Observation of the decay (B)over-bar(0)->Lambda(+)(c)(p)over-bar






Full text


Observation of the Decay






N. Gabyshev,8H. Kichimi,8K. Abe,8K. Abe,41R. Abe,28T. Abe,42I. Adachi,8H. Aihara,43M. Akatsu,22Y. Asano,48 T. Aso,47V. Aulchenko,1T. Aushev,12A. M. Bakich,38Y. Ban,32E. Banas,26A. Bay,18I. Bedny,1I. Bizjak,13A. Bondar,1 A. Bozek,26M. Bracˇko,20,13J. Brodzicka,26T. E. Browder,7B. C. K. Casey,7M.-C. Chang,25P. Chang,25Y. Chao,25 K.-F. Chen,25B. G. Cheon,37R. Chistov,12S.-K. Choi,6Y. Choi,37Y. K. Choi,37M. Danilov,12L. Y. Dong,10J. Dragic,21 A. Drutskoy,12S. Eidelman,1V. Eiges,12Y. Enari,22F. Fang,7C. Fukunaga,45A. Garmash,1,8T. Gershon,8B. Golob,19,13

J. Haba,8C. Hagner,50F. Handa,42T. Hara,30K. Hasuko,33H. Hayashii,23M. Hazumi,8I. Higuchi,42L. Hinz,18 T. Hojo,30T. Hokuue,22Y. Hoshi,41W.-S. Hou,25Y. B. Hsiung,25H.-C. Huang,25T. Igaki,22Y. Igarashi,8T. Iijima,22

K. Inami,22A. Ishikawa,22R. Itoh,8H. Iwasaki,8Y. Iwasaki,8H. K. Jang,36J. H. Kang,52J. S. Kang,15P. Kapusta,26 S. U. Kataoka,23N. Katayama,8H. Kawai,2H. Kawai,43T. Kawasaki,28D.W. Kim,37H. J. Kim,52H. O. Kim,37 Hyunwoo Kim,15J. H. Kim,37S. K. Kim,36K. Kinoshita,4S. Kobayashi,34S. Korpar,20,13P. Krizˇan,19,13P. Krokovny,1

R. Kulasiri,4A. Kuzmin,1Y.-J. Kwon,52J. S. Lange,5,33G. Leder,11S. H. Lee,36J. Li,35S.-W. Lin,25D. Liventsev,12 R.-S. Lu,25J. MacNaughton,11G. Majumder,39F. Mandl,11T. Matsuishi,22S. Matsumoto,3T. Matsumoto,45 W. Mitaroff,11Y. Miyabayashi,22H. Miyake,30H. Miyata,28G. R. Moloney,21T. Mori,3T. Nagamine,42Y. Nagasaka,9

T. Nakadaira,43E. Nakano,29M. Nakao,8J.W. Nam,37Z. Natkaniec,26S. Nishida,16O. Nitoh,46S. Noguchi,23 T. Nozaki,8S. Ogawa,40T. Ohshima,22T. Okabe,22S. Okuno,14S. L. Olsen,7Y. Onuki,28W. Ostrowicz,26H. Ozaki,8

P. Pakhlov,12H. Palka,26C.W. Park,15H. Park,17K. S. Park,37L. S. Peak,38J.-P. Perroud,18L. E. Piilonen,50 M. Rozanska,26K. Rybicki,26H. Sagawa,8S. Saitoh,8Y. Sakai,8T. R. Sarangi,49M. Satapathy,49A. Satpathy,8,4 O. Schneider,18S. Schrenk,4J. Schu¨mann,25A. J. Schwartz,4S. Semenov,12K. Senyo,22R. Seuster,7M. E. Sevior,21 H. Shibuya,40B. Shwartz,1V. Sidorov,1J. B. Singh,31N. Soni,31S. Stanicˇ,48,* M. Staricˇ,13A. Sugi,22A. Sugiyama,22

