Search for doubly charmed baryons and study of charmed strange baryons at Belle

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Search for doubly charmed baryons and studyof charmed strange baryons at BelleY. Kato,36 T. Iijima,37,36 I. Adachi,11 H. Aihara,62 D. M. Asner,47 T. Aushev,20 A. M. Bakich,56 A. Bala,48 Y. Ban,49V. Bhardwaj,38 B. Bhuyan,14 A. Bobrov,3 G. Bonvicini,68 A. Bozek,42 M. Brako,31,21 T. E. Browder,10 D. ervenkov,4V. Chekelian,32 A. Chen,39 B. G. Cheon,9 K. Chilikin,20 R. Chistov,20 K. Cho,24 V. Chobanova,32 Y. Choi,55 D. Cinabro,68J. Dalseno,32,58 M. Danilov,20,34 Z. Doleal,4 Z. Drsal,4 A. Drutskoy,20,34 D. Dutta,14 K. Dutta,14 S. Eidelman,3 H. Farhat,68J. E. Fast,47 T. Ferber,6 V. Gaur,57 N. Gabyshev,3 S. Ganguly,68 A. Garmash,3 R. Gillard,68 Y. M. Goh,9 B. Golob,29,21J. Haba,11 K. Hayasaka,37 H. Hayashii,38 X. H. He,49 Y. Horii,37 Y. Hoshi,60 W.-S. Hou,41 Y. B. Hsiung,41 K. Inami,36A. Ishikawa,61 Y. Iwasaki,11 T. Iwashita,38 I. Jaegle,10 T. Julius,33 J. H. Kang,70 E. Kato,61 T. Kawasaki,44 C. Kiesling,32D. Y. Kim,54 H. J. Kim,27 J. B. Kim,25 J. H. Kim,24 M. J. Kim,27 Y. J. Kim,24 J. Klucar,21 B. R. Ko,25 P. Kody,4 S. Korpar,31,21P. Krokovny,3 T. Kuhr,23 A. Kuzmin,3 Y.-J. Kwon,70 S.-H. Lee,25 J. Li,53 Y. Li,67 L. Li Gioi,32 J. Libby,15 Y. Liu,5D. Liventsev,11 D. Matvienko,3 K. Miyabayashi,38 H. Miyata,44 R. Mizuk,20,34 A. Moll,32,58 N. Muramatsu,51 R. Mussa,19Y. Nagasaka,12 E. Nakano,46 M. Nakao,11 H. Nakazawa,39 M. Nayak,15 E. Nedelkovska,32 C. Ng,62 M. Niiyama,26N. K. Nisar,57 S. Nishida,11 O. Nitoh,65 S. Ogawa,59 S. Okuno,22 P. Pakhlov,20,34 G. Pakhlova,20 C.W. Park,55 H. Park,27H. K. Park,27 T. K. Pedlar,30 T. Peng,52 R. Pestotnik,21 M. Petri,21 L. E. Piilonen,67 M. Ritter,32 M. Rhrken,23A. Rostomyan,6 H. Sahoo,10 T. Saito,61 Y. Sakai,11 S. Sandilya,57 L. Santelj,21 T. Sanuki,61 V. Savinov,50 O. Schneider,28G. Schnell,1,13 C. Schwanda,17 D. Semmler,7 K. Senyo,69 O. Seon,36 M. Shapkin,18 C. P. Shen,2 T.-A. Shibata,63J.-G. Shiu,41 B. Shwartz,3 A. Sibidanov,56 Y.-S. Sohn,70 A. Sokolov,18 E. Solovieva,20 S. Stani,45 M. Stari,21 M. Steder,6M. Sumihama,8 T. Sumiyoshi,64 U. Tamponi,19,66 K. Tanida,53 G. Tatishvili,47 Y. Teramoto,46 M. Uchida,63 S. Uehara,11T. Uglov,20,35 Y. Unno,9 S. Uno,11 C. Van Hulse,1 P. Vanhoefer,32 G. Varner,10 A. Vinokurova,3 V. Vorobyev,3M. N. Wagner,7 C. H. Wang,40 M.-Z. Wang,41 P. Wang,16 M. Watanabe,44 Y. Watanabe,22 K. M. Williams,67 E. Won,25Y. Yamashita,43 S. Yashchenko,6 Z. P. Zhang,52 V. Zhilich,3 V. Zhulanov,3 and A. Zupanc23(Belle Collaboration)1University of the Basque Country UPV/EHU, 48080 Bilbao2Beihang University, Beijing 1001913Budker Institute of Nuclear Physics SB RAS and Novosibirsk State University, Novosibirsk 6300904Faculty of Mathematics and Physics, Charles University, 121 16 Prague5University of Cincinnati, Cincinnati, Ohio 452216Deutsches ElektronenSynchrotron, 22607 Hamburg7Justus-Liebig-Universitt Gieen, 35392 Gieen8Gifu University, Gifu 501-11939Hanyang University, Seoul 133-79110University of Hawaii, Honolulu, Hawaii 9682211High Energy Accelerator Research Organization (KEK), Tsukuba 305-080112Hiroshima Institute of Technology, Hiroshima 731-519313IKERBASQUE, Basque Foundation for Science, 48011 Bilbao14Indian Institute of Technology Guwahati, Assam 78103915Indian Institute of Technology Madras, Chennai 60003616Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 10004917Institute of High Energy Physics, Vienna 105018Institute for High Energy Physics, Protvino 14228119INFN - Sezione di Torino, 10125 Torino20Institute for Theoretical and Experimental Physics, Moscow 11721821J. Stefan Institute, 1000 Ljubljana22Kanagawa University, Yokohama 221-868623Institut fr Experimentelle Kernphysik, Karlsruher Institut fr Technologie, 76131 Karlsruhe24Korea Institute of Science and Technology Information, Daejeon 305-80625Korea University, Seoul 136-71326Kyoto University, Kyoto 606-850227Kyungpook National University, Daegu 702-70128cole