PHYSICAL REVIEW D 1 JUNE 1998VOLUME 57, NUMBER 11

Study of semileptonic decays ofB mesons to charmed baryons

G. Bonvicini,1 D. Cinabro,1 R. Greene,1 L. P. Perera,1 G. J. Zhou,1 M. Chadha,2 S. Chan,2 G. Eigen,2

J. S. Miller,2 C. O’Grady,2 M. Schmidtler,2 J. Urheim,2 A. J. Weinstein,2 F. Wurthwein,2 D. W. Bliss,3 G. Masek,3

H. P. Paar,3 S. Prell,3 V. Sharma,3 D. M. Asner,4 J. Gronberg,4 T. S. Hill,4 D. J. Lange,4 R. J. Morrison,4

H. N. Nelson,4 T. K. Nelson,4 D. Roberts,4 A. Ryd,4 R. Balest,5 B. H. Behrens,5 W. T. Ford,5 H. Park,5 J. Roy,5

J. G. Smith,5 J. P. Alexander,6 R. Baker,6 C. Bebek,6 B. E. Berger,6 K. Berkelman,6 K. Bloom,6 V. Boisvert,6

D. G. Cassel,6 D. S. Crowcroft,6 M. Dickson,6 S. von Dombrowski,6 P. S. Drell,6 K. M. Ecklund,6 R. Ehrlich,6

A. D. Foland,6 P. Gaidarev,6 L. Gibbons,6 B. Gittelman,6 S. W. Gray,6 D. L. Hartill,6 B. K. Heltsley,6 P. I. Hopman,6

J. Kandaswamy,6 P. C. Kim,6 D. L. Kreinick,6 T. Lee,6 Y. Liu,6 N. B. Mistry,6 C. R. Ng,6 E. Nordberg,6

M. Ogg,6,* J. R. Patterson,6 D. Peterson,6 D. Riley,6 A. Soffer,6 B. Valant-Spaight,6 C. Ward,6 M. Athanas,7

P. Avery,7 C. D. Jones,7 M. Lohner,7 S. Patton,7 C. Prescott,7 J. Yelton,7 J. Zheng,7 G. Brandenburg,8 R. A. Briere,8

A. Ershov,8 Y. S. Gao,8 D. Y.-J. Kim,8 R. Wilson,8 H. Yamamoto,8 T. E. Browder,9 Y. Li, 9 J. L. Rodriguez,9

T. Bergfeld,10 B. I. Eisenstein,10 J. Ernst,10 G. E. Gladding,10 G. D. Gollin,10 R. M. Hans,10 E. Johnson,10 I. Karliner,10