K. Sumisawa,8T. Sumiyoshi,45S. Suzuki,51S. Y. Suzuki,8S. K. Swain,7T. Takahashi,29F. Takasaki,8K. Tamai,8 N. Tamura,28J. Tanaka,43M. Tanaka,8G. N. Taylor,21Y. Teramoto,29S. Tokuda,22T. Tomura,43T. Tsuboyama,8 T. Tsukamoto,8S. Uehara,8Y. Unno,2S. Uno,8G. Varner,7K. E. Varvell,38C. C. Wang,25C. H. Wang,24J. G. Wang,50 M.-Z. Wang,25Y. Watanabe,44E. Won,15B. D. Yabsley,50Y. Yamada,8A. Yamaguchi,42Y. Yamashita,27Y. Yamashita,43 M. Yamauchi,8H. Yanai,28Y. Yuan,10Y. Yusa,42C. C. Zhang,10Z. P. Zhang,35Y. Zheng,7V. Zhilich,1and D. Zˇ ontar19,13

(Belle Collaboration)

1Budker Institute of Nuclear Physics, Novosibirsk,

2Chiba University, Chiba

3Chuo University, Tokyo

4University of Cincinnati, Cincinnati, Ohio 45221

5University of Frankfurt, Frankfurt

6Gyeongsang National University, Chinju

7University of Hawaii, Honolulu, Hawaii 96822

8High Energy Accelerator Research Organization (KEK), Tsukuba

9Hiroshima Institute of Technology, Hiroshima

10Institute of High Energy Physics, Chinese Academy of Sciences, Beijing

11Institute of High Energy Physics, Vienna

12Institute for Theoretical and Experimental Physics, Moscow

13J. Stefan Institute, Ljubljana

14Kanagawa University, Yokohama

15Korea University, Seoul

16Kyoto University, Kyoto

17Kyungpook National University, Taegu

18Institut de Physique des Hautes E´ nergies, Universite´ de Lausanne, Lausanne

19University of Ljubljana, Ljubljana

20University of Maribor, Maribor

21University of Melbourne, Victoria

22Nagoya University, Nagoya

23Nara Women’s University, Nara

24National Lien-Ho Institute of Technology, Miao Li

25National Taiwan University, Taipei


27Nihon Dental College, Niigata

28Niigata University, Niigata

29Osaka City University, Osaka

30Osaka University, Osaka

31Panjab University, Chandigarh

32Peking University, Beijing

33RIKEN BNL Research Center, Upton, New York 11973

34Saga University, Saga

35University of Science and Technology of China, Hefei

36Seoul National University, Seoul

37Sungkyunkwan University, Suwon

38University of Sydney, Sydney New South Wales

39Tata Institute of Fundamental Research, Bombay

40Toho University, Funabashi

41Tohoku Gakuin University, Tagajo

42Tohoku University, Sendai

43University of Tokyo, Tokyo

44Tokyo Institute of Technology, Tokyo

45Tokyo Metropolitan University, Tokyo

46Tokyo University of Agriculture and Technology, Tokyo

47Toyama National College of Maritime Technology, Toyama

48University of Tsukuba, Tsukuba

49Utkal University, Bhubaneswer

50Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061

51Yokkaichi University, Yokkaichi

52Yonsei University, Seoul

(Received 20 December 2002; published 26 March 2003)

We report the measurement of the charmed baryonic decayBB0!

cpp with a branching fraction of 2:190:56

0:490:320:57 10

5 and a statistical significance of 5:8. The errors are statistical,

systematic, and the error of thec !pK decay branching fraction. This is the first observation

of a two-body baryonicBdecay. The analysis is based on78:2 fb1of data accumulated at the4S

resonance with the Belle detector at the KEKB asymmetricee collider.

DOI: 10.1103/PhysRevLett.90.121802 PACS numbers: 13.25.Hw, 14.20.Lq

Although the four- and three-body baryonicBdecays


B0 !

cpp andB!cpp are experimentally

well established [1,2], there has been, until now, no re-ported observation of any two-body mode, such asBB0 !

cpp. In a previous Belle analysis, based on a29:1 fb1

data sample [2], we obtained the following branching fractions for four-, three- and two-body decays:




1:171:982:87 10 4;


1:8700::43400:280:49 104;


cpp<0:31104 90%confidence level:

The measured branching fractions decrease rapidly with decreasing decay multiplicity. This suppression of lower multiplicity decays is a key issue in the understanding of the mechanism behind charmed baryonicBdecays.