Polytechnique Fdrale de Lausanne (EPFL), Lausanne 101529Faculty of Mathematics and Physics, University of Ljubljana, 1000 Ljubljana30Luther College, Decorah, Iowa 5210131University of Maribor, 2000 Maribor32Max-Planck-Institut fr Physik, 80805 MnchenPHYSICAL REVIEW D 89, 052003 (2014)1550-7998=2014=89(5)=052003(14) 052003-1 2014 American Physical Society33School of Physics, University of Melbourne, Victoria 301034Moscow Physical Engineering Institute, Moscow 11540935Moscow Institute of Physics and Technology, Moscow Region 14170036Graduate School of Science, Nagoya University, Nagoya 464-860237Kobayashi-Maskawa Institute, Nagoya University, Nagoya 464-860238Nara Womens University, Nara 630-850639National Central University, Chung-li 3205440National United University, Miao Li 3600341Department of Physics, National Taiwan University, Taipei 1061742H. Niewodniczanski Institute of Nuclear Physics, Krakow 31-34243Nippon Dental University, Niigata 951-858044Niigata University, Niigata 950-218145University of Nova Gorica, 5000 Nova Gorica46Osaka City University, Osaka 558-858547Pacific Northwest National Laboratory, Richland, Washington 9935248Panjab University, Chandigarh 16001449Peking University, Beijing 10087150University of Pittsburgh, Pittsburgh, Pennsylvania 1526051Research Center for Electron Photon Science, Tohoku University, Sendai 980-857852University of Science and Technology of China, Hefei 23002653Seoul National University, Seoul 151-74254Soongsil University, Seoul 156-74355Sungkyunkwan University, Suwon 440-74656School of Physics, University of Sydney, NSW 200657Tata Institute of Fundamental Research, Mumbai 40000558Excellence Cluster Universe, Technische Universitt Mnchen, 85748 Garching59Toho University, Funabashi 274-851060Tohoku Gakuin University, Tagajo 985-853761Tohoku University, Sendai 980-857862Department of Physics, University of Tokyo, Tokyo 113-003363Tokyo Institute of Technology, Tokyo 152-855064Tokyo Metropolitan University, Tokyo 192-039765Tokyo University of Agriculture and Technology, Tokyo 184-858866University of Torino, 10124 Torino67CNP, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 2406168Wayne State University, Detroit, Michigan 4820269Yamagata University, Yamagata 990-856070Yonsei University, Seoul 120-749(Received 3 December 2013; published 17 March 2014)We report results of a study of doubly charmed baryons and charmed strange baryons. The analysis isperformed using a 980 fb1 data sample collected with the Belle detector at the KEKB asymmetric-energyee collider. We search for doubly charmed baryons cc with the c K and 0c finalstates. No significant signal is observed. We also search for two excited charmed strange baryons,c3055 and c3123 with the c 2455K and c 2520K final states. The c3055 signal isobserved with a significance of 6.6 standard deviations including systematic uncertainty, while no signatureof the c3123 is seen. We also study properties of the c2645 and measure a width of2.6 0.2stat 0.4syst MeV=c2, which is the first significant determination.DOI: 10.1103/PhysRevD.89.052003 PACS numbers: 14.20.Lq, 14.20.-c, 14.20.GkI. INTRODUCTIONIn recent years, there has been significant progress incharmed baryon spectroscopy, mainly by the Belle andBABAR experiments [18]. In particular, all the ground statesof the single-charmed baryons predicted by the constituentquarkmodel andseveral excited stateshavebeenobserved [9].However, there are no experimentally established doublycharmed baryons. The lightest doubly charmed baryoncontains two charm quarks and one up or down quark(cc ccd, cc ccu), and the spin-parity of the groundstate is expected to be 12. The mass of the cc has beenextensively studied theoretically, and the prediction of thequark model ranges from 3.48 GeV=c2 to 3.74 GeV=c2KATO et al. PHYSICAL REVIEW D 89, 052003 (2014)052003-2[1022], whereas the mass predicted by lattice QCD rangesfrom 3.51 GeV=c2 to 3.67 GeV=c2 [2327]. The crosssections of the cc production