M. A. Marsh,10 M. Palmer,10 M. Selen,10 J. J. Thaler,10 K. W. Edwards,11 A. Bellerive,12 R. Janicek,12

D. B. MacFarlane,12 P. M. Patel,12 A. J. Sadoff,13 R. Ammar,14 P. Baringer,14 A. Bean,14 D. Besson,14 D. Coppage,14

C. Darling,14 R. Davis,14 S. Kotov,14 I. Kravchenko,14 N. Kwak,14 L. Zhou,14 S. Anderson,15 Y. Kubota,15

S. J. Lee,15 J. J. O’Neill,15 R. Poling,15 T. Riehle,15 A. Smith,15 M. S. Alam,16 S. B. Athar,16 Z. Ling,16

A. H. Mahmood,16 S. Timm,16 F. Wappler,16 A. Anastassov,17 J. E. Duboscq,17 D. Fujino,17,† K. K. Gan,17 T. Hart,17

K. Honscheid,17 H. Kagan,17 R. Kass,17 J. Lee,17 M. B. Spencer,17 M. Sung,17 A. Undrus,17,‡ R. Wanke,17

A. Wolf,17 M. M. Zoeller,17 B. Nemati,18 S. J. Richichi,18 W. R. Ross,18 H. Severini,18 P. Skubic,18 M. Bishai,19

J. Fast,19 J. W. Hinson,19 N. Menon,19 D. H. Miller,19 E. I. Shibata,19 I. P. J. Shipsey,19 M. Yurko,19 S. Glenn,20

S. D. Johnson,20 Y. Kwon,20,§ S. Roberts,20 E. H. Thorndike,20 C. P. Jessop,21 K. Lingel,21 H. Marsiske,21

M. L. Perl,21 V. Savinov,21 D. Ugolini,21 R. Wang,21 X. Zhou,21 T. E. Coan,22 V. Fadeyev,22 I. Korolkov,22

Y. Maravin,22 I. Narsky,22 V. Shelkov,22 J. Staeck,22 R. Stroynowski,22 I. Volobouev,22 J. Ye,22 M. Artuso,23 F. Azfar,23

A. Efimov,23 M. Goldberg,23 D. He,23 S. Kopp,23 G. C. Moneti,23 R. Mountain,23 S. Schuh,23 T. Skwarnicki,23

S. Stone,23 G. Viehhauser,23 X. Xing,23 J. Bartelt,24 S. E. Csorna,24 V. Jain,24,i K. W. McLean,24 S. Marka,24

R. Godang,25 K. Kinoshita,25 I. C. Lai,25 P. Pomianowski,25 and S. Schrenk25

~CLEO Collaboration!1Wayne State University, Detroit, Michigan 48202

2California Institute of Technology, Pasadena, California 911253University of California, San Diego, La Jolla, California 92093

4University of California, Santa Barbara, California 931065University of Colorado, Boulder, Colorado 80309-0390

6Cornell University, Ithaca, New York 148537University of Florida, Gainesville, Florida 32611

8Harvard University, Cambridge, Massachusetts 021389University of Hawaii at Manoa, Honolulu, Hawaii 9682210University of Illinois, Urbana-Champaign, Illinois 6180111Carleton University, Ottawa, Ontario, Canada K1S 5B6

and the Institute of Particle Physics, Canada12McGill University, Montreal, Quebec, Canada H3A 2T8

and the Institute of Particle Physics, Canada13Ithaca College, Ithaca, New York 14850

14University of Kansas, Lawrence, Kansas 6604515University of Minnesota, Minneapolis, Minnesota 55455

16State University of New York at Albany, Albany, New York 1222217Ohio State University, Columbus, Ohio 43210

18University of Oklahoma, Norman, Oklahoma 7301919Purdue University, West Lafayette, Indiana 47907

20University of Rochester, Rochester, New York 1462721Stanford Linear Accelerator Center, Stanford University, Stanford, California 94309

22Southern Methodist University, Dallas, Texas 7527523Syracuse University, Syracuse, New York 13244

24Vanderbilt University, Nashville, Tennessee 3723525Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061

~Received 2 December 1997; published 14 April 1998!

570556-2821/98/57~11!/6604~5!/$15.00 6604 © 1998 The American Physical Society

57 6605STUDY OF SEMILEPTONIC DECAYS OFB MESONS TO . . .

Using data collected by the CLEO II detector at a center-of-mass energy on or near theY(4S) resonance,

we have determined the 90% confidence level upper limitB(B→Lc1e2X)/B„B→(Lc

1 or Lc2)X…,0.05 for

electrons with momentum above 0.6 GeV/c. We have also obtained the limitB(B2→Lc1pe2ne)/B(B

→Lc1pX),0.04 at the 90% confidence level and measured the ratioB(B→Lc

1pX)/B„B→(Lc1 or Lc

2)X…50.5760.0560.05. @S0556-2821~98!03111-7#

PACS number~s!: 13.20.He

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I. INTRODUCTION

In the naive spectator model, mostB mesons decaythrough the spectator diagram with semileptonic decays

curring by ‘‘external’’ W-emission:b→cW; W→l n l . Inthis picture, charmed baryon production occurs when tquark-antiquark pairs from the vacuum bind with the chaquark and the spectator antiquark to form aLc

1(cud) plus an

antinucleonN. In this paper we attempt to isolate the manitude of this externalW-emission spectator diagram icharmed baryon decays by measuringB→Lc