There are several different theoretical calculations for the BB0 !

cpp branching fraction, based on a diquark

model [3], a QCD sum rule model [4], and pole models

[5,6]. They differ by an order of magnitude. Thus, a measurement of the two-body decayBB0!


branch-ing fraction would distbranch-inguish between these different theoretical approaches and provide important insight into the underlying physics.

In this Letter we report the first observation of the two-body decay BB0 !

cpp. The analysis is based on a data

sample of78:2 fb1 accumulated at the4Sresonance with the Belle detector at the KEKB 8 GeVeon 3.5 GeV

e asymmetric collider.

The Belle detector is a large-solid-angle magnetic spectrometer that consists of a three-layer silicon vertex detector (SVD), a 50-layer cylindrical drift chamber (CDC), a mosaic of aerogel threshold Cˇ erenkov counters (ACC), a barrel-like array of time-of-flight scintillation counters (TOF), and an array of CsI(Tl) crystals (ECL) located inside a superconducting solenoidal coil that pro-vides a 1.5 T magnetic field. An iron flux return located outside the coil is instrumented to detect muons andKL


We detect thec via the c !pK decay chan-nel. Inclusion of charge conjugate states is implicit unless otherwise stated. The particle identification (PID) infor-mation from the CDC, ACC, and TOF is used to construct likelihood functionsLp,LK, andLfor the proton, kaon, and pion assignment for all charged tracks, respectively. Likelihood ratiosLRA=B LA=LALBare required

to be greater than 0.6 to identify a particle fromc decay

as typeA, whereBdenotes the other two possible assign-ments among kaon, pion, or proton. In order to maintain high efficiency for the high momentum prompt antiproton that comes directly from the primaryBB0meson decay, we rely more heavily on the kinematic reconstruction and loosen the PID requirement to LRA=B>0:2, which improves the efficiency by a factor of about 20%. Electron and muon candidates are removed if their com-bined likelihood ratios from the ECL, CDC, and KLM information are greater than 0.95. A c candidate is selected if the invariant mass MpK is within 0:010 GeV=c2 (2:5) of the2:285 GeV=c2 c mass. A

c mass constrained fit is carried out at the

recon-structedc decay vertex to remove background

includ-ing secondary particles fromorKSdecay. The BB0 !

cpp events are identified by their energy

difference E PEi Ebeam, and the beam-energy

constrained mass Mbc

E2beam P


~ p

pi2, where Ebeam

is the beam energy, andpp~iiandEiare the three-momenta and energies of theBmeson decay products, all defined in the center-of-mass system of theeecollision. We select events with Mbc >5:20 GeV=c2 and jEj<0:20 GeV.

The prompt antiproton track and the virtual c track are required to form a commonBmass constrained decay vertex. To suppress continuum background, we impose requirements on event-shape variables. We require jcosthrj<0:80, wherethris the angle between the thrust

axis of theBcandidate tracks and that of the other tracks. This requirement eliminates 80% of the continuum back-ground and retains 80% of the signal events. We also requireR2<0:35, whereR2 is the ratio of the second to the zeroth Fox-Wolfram moments [9]. This requirement rejects 50% of the remaining continuum background and retains 90% of the signal. If there are multiple candidates in an event, the candidate with the best 2

B for the B

vertex fit is selected.

Figure 1 shows a scatter plot ofEversusMbcand their

projections for selected events. The E projection is shown for Mbc>5:270 GeV=c2 and the Mbc projection

is shown forjEj<0:030 GeV. The widths determined from single Gaussian fits to signal MC events are 2:7 MeV=c2and10:3 MeVforMbcandE, respectively.

A two-dimensional binned maximum likelihood fit is performed to determine the signal yield. For this fit, the E distribution is represented by a double Gaussian for the signal plus a first order polynomial for the back-ground, and theMbcdistribution is represented by a single Gaussian for the signal plus the ARGUS function [10] for the background. The regionE <0:1 GeVis excluded from the fit to avoid feed down from modes including extra pions, which produces the bump structure observed in the region E 0:15 GeV. In the fit, the signal shape parameters are fixed to the values fitted to the

-0.2 -0.1 0 0.1 0.2

5.2 5.22 5.24 5.26 5.28 5.3 Mbc (GeV/c2)

E (GeV)


0 1 2 3 4 5 6

-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2E (GeV)

Events/(5 MeV)


0 1 2 3 4 5 6 7

5.2 5.22 5.24 5.26 5.28 5.3 Mbc (GeV/c2)

Events/(1 MeV/c

2 )


FIG. 1. CandidateBB0!

cpp events: (a) scatter plot ofEversusMbc, (b)Edistribution forMbc>5:270 GeV=c2, and (c)Mbc


signal MC, and the signal yield and the background parameters are allowed to float. The curves in Fig. 1(b) and 1(c) indicate the results of this two-dimensional fit.

