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Observation of B0bar-Lambda_c+pbar decay

a r X i v :h e p -e x /0212052v 2 6 F e

b 2003

KEK preprint 2002-127hep-ex/0212052

BELLE Observation of ˉB 0→Λ+c ˉp decay

N.Gabyshev,8H.Kichimi,8K.Abe,8K.Abe,41R.Abe,28T.Abe,42I.Adachi,8H.Aihara,43M.Akatsu,22Y.Asano,48T.Aso,47V.Aulchenko,1T.Aushev,12A.M.Bakich,38Y.Ban,32E.Banas,26A.Bay,18I.Bedny,https://www.wendangku.net/doc/b78165937.html,jak,13A.Bondar,1A.Bozek,26M.Braˇc ko,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,21A.Drutskoy,12S.Eidelman,1V.Eiges,12Y.Enari,22F.Fang,7C.Fukunaga,45A.Garmash,1,8T.Gershon,8B.Golob,19,13J.Haba,8C.Hagner,50F.Handa,42T.Hara,30K.Hasuko,33H.Hayashii,23M.Hazumi,8I.Higuchi,42L.Hinz,18T.Hojo,30T.Hokuue,22Y.Hoshi,41W.-S.Hou,25Y.B.Hsiung,25

H.-C.Huang,25T.Igaki,22Y.Igarashi,8T.Iijima,22K.Inami,22A.Ishikawa,22

R.Itoh,8H.Iwasaki,8Y.Iwasaki,8H.K.Jang,36J.H.Kang,52J.S.Kang,15

P.Kapusta,26S.U.Kataoka,23N.Katayama,8H.Kawai,2H.Kawai,43T.Kawasaki,28

D.W.Kim,37H.J.Kim,52H.O.Kim,37Hyunwoo Kim,15J.H.Kim,37S.K.Kim,36K.Kinoshita,4S.Kobayashi,34S.Korpar,20,13P.Kriˇz an,19,13P.Krokovny,1R.Kulasiri,4

A.Kuzmin,1Y.-J.Kwon,https://www.wendangku.net/doc/b78165937.html,nge,5,33G.Leder,11S.H.Lee,36J.Li,35S.-W.Lin,25

D.Liventsev,12R.-S.Lu,25J.MacNaughton,11G.Majumder,39F.Mandl,11T.Matsuishi,22

S.Matsumoto,3T.Matsumoto,45W.Mitaro?,11Y.Miyabayashi,22H.Miyake,30H.Miyata,28G.R.Moloney,21T.Mori,3T.Nagamine,42Y.Nagasaka,9T.Nakadaira,43

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

Y.Onuki,28W.Ostrowicz,26H.Ozaki,8P.Pakhlov,12H.Palka,26C.W.Park,15H.Park,17K.S.Park,37L.S.Peak,38J.-P.Perroud,18L.E.Piilonen,50M.Rozanska,26K.Rybicki,26H.Sagawa,8S.Saitoh,8Y.Sakai,8T.R.Sarangi,49M.Satapathy,49

A.Satpathy,8,4O.Schneider,18S.Schrenk,4J.Sch¨u mann,25A.J.Schwartz,4

S.Semenov,12K.Senyo,22R.Seuster,7M.E.Sevior,21H.Shibuya,40B.Shwartz,1

V.Sidorov,1J.B.Singh,31N.Soni,31S.Staniˇc ,48,?M.Stariˇ

c ,13A.Sugi,22A.Sugiyama,22K.Sumisawa,8T.Sumiyoshi,45S.Suzuki,51S.Y.Suzuki,8S.K.Swain,7T.Takahashi,29

F.Takasaki,8K.Tamai,8N.Tamura,28J.Tanaka,43M.Tanaka,8

G.N.Taylor,21 Y.Teramoto,29S.Tokuda,22T.Tomura,43T.Tsuboyama,8T.Tsukamoto,8S.Uehara,8 Y.Unno,2S.Uno,8G.Varner,7K.E.Varvell,38C.C.Wang,25C.

H.Wang,24

J.G.Wang,50M.-Z.Wang,25Y.Watanabe,44E.Won,15B.D.Yabsley,50Y.Yamada,8 A.Yamaguchi,42Y.Yamashita,27Y.Yamashita,43M.Yamauchi,8H.Yanai,28Y.Yuan,10 Y.Yusa,42C.C.Zhang,10Z.P.Zhang,35Y.Zheng,7V.Zhilich,1and D.ˇZontar19,13

(The Belle Collaboration)

1Budker Institute of Nuclear Physics,Novosibirsk

2Chiba University,Chiba

3Chuo University,Tokyo

4University of Cincinnati,Cincinnati,Ohio45221

5University of Frankfurt,Frankfurt

6Gyeongsang National University,Chinju

7University of Hawaii,Honolulu,Hawaii96822

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′Energies,Universit′e 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

26H.Niewodniczanski Institute of Nuclear Physics,Krakow

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 York11973

34Saga University,Saga

35University of Science and Technology of China,Hefei

36Seoul National University,Seoul

37Sungkyunkwan University,Suwon

38University of Sydney,Sydney NSW

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,Virginia24061

51Yokkaichi University,Yokkaichi

52Yonsei University,Seoul

Abstract

We report the measurement of the charmed baryonic decayˉB0→Λ+cˉp with a branching fraction ±0.32±0.57)×10?5and a statistical signi?cance of5.8σ.The errors are statistical, of(2.19+0.56

?0.49

systematic,and the error of theΛ+c→pK?π+decay branching fraction.This is the?rst observa-tion of a two-body baryonic B decay.The analysis is based on78.2fb?1of data accumulated at theΥ(4S)resonance with the Belle detector at the KEKB asymmetric e+e?collider.