1e2X and

B2→Lc1pe2ne . For normalization modes, we also measu

B→Lc1pX and B→(Lc

1 or Lc2)X. Throughout this pape

charged conjugate modes are implicit.If B→baryons does indeed occur through exter

W-emission as outlined above, then the decB→Lc

1NXe2n l will occur @1#. We can estimate the magn

tude of R5B(B→Lc1Ne2n l )/B(B→Lc

1NX) by using thenaive expectation for the semileptonic branching ratiothese decays. The (cs) and (tnt) contributions are absendue to the limited available phase space, and so a maximof 20% is expected for the ratioR. Alternately, one mightanticipate thatB(B→Lc

1Xe2ne)/B(B→Lc1X) is compa-

rable to the measurements ofB(B→DXe2ne)/B(B→DX).12% @2#.

There are two other baryon production mechanisms inBdecay, neither making a contribution to semileptonic decIn one, theW is emitted internally and decays to (cs), lead-ing to JcLc final states. This mechanism was studied inprevious CLEO paper, which looked at the charge corretions between Lc’s and leptons from B decay andfound RLc

5NLc2l 1 /NL

c1l 15B(B→Lc

2X)B(B→Xl 1n l )/

B(B→Lc1X)B(B→Xl 1n l )50.1960.1360.04 which is

directly related toB(b→ccs)/B(b→cud) @3#. For LcX fi-nal states, we cannot rule out the possibility in our analythat we are observing decays of the typeB→JcLc , as wecannot tag the parentB meson in theB→Lc

1X analysis.Therefore, the yields for this mode will be quoted as decof the typeB→(Lc

1 or Lc2)X. Another mechanism is the

*Permanent address: University of Texas, Austin TX 78712.†Permanent address: Lawrence Livermore National Laborat

Livermore, CA 94551.‡Permanent address: BINP, RU-630090 Novosibirsk, Russia.§Permanent address: Yonsei University, Seoul 120-749, KoreiPermanent address: Brookhaven National Laboratory, Up

NY 11973.

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internal emission of aW followed by its decay to (ud).Measurements ofB mesons decaying hadronically tcharmed baryons indicate that this internalW-emission dia-gram may contribute significantly@4#. A substantial contri-bution from this diagram would reduce the semileptonic dcay width.

The semileptonic branching ratio ofB mesons is knownto have a lower value than theoretical predictions@5#. Thesepredictions assume a large externalW-emission contributionin baryon decays. The suggestion has been made that thmay underestimate theB-hadronic width by neglectingBdecay channels to baryon states@6#. If this is the case, hadronic decays to charmed baryons could explain the lowclusive semileptonic branching ratio. The measuremensemileptonic decays ofB mesons toLc will provide vitalinformation on baryon production inB decays.

A. Data sample and event selection

The data were taken with the CLEO II detector@7# at theCornell Electron Storage Ring~CESR!, and consist of3.2 fb21 on theY(4S) resonance and 1.6 fb21 at a center-of-mass energy 60 MeV below the resonance. The on-resonsample contains 3.43106 BB events and 103106 continuumevents. We select hadronic events containing at leascharged tracks. To suppress continuum background, wequire the ratio of Fox-Wolfram moments@8# R25H2 /H0 tosatisfy R2<0.35. We reconstructLc’s in the pKp decaymode. For the hadronic particle identification, a probabilcut for each target hadron is made which uses informaobtained fromdE/dx and time-of-flight detectors. For particle consistency, the probability cuts are chosen togreater than 0.0027~within three standard deviations of thexpected value! for pions, 0.0001 for kaons, and 0.0003 fprotons. Continuum data are used to directly subtract bagrounds from non-BB events.