The signal peak positions determined from fits to the data,5279:50:3MeV=c2 forM

bc and0:91:8MeV

forE, are consistent with the world averageB0mass [11]

and zero, respectively. When we use single Gaussians for

Mbc andE signal functions and fit with the widths as

free parameters, the fitted values in the data are found to be1:30:3MeV=c2 and 6:91:5MeV, respectively,

which are narrower than those determined with the signal MC. The probability of obtaining such narrow widths is

O1%and is attributed to a statistical fluctuation. We also investigate the decays BB0!cpp , B!cpp

and BB0!J= KK8920, J= !ppp as control samples,

and find that for these modes Mbc and E widths are consistent between the data and signal MC. The effect of the narrow widths to the signal yield is investigated by applying fits where single Gaussians with widths allowed to float are used for the signal shapes. The difference in the fitted yields is taken into account in a systematic error as discussed below.

From the fit we obtain 19:65:0

4:4 signal events. From

separate fits to the charge conjugate modes we obtain signal yields of 6:43:0

2:4 and 13:343::25 events for BB0 !

cpp and B0 !cp, respectively. These are consistent

within statistical errors.

The branching fraction is calculated asNS="NBBB Bc !pK, using the measured signal yieldNS

and the decay branching fraction Bc !pK

5:01:3%[11]. The detection efficiency"is evaluated to be 21.1% from the signal MC. The number ofBBB pairs

NBBB is85:00:5 106. The fractions of charged and

neutralBmesons are assumed to be the same. We obtain a branching fraction of

BBB0 !

cpp 2:1900::56490:320:57 105;

where the first and the second errors are statistical and systematic, respectively. The last error of 26% is due to uncertainty in the branching fractionBc !pK.

The total systematic error of 14.8% is determined as follows. The tracking systematic error is estimated to be 8% in total, assuming a correlated systematic error of 2% per charged track, based on tracking efficiency studies

with!and!0 samples. The PID

tematic error is 10% in total, assuming a correlated sys-tematic error of 3% per proton and 2% per pion or kaon, based on studies with a !p sample for protons;

and with a D!D0, D0 !K sample for

kaons and pions. The systematic error in the fitting pro-cedure and signal shape is estimated to be 7.3%, which is half of the maximum deviation in the branching fractions obtained with various modifications to the fitting func-tions: with a single or a double Gaussian for the E

signal, with the widths and means for bothMbc and E

signals fixed to MC determined values or fitted in the

data. Finally, the systematic error in the detection effi-ciency due to MC statistics is 1.3%.

Figure 2 shows the invariant mass distribution

MpKforBcandidates in the signal regionjEj<

0:030 GeVandMbc>5:27 GeV=c2. The curve indicates

a fit result with a Gaussian over linear background. The fitted width of4:01:0MeV=c2and the fitted mean of

2286:41:3MeV=c2 are consistent with values

ob-tained from fits to signal MC events, which are generated assuming the world averagec mass [11]. We obtain ac

yield of17:55:2

4:6events, consistent with theBsignal yield

mentioned above.

We consider a contribution in the BB0 !

cpp signal

yield from otherBdecays, which gives a uniform distri-bution in the c invariant mass. We analyze the c

sideband 0:015<jMpK M

cj<0:050 GeV=c


and obtain a B signal yield of 1:232::24 events, which is

consistent with expectation of 1:40:4events from the



cpp decay MC, assuming our observed branching

fraction. From this we estimate the other B decay con-tribution of 0:10:9

0:7 events in the

c signal region,

which is negligibly small. From a simultaneous fit of the



cpp signal yield and the otherB decay

contribu-tion in the c signal and sideband regions, we obtain a

statistical significance of5:8. The significance is calcu-lated as 2 lnL0=Lmax


, whereLmaxandL0denote the

maximum likelihoods with the fitted signal yield and with the yield fixed at zero, respectively.

