In this article the current status of the partial width R/sub c//sup 0/ of the Z/sup 0/ to cc and of the multiplicity n/sub c/ of charm quarks per b decay are discussed. Final results presented by the LEP experiments using their full LEP 1 data sets lead to a more precise determination of both quantities. The new LEP and SLC averages are R /sub c//sup 0/=0.1674+or-0.0038 and n/sub c/=1.149+or-0.036. (18 refs)

Elsing, M

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DELPHI Collaboration DELPHI 99-160 Talk 514 October 1999Charm Counting in b and c Events at LEPM. ElsingCERNTalk given at the EPS-HEP ’99 - International Conference on HighEnergy Physics, Tampere, Finland, 15-21 July 1999.1Charm Counting in b and c Events at LEPM.Elsing 11 CERN, CH-1211 Geneva 23, SwitzerlandE-mails: markus.elsing@cern.chAbstractIn this article the current status of the partial width R0c of the Z0→ cc¯ andof the multiplicity nc of charm quarks per b decay are discussed. Final resultspresented by the LEP experiments using their full LEP 1 data sets lead to a moreprecise determination of both quantities. The new LEP and SLC averages areR0c = 0.1674± 0.0038 and nc = 1.149± 0.036.1. Charm counting analysisR0c and nc can be obtained from the sum of allweak decaying charm hadron rates in Z0 → cc¯ andZ0 → bb¯ events. ALEPH and DELPHI presentedfinal updates based on their full LEP 1 data set.The rates of D0, D+, D+s and Λc are extracted fromthe invariant mass signals of exclusive reconstructeddecays. The contributions from b and c events needto be separated after subtraction of background andg → cc¯. ALEPH uses a high purity b tag veto inthe hemisphere opposite to the reconstructed charmhadron and applies an energy cut of XE > 0.5to select a 79 % pure c sample. DELPHI uses acombined fit to the b tagging and to the charmhadron energy spectra to simultaneously measurethe rates in b and c events with a small correlationof -35 %. They remove the charm hadron decayproducts from the event to improve the separationpower of the b tagging, especially for the D+channel.2. Charm counting results for c eventsThe measured rates for D0, D+, D+s and Λc aredivided by the PDG [1] branching ratios for thecorresponding decay modes. The Λc rates arecorrected by 15 ± 5 % for Ωc and Ξc productionassuming strange baryon production ratios. Fromthe sum ALEPH [2] and DELPHI [3] obtain:R0c = 0.1738± 0.0047± 0.0113(A) (1)R0c = 0.1692± 0.0047± 0.0097(D), (2)respectively. A previous result from OPAL [4] usingonly data up to 1993 gives R0c = 0.1670± 0.0110±0.0130.The systematic uncertainty due to the branch-ing ratio uncertainty dominates the total error ofthe individual measurements, especially for D+s andΛc. Using the branching ratio error in the calcula-tion of the averages would bias the results towardssmall rates, because the branching ratio enters asa factor and hence the constant relative error ofthe branching ratio results in smaller absolute er-rors for downwards fluctuating measurements. Thisproblem is present in the current LEP electroweakworking group average which includes these mea-surements.3. The DELPHI R0cdouble tagging resultDELPHI has presented a final update of the R0cmeasurement using reconstructed D∗+ in the decaymode D∗+ → (K−pi+)pi+. The same b/c separationmethod as for the charm counting analysis wasapplied to obtain the rates in c and b events [3]:RcPc→DBRD∗+→D0pi+BRD0→K−pi+= 0.01089± 0.00027± 0.00039 (3)RbPb→DBRD∗+→D0pi+BRD0→K−pi+= 0.01315± 0.00035± 0.00053 (4)with a correlation of -34 %. The ratePb→DBRD∗+→D0pi+ was determined using a doubletagging method. In one event hemisphere a 81 %pure charm sample was selected using exclusivelyreconstructed D mesons from 7 different decaymodes and applying a b tagging veto. In theopposite hemisphere the rate of slow pion from thedecay D∗+ → D0pi+ was determined from the p2tspectrum. From this rate DELPHI obtains [5]:Pb→DBRD∗+→D0pi+ = 0.174± 0.010± 0.004. (5)2 Γc/ΓhadLEP+SLD 0.1674 ± 0.0038OPALcharm count. 1991-930.167 ± 0.011 ± 0.012DELPHIcharm count. 1991-950.1692 ± 0.0047 ± 0.0097ALEPHcharm count. 1991-950.1738 ± 0.0047 ± 0.0113SLDmass+lifetime 1993-980.169 ± 0.005 ± 0.004ALEPHD excl/excl 1990-950.173 ± 0.014 ± 0.009OPALD* incl/excl 1990-950.180 ± 0.010 ± 0.012DELPHID* incl/excl 1991-950.161 ± 0.010 ± 0.009ALEPHD* incl/excl 1990-950.166 ± 0.012 ± 0.009ALEPHlepton 1992-950.1675 ± 0.0062 ± 0.01030.14 0.16 0.18Γc/ΓhadFigure 1. Summary of results on R0c from LEP andSLC.Combining both results and using the PDGBRD0→K−pi+ [1] yields:R0c = 0.1610± 0.0104± 0.0088. (6)4. LEP and SLC results on R0cA summary of the LEP and SLC results on R0cis given in figure 1. Good agreement of theresults with the Standard Model prediction of 0.172is found. Using the charm counting results thefractions of different charm hadrons per c quark aredetermined to be :f(D+) = 0.240± 0.016 (7)f(D+s ) = 0.118± 0.025 (8)f(Λc) = 0.084± 0.022, (9)assuming f(D0) = 1 − f(D+) − f(D+s ) − f(Λc).The LEP combined value for Pc→D∗+BRD∗+→D0pi+is 0.1652± 0.0053.5. Charm counting results for b eventsThe measured rates of D0, D+, D+s and Λc inb events are divided by the PDG [1] branchingratios for the corresponding decays modes. UsingR0b= 0.21642 ± 0.00073 [6] one obtains theprobabilities Pb→D for a b to produce a charmhadron. Charmonia states have been measured atLEP [7]. Assuming ηc : J/ψ : χc : ψ′c = 0.57 :1 : 0.27 : 0.31 [8] the charmonia states contribute4.0 ± 1.3 % to the charm rate per b decay. Thebranching ratio of B into Ξc has been measured byCLEO [9]. One needs to add 4.0 ± 1.6 % to thecharm rate taking Ξc production from Bs and Λbinto account. The new DELPHI result [3] is:nc = 1.166± 0.031± 0.049± 0.054(BR). (10)This compares to previous results nc = 1.190 ±0.034 ± 0.065(sys + BR) from ALEPH [10] andnc = 1.137±0.048±0.084(sys+BR) from OPAL [4].The correlated LEP average over all charm countingresults is:nc = 1.151± 0.022± 0.022± 0.051(BR). (11)6. Inclusive nc measurementDELPHI has performed an inclusive measurementof the rate of double open charm n2c and no opencharm n0c decays per b. The analysis was basedon the high purity impact parameter b tagging.By tagging one hemisphere a 84 % pure b samplewas selected, while in the opposite hemispherethe different impact parameter signatures of theadditional production of charm hadrons from opencharm was used to separate the classes. Aftersubtraction of the light quark background and usingthe normalisation to fix the single charm ratesDELPHI obtains [11] rates of n0c = 3.3 ± 2.1 %and n2c = 13.9± 4.2 %. From this they deduce anupper limit on the rate of charmless b decays of 3.5% at 95 % CL. From this measurement DELPHIobtains:nc = 1.147± 0.041± 0.008. (12)7. Exclusive double charmTwo measurements of exclusive double charm havebeen performed so far. CLEO [12] uses D-leptoncorrelation and obtains Γ(B → DX)/Γ(B →D¯X) = 0.100 ± 0.031, which translates intoa branching ratio for upper vertex charm ofBRB→D¯X = 7.9 ± 2.2 %. This measurementcorresponds to BRb→cc¯s = 21.9±3.7 %, taking D+s ,charmonia and baryon production into account.ALEPH [13] measures exclusively reconstructeddouble D decays and finds BRb→D(s)D¯(s)X = 20.9±3.0±2.4±3.7 %. Adding charmonia this correspondsto BRb→cc¯s = 22.1±3.1 %. If one uses the Standard30.250.51.0 1.50.250.290.33µ/mbmc/mb8 9 10 11 12 13 14BSL (%)11.11.21.31.4ncY4SLEPdouble charmFigure 2. Y 4S and LEP results on nc and BSLcompared to HQET predictions. The band is givenby the indirect result from exclusive double charmmeasurements.Model upper limit of 2 % [14] for charmless b decaysone obtains a combined indirect measurement of:nc = 1.201± 0.037. (13)A check on these results can be obtained from arecent DELPHI result of BRb→c¯→l = 1.68 ± 0.46% [15]. Using the results above and the PDGcharm semileptonic branching ratios one can deduceBRb→c¯→l = 1.67± 0.57 % [16].8. nc vs BR(B → l)Figure 2 shows the HQET predictions [17] aftercorrection for spectator effects for the charmcounting in b decays and the B semileptonicbranching ratio for different values of µ/mb andmc/mb. The combined LEP average of the charmcounting analyses and the inclusive DELPHI resulton the number of charm quarks per b decay is:nc = 1.149± 0.036. (14)The LEP average for the semileptonic branchingratio BRb→l = 10.63 ± 0.17 [18] needs to becorrected by BSL ' τB/τb × BRb→l to account forthe different b hadron production at LEP.The Y 4S results are nc = 1.10 ± 0.05 [9]and BSL = 10.45 ± 0.21 % [1]. The band infigure 2 is given by the indirect result from theexclusive double charm production. No majordisagreement between the measurements and theHQET prediction is found within errors.References[1] C.Caso et al., Eur. Phys. J. C3 (1998) 1[2] R.Barate et al., ALEPH Coll.,CERN-EP/99-94[3] P.Abreu et al., DELPHI Coll.,CERN-EP/99-66[4] G.Alexander et al.,OPAL Coll., Z. Phys. C72(1996) 1[5] P.Abreu et al., DELPHI Coll.,CERN-EP/99-67[6] J.Mnich, Tests of the Standard Model,Proceedings EPS-HEP’99[7] P.Abreu et al., DELPHI Coll., Phys. Lett.B341 (1994) 109,D.Buskulic et al., ALEPH Coll., Phys. Lett.B295 (1992) 396[8] J.H.Ku¨hn, S.Nussinov, R.Ru¨ckl, Z. Phys. C5(1980) 117[9] L.Gibbons el at., CLEO Coll., Phys. Rev.D56 (1997) 3783[10] D.Buskulic et al., ALEPH Coll., Phys. Lett.B388 (1996) 648[11] P.Abreu et al., DELPHI Coll., Phys. Lett.B426 (1998) 193[12] T.E.Coan, CLEO Coll., Phys. Rev. Lett. 80(1998) 1150[13] R. Barate et al., ALEPH Coll., Eur. Phys. J.C4 (1998) 387[14] G.Buchalla, I.Dunietz, H.Yamamoto,Hadronization of b→ cc¯s,FEMILAB-PUB-95/167-T[15] P.Abreu et al., DELPHI Coll., Measurementof the semileptonic b branching ratios in Zdecays, HEP’99 # 5 522[16] D.Abbaneo et al., LEP Heavy FlavourElectroweak Working Group, LEPHF/98-01[17] M.Neubert, C.T.Sachrajda, Nucl. Phys. B483(1997) 339[18] P.Gagnon, Semileptonic b branching fractionsat LEP, Proceedings EPS-HEP’99

Charm counting in b and c events at LEP

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