in the process ee ccXatffiffiffisp 10.58 GeV, where X denotes the remainingparticles produced in the fragmentation, is predicted tobe 70 fb in Ref. [28] and 230 fb in Ref. [29]. The crosssection of the pair production of the cc and c c diquarks ispredicted to be 7 fb [30].There have been several experimental studies tosearch for the cc. The SELEX collaboration reportedevidence for the cc in the c K [31] and pDK[32] final states with a mass of about 3.52 GeV=c2using a 600 GeV=c charged hyperon beam. However,the results have not been supported by FOCUS [33],BABAR [34], Belle [2] nor LHCb [35]. The BABARcollaboration searched for the cc in the c Kand 0c decay modes with a 232 fb1 data sampleof ee collisions at or near the 4S. They found noevidence for the cc and set an upper limit on theproduct of the production cross section and branchingfractions of cc and c or 0c to be a few fb, dependingon the decay mode. In our search for the cc in thec K final state with a 462 fb1 data sample of Belleat or near the 4S [2], Belle also found no evidencefor the cc and set an upper limit on ee ccX Bcc c K=ee c X of 1.5 104 witha pc > 2.5 GeV=c requirement. Here, pc is the momentum of the c in the center-of-mass(CM) frame.In this paper, we report on an improved search for the ccin its weak decays to the c K and 0c finalstates. The Belle collaboration has collected a data samplewith a total integrated luminosity of 980 fb1, which isaround two (four) times the statistics of the previous ccsearch by Belle [2] (BABAR [34]) and supersedes the resultsin Ref. [2]. Furthermore, in the previous studies, the cand the 0c states have been reconstructed only fromdecay modes of pK and , respectively. Weincorporate additional decay modes to improve the stat-istical sensitivity.The same data sample can be used to study charmedstrange baryons, as the c K and the 0c final statesare strong decay modes of excited c (c ) states. TheBABAR collaboration found two c states, c3055 andc3123, decaying to the c K final state throughintermediate c2455 or c2520 states using adata sample of 384 fb1 [6]. Their statistical significancewas 6.4 standard deviations () and 3.6, respectively. Aconfirmation of these states in other experiments isnecessary. The 0c is a strong decay mode of thec2645. Currently, only the upper limit of3.1 MeV=c2 exists for its width [36]. In this paper, wealso report on a search for the c3055 and c3123 inthe c K final state, and the measurement of the widthof the c2645.The remaining sections of the paper are organized asfollows. In Sec. II, the data samples and the Belle detectorare described. In Sec. III, a study of the final states with c ,i.e., the cc search and the c3055 and c3123search, are reported. In Sec. IV, a study of the final statewith 0c i.e., the cc search and measurement of the width ofthe c2645, are described. Finally, conclusions aregiven in Sec. V.II. DATA SAMPLES AND THE BELLE DETECTORWe use a data sample with a total integrated luminosityof 980 fb1 recorded with the Belle detector at the KEKBasymmetric-energy ee collider [37]. The data sampleswith different beam energies at or near the 1S to 5Sare combined in this study. The beam energies andintegrated luminosities are summarized in Table I. Theluminosity-weighted average offfiffiffispis 10.59 GeV.The Belle detector is a large-solid-angle magneticspectrometer that consists of a silicon vertex detector(SVD), a 50-layer central drift chamber (CDC), an arrayof aerogel threshold Cherenkov counters (ACC), a barrel-like arrangement of time-of-flight scintillation counters(TOF), and an electromagnetic calorimeter comprised ofCsI(Tl) crystals (ECL) located inside a superconductingsolenoid coil that provides a 1.5 T magnetic field. An ironflux return located outside of the coil is instrumented todetect K0L mesons and to identify muons (KLM). Thedetector is described in detail elsewhere [38]. Two innerdetector configurations were used. A 2.0 cm radius beam-pipe and a 3-layer silicon vertex detector were used for thefirst sample of 156 fb1, while a 1.5 cm radius beampipe, a4-layer silicon detector and a small-cell inner drift chamberwere used to record the remaining 824 fb1 [39].The selection of charged hadrons is based on informationfrom the tracking system (SVD and CDC) and hadronidentification system (CDC, ACC, and TOF). The chargedproton, kaon, and pion that are not associated with long-lived particles like K0S, and , are required to have apoint of closest approach to the interaction point that iswithin 0.2 cm in the transverse (r-) direction and within2 cm along the z-axis. (The z-axis is opposite the positronbeam direction.) For each track, the likelihood values Lp,TABLE I. Summary of the integrated luminosities and beam energies.ffiffiffisp5S/near it 4S/near it 3S/near it 2S/near it 1S/near itIntegrated luminosity (fb1) 121.0=29.3 702.6=89.5 2.9=0.3 24.9=1.8 5.7=1.8SEARCH FOR DOUBLY CHARMED BARYONS AND STUDY PHYSICAL REVIEW D 89, 052003 (2014)052003-3LK , and L are provided for the assumption of proton,kaon, and pion, respectively, from the hadron identificationsystem, based on the ionization energy loss in the CDC, thenumber of detected Cherenkov photons in the ACC, and thetime of flight measured by the TOF. The likelihood ratio isdefined as Lij Li=Li Lj and a track is identifiedas a proton if the likelihood ratios Lp and LpK aregreater than 0.6. A track is identified as a kaon if thelikelihood ratios LK and LKp are greater than 0.6.A track is identified as a pion if the likelihood ratiosLK and Lp are greater than 0.6. In addition,electron (Le) likelihood is provided based on informationfrom the ECL, ACC, and CDC [40]. A track with anelectron likelihood greater than 0.95 is rejected.The momentum averaged efficiencies of hadron identi-fication are about 90%, 90%, and 93% for pions, kaons, andprotons, respectively. The momentum averaged probabilityto misidentify a pion (kaon) track as a kaon (pion) track isabout 9 (10)%, and the momentum averaged probability tomisidentify a pion or kaon track as a proton track isabout 5%.We use a Monte-Carlo (MC) simulation events generatedwith EVTGEN [41], JETSET [42] with final QED finalstate radiation by PHOTOS [43] and then processed by aGEANT3 [44] based detector simulation to obtain thereconstruction efficiency and the mass resolution.III. FINAL STATE WITH THE CIn this section, the analysis using the final states with thec baryon is described. Reconstruction of the c candi-date is explained first, followed by the description ofthe cc search in its decay into c K and thestudy of two charmed strange baryons, c3055 andc3123. Throughout this paper, the selection criteria aredetermined to maximize the figure of merit (FOM), definedas =ffiffiffiffiffiffiffiffiNbgp, where is the cc efficiency for the selectioncriteria and Nbg is the number of background events underthe signal peak except for the scaled momentum selectionfor the c3055 and c3123 search, which followedBABARs analysis. The distribution of background events isestimated based on data. When the selection criteria aredetermined, we hide the possible signal peak by smearingthe invariant mass of the cc candidates event by event witha Gaussian having a 50 MeV=c2 width in order to avoidany biases.A. Reconstruction of the cThe c candidates are reconstructed in the pK andpK0S decay modes [45]. The K0S candidate is reconstructedfrom its decay into . A pair of oppositely chargedpions that have an invariant mass within 8 MeV=c2 of thenominal K0S mass, which corresponds to approximately3.5 of the mass resolution, is used. Two pion tracks arefitted to a common vertex. The result of the fit is used tosuppress misreconstructed K0S candidates and to performfurther vertex fit of the c pK0S. The vertex of the twopions for the K0S is required to be displaced from theinteraction point (IP) in the direction of the pion pairmomentum [46]. The daughters of the c are fitted to acommon vertex; the invariant mass of the daughters must bewithin 56 MeV=c2, or 1.5, nominal c mass for thepK (pK0S) decay mode. The 2 value of the commonvertex fit of the c is required to be less than 50. For theremaining candidate, a mass constraint fit to the c mass isperformed to improve the momentum resolution. As thesignal-to-background ratio for the c candidates is similarfor the pK and pK0S decay modes, they are combined inthe following analysis. By including the pK0S mode inaddition to the pK mode, the yield of the c isincreased by about 20%.B. Search for doubly charmed baryons in c KWe search for the cc in its decay intoc K inthe mass range of 3.24.0 GeV=c2. The expected mass re-solution of the cc estimated from MC is 2.03.5MeV=c2,depending on the mass of the cc (degrading with in-creasing mass). In order to reduce the combinatorialbackground, a selection on the scaled momentum xp pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffis=4 m2pis used, where p is the CM momentum of acc candidate and s is CM energy squared andm is mass ofthe cc candidate. As there is no measurement of the xpspectrum for cc production, we assume it to be the same asthat of the c , which has been precisely measured [47].The xp spectrum is represented by a smooth polynomialfunction and is used to generate a MC sample for the ccsignal. Decays of the cc and c are generated accordingto the available phase space distribution. The number ofbackground events as a function of the xp cut is estimatedbased on smeared data. The FOM as a function of the xp cutis surveyed in the search region. The optimization pro-cedure yields 0.5 < xp < 1.0 regardless of the cc mass.To check the validity of our analysis, we independentlyexamine the xp spectrum of the c and confirm that it isconsistent with that presented in Ref. [47].Figure 1(a) and 1(b) show the Mc K andMc K distributions, respectively, for data afterall the event selections are applied. No significant signal isseen in the data for either cc or cc . The statisticalsignificance for a given mass is evaluated with an unbinnedextended maximum likelihood (UML) fit. The probabilitydensity function (PDF) for the signal is describedwith signalMCgenerated for eachgivenccmass,whereas a third-orderpolynomial function is used as the background PDF. Thestatistical significance is defined asffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2 ln L0=Lp, whereL0 is the likelihood for the fit without the signal componentand L is the likelihood for the fit with the signal componentincluded. The significance is evaluated for the cc massKATO et al. PHYSICAL REVIEW D 89, 052003 (2014)052003-4scanned with a 1 MeV=c2 step in the search region. Noneof the mass points give a significance exceeding the3 level.The 95% confidence level (C.L.) upper limit for theproduct of the cross section and branching fraction to thecK state produced with the 0.5 < xp < 1.0condition,B ee cc Xis evaluated. Here, L is the total integrated luminosity, Nsigis the cc signal yield, BpK is the branching fraction ofthec pK (which amounts to 0.050 0.013), BpKSis the branching fraction of c pK0S measured relativeto the pK mode (BpKS=BpK 0.24 0.02), andpKpKS is the reconstruction efficiency for the c pK (c pK0S) decay mode evaluated as a functionof the cc mass. The efficiencies for the cc as a functionof their masses are shown in Fig. 2. The factor of two in thedenominator comes from inclusion of the charge-conjugatemode. By including this factor, our measurement can becompared with the theoretical predictions [28,29]; while tocompare with the prediction in Ref. [30], it is necessary tomultiply our B measurement by 2 because they predictedthe cross section of the pair production of the cc and c cdiquarks. In BABARs measurement [34], they do notintroduce the factor of two, i.e., they report an upper limitfor the sum of the ee cc X and its charge-conjugate mode). Therefore, our measurement should bedoubled when comparing with BABARs result. We notethat the cross section reported here and elsewhere in thispaper is a visible cross section (i.e., a radiative correction isnot applied.The upper limit is evaluated following the Bayesianapproach. First, we scan the likelihood profile by determin-ing the likelihood values as a function of the B (LB),M(c+K-+) (GeV/c2)020040060080010001200 (a) (b)(c) (d)3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4Events/(0.001GeV/c2 )M(c+K-++) (GeV/c2)020040060080010003.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4Events/(0.001GeV/c2 )0510152025303.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4Mass (GeV/c2) B (fb)0510152025303.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4Mass (GeV/c2) B (fb)FIG. 1 (color online). Invariant mass distribution of the cc candidates for (a)Mc K, (b)Mc K; the vertical error barsare from data and the dashed histogram are from signal MC for the cc signal generated with a mass of 3.60 GeV=c2 and a productioncross section ee cc X of 500 fb and Bcc c K of 5%. 95% C.L. upper limit of B as a function of the masswith a 1 MeV=c2 step for (c) cc and (d) cc .SEARCH FOR DOUBLY CHARMED BARYONS AND STUDY PHYSICAL REVIEW D 89, 052003 (2014)052003-5varying Nsig from zero up to the Nsig value for which thelikelihood drops to zero. Then, LB is convolved with aGaussian whose width equals the systematic uncertaintiesof B. The B value for which the integral (starting fromB 0) becomes 95% of the entire area is regarded as the95% C.L. upper limit.We consider the following systematic uncertainties in thecc search. The systematic uncertainty due to the efficiencyof pion and kaon identification is estimated from the ratioof the yield of the D D0, D0 K with andwithout the pion/kaon identification requirements for dataandMC. The difference of the ratio between data andMC iscorrected and the statistical error of the ratio is regarded asthe systematic uncertainty. The systematic uncertainty dueto the efficiency of proton identification is estimated usingthe ratio of the yield of the p with and without theproton identification requirement. The difference of theratio between data and MC is corrected and the statisticalerror of the ratio is regarded as the systematic uncertainty.The systematic uncertainty due to the charged trackreconstruction efficiency is estimated using the decay chainD D0, D0 K0S, and K0S , whereK0S is either partially or fully reconstructed. Theratio of the yields for partially and fully reconstructedsignals in data and MC is compared, and the difference istaken as the systematic uncertainty. This amounts to 0.35%per track. The systematic uncertainty of the total integratedluminosity is 1.4%. To check the systematic error due to thesignal PDF, we compare the mass resolution of the c indata and MC. We find that the resolution for data is 5%larger than in MC. To monitor the effect of this discrepancy,we perform a pseudoexperiment test in which we extractthe signal yield with correct PDF and one that is narrowerby 5%. The largest difference of 3% measured in this test isregarded as the systematic uncertainty. The systematicuncertainty related to the c branching fraction is propa-gated from the errors taken from the PDG [9]. To estimatethe systematic uncertainty of the reconstruction efficiencydue to the possible difference of xp spectrum between ourassumption (the same as that of c ) and the actual one, weexamine the xp dependence of the reconstruction effi-ciency. The root mean square of the reconstruction effi-ciency in the region of 0.5 < xp < 1.0 is regarded as thesystematic uncertainty. The elements of the systematicuncertainty for the measurement of the B are enumeratedin the first and second columns of Table II.Figures 1(c) and 1(d) show the 95% C.L. upper limit onB for cc and cc , respectively, as a function of the mass00. (b)3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9Mass (GeV/c2)Efficiency00. 3.3 3.4 3.5 3.6 3.7 3.8 3.9Mass (GeV/c2)EfficiencyFIG. 2. