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
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
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
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 , we obtained the following branching fractions for four-, three- and two-body decays:
1:171:982:87 10 4;
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 , a QCD sum rule model , 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
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 . 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  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)
0 1 2 3 4 5 6
-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 ∆E (GeV)
0 1 2 3 4 5 6 7
5.2 5.22 5.24 5.26 5.28 5.3 Mbc (GeV/c2)
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
forE, are consistent with the world averageB0mass 
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%. 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
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 . We obtain ac
4:6events, consistent with theBsignal yield
We consider a contribution in the BB0 !
yield from otherBdecays, which gives a uniform distri-bution in the c invariant mass. We analyze the c
sideband 0:015<jMpK M
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 . 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)
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
3, which is consistent with
pre-vious measurements [11,12].
In summary, we report the measurement of the charmed baryonicBdecayBB0 !
cpp with a branching
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  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 . 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 .
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.  CLEO Collaboration, S. A. Dytmanet al., Phys. Rev. D
66, 091101(R) (2002).
 Belle Collaboration, N. Gabyshevet al., Phys. Rev. D66, 091102(R) (2002).
 P. Ball and H. G. Dosch, Z. Phys. C51, 445 (1991).  V. Chernyak and I. Zhitnitsky, Nucl. Phys. B345, 137
 M. Jarfi et al., Phys. Lett. B 237, 513 (1990); M. Jarfi
et al., Phys. Rev. D 43, 1599 (1991); N. Deshpande, J. Trampetic, and A. Soni, Mod. Phys. Lett. A 3, 749 (1988).
 H. Y. Cheng and K. C. Yang, Phys. Rev. D 65, 054028 (2002);65, 099901(E) (2002).
 Belle Collaboration, A. Abashianet al., Nucl. Instrum. Methods Phys. Res., Sect. A479, 117 (2002).
 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.
 G. C. Fox and S. Wolfram, Phys. Rev. Lett. 41, 1581 (1978).
 ARGUS Collaboration, H. Albrechtet al., Phys. Lett. B
229, 304 (1989);241, 278 (1990).
 Particle Data Group, K. Hagiwaraet al., Phys. Rev. D66, 010001 (2002).
 Belle Collaboration, K. Abeet al., Phys. Lett. B538, 11 (2002).