PACS numbers:13.25.Hw,14.20.Lq

Although the four-and three-body baryonic B decaysˉB0→Λ+cˉpπ+π?and B?→Λ+cˉpπ?are experimentally well established[1,2],there has been,until now,no reported observation of any two-body mode,such asˉB0→Λ+cˉp.In a previous Belle analysis,based on a29.1fb?1 data sample[2],we obtained the following branching fractions for four-,three-and two-body decays:

B(ˉB0→Λ+cˉpπ+π?)=(11.04+1.22

±1.98±2.87)×10?4,

?1.17

±0.28±0.49)×10?4,

B(B?→Λ+cˉpπ?)=(1.87+0.43

?0.40

B(ˉB0→Λ+cˉp)<0.31×10?4(90%con?dence 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 baryonic B decays.

There are several di?erent theoretical calculations for theˉB0→Λ+cˉp branching fraction, based on a diquark model[3],a QCD sum rule model[4]and pole models[5,6].They di?er by an order of magnitude.Thus,a measurement of the two-body decayˉB0→Λ+cˉp branching fraction would distinguish between these di?erent theoretical approaches and provide important insight into the underlying physics.

In this paper we report the?rst observation of the two-body decayˉB0→Λ+cˉp.The analysis is based on a data sample of78.2fb?1accumulated at theΥ(4S)resonance with the Belle detector at the KEKB8GeV e?on3.5GeV e+asymmetric collider.

The Belle detector is a large-solid-angle magnetic spectrometer that consists of a three-layer silicon vertex detector(SVD),a50-layer cylindrical drift chamber(CDC),a mosaic of aerogel thresholdˇCerenkov counters(ACC),a barrel-like array of time-of-?ight scintillation counters(TOF),and an array of CsI(Tl)crystals(ECL)located inside a superconducting solenoidal coil that provides a1.5T magnetic?eld.An iron?ux return located outside the coil is instrumented to detect muons and K L mesons(KLM).The detector is described in detail elsewhere[7].We use a GEANT based Monte Carlo(MC)simulation to model the response of the detector and determine its acceptance[8].

We detect theΛ+c via theΛ+c→pK?π+decay channel.Inclusion of charge conjugate states is implicit unless otherwise stated.The particle identi?cation(PID)information from the CDC,ACC and TOF is used to construct likelihood functions L p,L K and Lπfor the proton,kaon and pion assignment for all charged tracks,respectively.Likelihood ratios

LR(A/B)=L A/(L A+L B)are required to be greater than0.6to identify a particle fromΛ+c decay as type A,where B denotes the other two possible assignments among kaon,pion or proton.In order to maintain high e?ciency for the high momentum prompt antiproton that comes directly from the primaryˉB0meson decay,we rely more heavily on the kinematic reconstruction and loosen the PID requirement to LR(A/B)>0.2,which improves the e?ciency by a factor of about20%.Electron and muon candidates are removed if their combined likelihood ratios from the ECL,CDC and KLM information are greater than0.95. AΛ+c candidate is selected if the invariant mass M(pK?π+)is within0.010GeV/c2(2.5σ) of the2.285GeV/c2Λ+c mass.AΛ+c mass constrained?t is carried out at the reconstructed Λ+c decay vertex to remove background including secondary particles fromΛor K S decay. TheˉB0→Λ+cˉp events are identi?ed by their energy di?erence?E=( E i)?E beam,

and the beam-energy constrained mass M bc=

M bc (GeV/c 2)?E (G e V )01

3456-0.2-0.15-0.1-0.0500.050.10.150.2

?E (GeV)

E v e n t s /(5 M e V )(b)012345675.2 5.22 5.24 5.26 5.28 5.3

M bc (GeV/c 2)E v e n t s /(1 M e V /c 2)(c)FIG.1:Candidate ˉB 0→Λ+c ˉp events:(a)scatter plot of ?E versus M bc ,(b)?E distribution for M bc >5.270GeV /c 2and (c)M bc distribution for |?E |<0.030GeV.The curves indicate the result of a two-dimensional ?t.

the bump structure observed in the region ?E ≤?0.15GeV.In the ?t,the signal shape parameters are ?xed to the values ?tted to the signal MC,and the signal yield and the background parameters are allowed to ?oat.The curves in Figure 1(b)and (c)indicate the results of this two-dimensional ?t.

