, our R:c, measurement corre- sponds to: R!cl = 0.75 f 0.19 f 0.15, which is signifi- cantly higher than the color singlet model expectation Ric, cz’ 0.2.

4. Summary

With a sample of 3.2 million hadronic Z decays we obtain:

Br(Z--+J+X)

= (3.40f0.23 (stat.) f0.27 (sys.)) x 10w3, Br(Z --+ @’ + X)

= ( 1.6 f. 0.5 (stat.) i 0.3 (sys.)) x 10e3, Br(Z -+ xc1 + X)

= (2.7 & 0.6 (stat.) f 0.5 (sys.)) x lo-‘.

These results improve and supersede our previous m~urements [ 13,171. The results on J and J/’ are consistent with the other LEP measurements [ 27,141.

The result on xc1 is the most accurate today, We obtain a 90% C.L. upper limit:

BrfZ --+ xc2 f X) < 3.2 x lo-“.

360 L3 Cuiiabora~~~ /Physics L+etters B 407 (1997) 3Sf-360

If assumptions on the contribution to charmo- nium production due to fragmentation processes are made, the measured inclusive Z decay rates can be related to b decays. While the inferred b -+ J (+‘, xcl) + X rates are consistent with mea- surements performed at the Y (4s) [ 25,28 J, the Br(b 4 xc1 + X)/Br(b -+ J + X) ratio is higher than expected from first-order color singlet model calculations, and suggests the existence of additional mechanisms of quarkonia production,

We wish to express our gratitude to the CERN ac- celerator divisions for the excellent performance of the LEP machine. We acknowledge with appreciation the effort of all engineers, technicians and support staff who have participated in the construction and mainte- nance of this experiment.

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