Tagged signal Monte Carlo simulated events were useobtain the signal efficiencies whileBB Monte Carlo simu-lated events, with the signal channel removed, were useestimate the background fromB decays to non-signal modesThe CLEOBB Monte Carlo simulation generates baryondecays with a phenomenological model which is tunedmatch the observedLc momentum spectrum. We useBB

Monte Carlo events where we force theB→Lc1X,

Lc1→pK2p1 decay chain to determine a detection ef

ciency of 0.3660.01.The pK2p1 invariant mass distributions are measur

separately for the resonance and continuum data. The rnance data are fitted to a double Gaussian signal atop aorder polynomial background. In these fits, the width of tGaussianLc signal function is constrained to the value d

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6606 57G. BONVICINI et al.

rived from the Monte Carlo simulation. After subtractinnon-BB contributions using off-resonance data scaled forminosity and cross section, we obtain a total sample48796296B→(Lc

1 or Lc2)X events from data. After scal

ing by the efficiency, we find a yield of 1355268226802events, where the second~systematic! error includes contri-butions from the efficiency correction.

B. Study of B˜Lc1e2X

Because of the soft lepton momentum spectrum fromdecay and the limited reconstruction efficiency for low mmentum muons, we use only electrons in our analysis. Etron identification relies onE/p measurements derived fromthe calorimeter and drift chamber, as well as specific ionition loss measurements from the drift chamber. The requment of ln(Pe/Pe”).3.0 is imposed, wherePe(Pe”) is the

FIG. 1. The fit ~line! to the pK2p1 invariant mass spectrumfrom on resonance data events~points with error bars! and the

scaled off resonance data~histogram! for the ~a! B→Lc1e2X and

~b! B2→Lc1pe2ne analyses.

-f

is-c-

-e-

probability that a given charged track is an electron~not anelectron!. We choose a minimum momentum cutoff o0.6 GeV/c for these electrons to limit fake and secondaelectron background sources. The maximum possible etron momentum for this decay is 1.5 GeV/c. Electron candi-dates are restricted to the polar angular regionucosuu<0.71. We pair allpK2p1 candidates, selected as describabove, with additional tracks in the events passing the lepidentification requirement. We then fit thepK2p1 invariantmass distributions on and off resonance for combinatipassing these cuts.

Figure 1~a! shows the fit to thepK2p1 invariant massdistribution for events that satisfy the above selection crria. The resonance data~points! are fit to a double Gaussiasignal over a second order polynomial background. Thenal shape is fixed to that from the data in theB→Lc

1Xanalysis. A similar fit has been performed on thepK2p1

invariant mass distribution from the continuum data~shownby the scaled histogram in the figure!. The yields are given inTable I.

In addition to continuumLc’s, other sources of background are fake leptons and uncorrelatedLc

12e2 pairs. Thenumber of fake leptons is obtained by running the saanalysis, but using an electron anti-identification criterioln(Pe/Pe”),0. The pK2p1 invariant mass is refit and thiyield is scaled by the measured lepton misidentificatprobabilities. The uncorrelated background includes comnations where theLc

1 originates fromB decay and the lepton

originates fromB decay or from aB if from an event wheremixing took place. This background is estimated usiB→Lc

1X Monte Carlo events and examining decays whthe Lc

1 and e2 have opposite charges, but do not originafrom the signal mode. We check this procedure by comping the data and Monte Carlo results obtained usingLc

1e1

~wrong sign! combinations. Wrong sign combinations wiinclude primary leptons from oneB paired withLc

1’s fromthe other B. We find consistency between the data aMonte Carlo wrong sign yields. The background predictioare given in Table I.

The lepton minimum momentum cut of 0.6 GeV/c resultsin a model dependence. Larger multiplicity final states whave a lower efficiency due to the minimum momentum cWe find the efficiency usingB2→Lc

1pe2ne Monte Carloevents where theB1 decays generically. This efficiency fo

tsto the

TABLE I. Results of theLc1e2X, Lc

1pe2ne , andLc1pX analyses. The ‘‘Data on’’ row shows resul

from fits to the on resonance data sample, while the ‘‘Scaled off’’ row shows the results of the fitsnearby continuum data scaled for luminosity and cross section.