The signal peak positions determined from ?ts to the data,(5279.5±0.3)MeV /c 2for M bc and (0.9±1.8)MeV for ?E ,are consistent with the world average B 0mass [11]and zero,respectively.When we use single Gaussians for M bc and ?E signal functions and ?t with the widths as free parameters,the ?tted values in the data are found to be (1.3±0.3)MeV /c 2and (6.9±1.5)MeV,respectively,which are narrower than those determined with the signal MC.The probability of obtaining such narrow widths is O (1%)and is attributed to a

statistical ?uctuation.We also investigate the decays ˉB 0→Λ+c ˉp π+π?,B ?→Λ+c ˉp

π?and ˉB 0→J/ψˉK ?(892)0,J/ψ→p ˉp as control samples,and ?nd that for these modes M bc and ?E widths are consistent between the data and signal MC.The e?ect of the narrow widths to the signal yield is investigated by applying ?ts where single Gaussians with widths allowed to ?oat are used for the signal shapes.The di?erence in the ?tted yields is taken into account in a systematic error as discussed below.

From the ?t we obtain 19.6+5.0?4.4signal events.From separate ?ts to the charge conjugate

modes we obtain signal yields of 6.4+3.0?2.4and 13.3+4.2?3.5events for ˉB 0→Λ+c ˉp and B 0→ˉΛ?c

p ,7

respectively.These are consistent within statistical errors.

The branching fraction is calculated as N S /(ε×N B ˉB ×B (Λ+c →pK ?π+)),using the measured signal yield N S and the decay branching fraction B (Λ+c →pK ?π+)=(5.0±

1.3)%[11].The detection e?ciency εis evaluated to be 21.1%from the signal MC.The

number of B ˉB pairs N B ˉB is (85.0±0.5)×106.The fractions of charged and neutral B

mesons are assumed to be the same.

We obtain a branching fraction of

B (ˉB 0→Λ+c ˉp )=(2.19+0.56?0.49±0.32±0.57)×10?5,

where the ?rst and the second errors are statistical and systematic,respectively.The last

error of 26%is due to uncertainty in the branching fraction B (Λ+c →pK ?π+).

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 e?ciency studies with η→γγand η→π+π?π0samples.The PID systematic error is 10%in total,assuming a correlated systematic error of 3%per proton and 2%per pion or kaon,based on studies with a Λ→pπ?sample for protons;and with a D ?+→D 0π+,D 0→K ?π+sample for kaons and pions.The systematic error in the ?tting procedure and signal shape is estimated to be 7.3%,which is half of the maximum deviation in the branching fractions obtained with various modi?cations to the ?tting functions:with a single or a double Gaussian for the ?E signal,with the widths and means for both M bc and ?E signals ?xed to MC determined values or ?tted in the data.Finally,the systematic error in the detection e?ciency due to MC statistics is 1.3%.

23 2.24 2.26 2.28 2.3

2.32M(pK -π+) (GeV/c 2

)E v e n t s /(1 M e V /c 2)FIG.2:Invariant mass M (pK ?π+)distribution for ˉB 0→Λ+c ˉp candidates in the B signal region.

Figure 2shows the invariant mass distribution M (pK ?π+)for B candidates in the signal region |?E |<0.030GeV and M bc >5.27GeV/c 2.The curve indicates a ?t result with a Gaussian over linear background.The ?tted width of (4.0±1.0)MeV /c 2and the ?tted

mean of(2286.4±1.3)MeV/c2are consistent with values obtained from?ts to signal MC events,which are generated assuming the world averageΛ+c mass[11].We obtain aΛ+c yield of17.5+5.2

?4.6

events,consistent with the B signal yield mentioned above.

We consider a contribution in theˉB0→Λ+cˉp signal yield from other B decays,which gives an uniform distribution in theΛ+c invariant mass.We analyze theΛ+c sideband0.015<

|M(pK?π+)?M

Λ+c |<0.050GeV/c2,and obtain a B signal yield of1.2+3.2

?2.4

events,which

is consistent with expectation of1.4±0.4events from theˉB0→Λ+cˉp decay MC,assuming our observed branching fraction.From this we estimate the other B decay contribution of (?0.1+0.9

?0.7

)events in theΛ+c signal region,which is negligibly small.From a simultaneous ?t of theˉB0→Λ+cˉp signal yield and the other B decay contribution in theΛ+c signal and sideband regions,we obtain a statistical signi?cance of5.8σ.The signi?cance is calculated as

predicts a value of the branching fraction of≤(1.1?3.1)×10?5forˉB0→Λ+cˉp,which is consistent with our measurement,while the other models[3,4,5]give substantially larger values.Charmless baryonic two-body decays are expected to be suppressed by an additional factor of|V ub/V cb|2[11].The result reported here implies that their branching fractions should not be much above the10?7level,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 acknowledge support from the Ministry of Education,Culture,Sports,Sci-ence,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 Re-sources;National Science Foundation of China under contract No.10175071;the Department of Science and Technology of India;the BK21program of the Ministry of Education of Korea and the CHEP SRC program of the Korea Science and Engineering Foundation;the Polish State Committee for Scienti?c Research under contract No.2P03B17017;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

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