We investigate the BB0!cpp decay in MC with all

known c decay modes [11]. The overall detection

effi-ciency for thec !pKfinal state, including

inter-mediate resonances, is found to be consistent with that calculated for the nonresonant c !pK decay alone. The other c decays show no peaking structure in theEandMbc distributions.

Finally, the overall systematics are checked by ana-lyzing the decay mode BB0!J= KK8920 observed as J= !ppp and KK8920 !K, which contains the

same final state particles as the mode under study. A simi-lar analysis procedure is applied, except for thec vertex

fit. A J= is tagged if the measured invariant mass

0 1 2 3

2.24 2.26 2.28 2.3 2.32 M(pK-π+) (GeV/c2)

Events/(1 MeV/c

2 )

FIG. 2. Invariant mass MpK distribution for BB0!


Mppp is within 0:019 GeV=c2 of the J= mass. A


K8920 is tagged if the measured invariant mass MK is within 0:2 GeV=c2 of the K8920 mass.

Vertex fits are also carried out at the J= and

B vertices. We obtain a signal of 23:75:6

4:9 events and

a branching fraction of BBB0!J= KK8920


0:210:14 10

3, which is consistent with

pre-vious measurements [11,12].

In summary, we report the measurement of the charmed baryonicBdecayBB0 !

cpp with a branching

fraction of2:190:56

0:490:320:57 105and a

statis-tical significance of5:8. This is the first observation of a two-body baryonic B decay. The branching fraction is found to be about an order-of-magnitude smaller than that of the three-body decayB!cpp . This suppression

is a unique feature of two-body baryonic decays; in con-trast, the two- and three-body mesonic B decays are comparable. A pole model [6] predicts a value of the branching fraction of 1:13:1 105 for BB0 ! cpp, which is consistent with our measurement, while

the other models [3–5] give substantially larger values. Charmless baryonic two-body decays are expected to be suppressed by an additional factor ofjVub=Vcbj2 [11]. The

result reported here implies that their branching fractions should not be much above the 107 level, which are

consistent with the present upper limits [13].

We wish to thank the KEKB accelerator group for their excellent operation of the KEKB accelerator. We acknowl-edge support from the Ministry of Education, Culture, Sports, Science, and Technology of Japan and the Japan Society for the Promotion of Science; the Australian Research Council and the Australian Department of Industry, Science and Resources; National Science Foundation of China under Contract No. 10175071; the Department of Science and Technology of India; the BK21 program of the Ministry of Education of Korea and the CHEP SRC program of the Korea Science and Engineering Foundation; the Polish State Committee

for Scientific Research under Contract No. 2P03B 17017; the Ministry of Science and Technology of Russian Federation; the Ministry of Education, Science and Sport of Slovenia; the National Science Council and the Ministry of Education of Taiwan, and the U.S. Department of Energy.

*On leave from Nova Gorica Polytechnic, Nova Gorica. [1] CLEO Collaboration, S. A. Dytmanet al., Phys. Rev. D

66, 091101(R) (2002).

[2] Belle Collaboration, N. Gabyshevet al., Phys. Rev. D66, 091102(R) (2002).

[3] P. Ball and H. G. Dosch, Z. Phys. C51, 445 (1991). [4] V. Chernyak and I. Zhitnitsky, Nucl. Phys. B345, 137


[5] M. Jarfi et al., Phys. Lett. B 237, 513 (1990); M. Jarfi

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[6] H. Y. Cheng and K. C. Yang, Phys. Rev. D 65, 054028 (2002);65, 099901(E) (2002).

[7] Belle Collaboration, A. Abashianet al., Nucl. Instrum. Methods Phys. Res., Sect. A479, 117 (2002).

[8] Events are generated with the CLEO group’s QQ program (http://www.lns.cornell.edu/public/CLEO/soft/ QQ). The detector response is simulated using GEANT: R. Brunet al.,GEANT 3.21, CERN Report No. DD/EE/ 84-1, 1984.

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FIG. 2.Invariant mass M�pK���� distribution for B�0 !��c p� candidates in the B signal region.
FIG 2 Invariant mass M pK distribution for B 0 c p candidates in the B signal region . View in document p.4