Type Lc1e2X Lc

1pe2ne Lc1pX

Data on 176641 20610 25016121Scaled off 9622 667 4406105

Fakes 1064 (e) 261 ~e and p! 3221516 ( p’s)

MC pred. uncorr. 9567633 1166 5866658Bkgd. sub. 62647634 161266 19716160260

159

Efficiencies 0.23960.00560.011 0.09460.00360.003 0.25760.00360.010Yield 25961966143 116132682 766966236385

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57 6607STUDY OF SEMILEPTONIC DECAYS OFB MESONS TO . . .

B2→Lc1pe2ne , where the electron is prompt from theB

decay, is found to be 17%. Monte Carlo events from

chain B0→Lc1D0e2ne , D0→ pp1 were also generated t

measure the efficiency. This mode adds one extra pion tototal decay chain although more could be present in o

decays such asB→ScDen. Differences between efficiencie

for B2 andB0 are found to be negligible. After all other cutwe find that 73% of the events fromB2→Lc

1pe2ne passour electron momentum cut while only 45% of the evefrom B0→Lc

1D0e2ne pass. Because the total efficiencydependent on the number of pions in the final state,choose to quote a partial branching fraction where the lepmomentum is greater than 0.6 GeV/c. In this electron mo-mentum range, the efficiency forB→Lc

1pe2ne is0.23960.005 which is consistent with the efficiency foother modes with extra pions. In addition to assigning a stematic error due to efficiency determination, we addquadrature errors from the fake lepton and uncorrelabackground source estimates to obtain the total systemerror.

C. Search for B2˜Lc

1pe2ne

The signature ofB2→Lc1pe2ne is a baryon-lepton-

antiproton combination which has a recoil mass consiswith that of a neutrino, approximating theB momentum aszero. CandidateLc

1’s, electrons, and antiprotons for thanalysis must satisfy requirements similar to those discusabove. We then require that the approximation of the squamass of the neutrino, M n

2[(Ebeam2ELc2Ee)

22(pLc

1pe)2, be greater than22(GeV/c2)2. In addition, we place

an angular cut of cosuLc-e,20.2, whereuLc-e is the angle

between theLc1 and electron. In Fig. 1~b! we show theLc

1

invariant mass distribution for combinations passing allthese cuts. This distribution is fit as before; results are giin Table I.

Backgrounds to this process stem from three sources:antiprotons or electrons, non-BB events, and secondary eletrons or antiprotons. Fake antiprotons and electrons aresidered separately. We use the same methods as descabove to determine each contribution. The continuum baground is measured using the off-resonance data scaleluminosity and cross section. The remaining backgrouevents, in which electrons come from the decay chb→ c→ sen, can be estimated using a Monte Carlo simution. The wrong sign data and Monte Carlo results are copared and again found to agree well.

We find efficiency using theB2→Lc1pe2ne Monte

Carlo events where theB1 decays generically. The efficiency is found to be 0.09460.003. Systematic errors arassigned for each of the background source estimates anefficiency determination as described above.

D. Study of B˜Lc1pX

We pair allLc1 and p candidates using theLc

1 selection

as described above. For thep, in addition to the cut on theproton probability of greater than 0.0003, we employ ad

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tional veto cuts on the particle identification of 2s for thep,K, and electron to reduce fake antiprotons. We then fitLc

1 invariant mass. The observedLc1 signal area then mea

sures the number ofLc12 p correlations. Figure 2 shows th

fit to the data.We are looking for decays where theLc

1 and p haveopposite charge and both are primary from theB decay.Backgrounds are categorized into two sources: secondap

~not primary from aB decay! and fakep. The first back-ground source is estimated by usingB→Lc

1X Monte Carloevents as above. Once again we check this procedurecomparing wrong sign data yields to our Monte Carlo wrosign prediction. Proton misidentification probabilities aalso measured directly from the data by using a pion samfrom KS

0→p1p2 where KS0’s are selected by a seconda

vertex finder. After applying the veto cuts for kaons, pionand electrons, the fake probability derived from just usithe pion rate is found to be consistent with the species aaged rate. The background contribution of fakep’s is ob-tained by running the same analysis without the partiidentification cuts for thep correlated withLc

1 . The yieldsobtained from fits to thepK2p1 mass are then multiplied bythe fakingp probabilities and weighted by momentum. Thprocedure will yield an upper limit on the number of fakeas real proton tracks are double counted in our procedWe studied the overcounting rate and assign a systemerror based on the difference between the number of procounted with no identification criteria and the numbcounted using the anti-identification criteria.

We have also measured the absolute proton identificaefficiency as a function of momentum for the cuts in thanalysis using a sample ofL→pp2 events. The differencesin the identification efficiencies between the data and MoCarlo prediction are then used to calculate the system

FIG. 2. The fit ~line! to the pK2p1 invariant mass spectrumfrom on resonance data events~points with error bars! and the

scaled off resonance data~histogram! for the B→Lc1pX analysis.

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6608 57G. BONVICINI et al.

error in the B(B→Lc1pX)/B(B→Lc

1X) measuremen~Table I!. The total systematic error for this mode is thderived from errors in the efficiency as well as backgroudeterminations.

II. SUMMARY

Table I summarizes the final numbers of candidatesthe three signal modes. After background subtraction,number of wrong sign candidates observed in the datconsistent with our expectations based on Monte Carlo sies.

We measure the ratio

B~B→Lc1pX!

B„B→~Lc1 or Lc

2!X…5

~766966236385!

~1355268226802!

50.5760.0560.05, ~1!

consistent with the naively expected value of 50%.For the electron channels, the number of signal candid

is fully consistent with the expected background level, aso we derive a 90% confidence level upper limit for the raR, for pe>0.6 GeV/c:

R5B~B→Lc

1e2X!

B„B→~Lc1 or Lc

2!X…5

~25961966143!

~1355268226802!

,0.05 at 90% C.L. ~2!

If one assumes that all of the semileptonic decays procvia the channelB2→Lc

1pe2ne , the upper limit onR wouldbe 0.07 for the entire electron momentum range. Similarlyall of the semileptonic decays wereB0→Lc

1D0e2ne , thelimit on R would be 0.11. Our result is consistent with thderived limit based on a previous measurement wherecharmed baryon is not observed and the lepton spectru

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extrapolated from a model ofB(B→Xpe2ne),0.16% at90% C.L. @9#. This implies a limit onB(B→Lc

1e2X)/B„B→(Lc

1 or Lc2)X…,5% at 90% C.L.

For theLcpene channel, we find

B~B2→Lc1pe2ne!

B~B→Lc1pX!

,0.04 at 90% C.L. ~3!

for the entire electron momentum range.Our limits on the semileptonic branching ratios do n

support the hypothesis that the externalW-emission diagramsaturates charmed baryon production inB decays. While theB(B→Lc

1e2X) measurement is limited by our knowledgof the possible decay states, the exclusive limit constrainsexpected dominant mode below the corresponding rate msured forB decays to charmed mesons. The semileptodecay rate fromB to baryons does not add a large contribtion to the total semileptonicB decay rate if these semileptonics decays are dominated by modes of the tyB→Lc

1Ne2ne .

ACKNOWLEDGMENTS

We gratefully acknowledge the effort of the CESR staffproviding us with excellent luminosity and running condtions. J.P.A., J.R.P., and I.P.J.S. thank the NYI programthe NSF, M.S. thanks the PFF program of the NSF, Gthanks the Heisenberg Foundation, K.K.G., M.S., H.N.T.S., and H.Y. thank the OJI program of the DOE, J.R.K.H., M.S. and V.S. thank the A.P. Sloan Foundation, R.thanks the Alexander von Humboldt Stiftung, M.S. thanResearch Corporation, and S.D. thanks the Swiss NatioScience Foundation for support. This work was supportedthe National Science Foundation, the U.S. DepartmentEnergy, and the Natural Sciences and Engineering ReseCouncil of Canada.

tt.

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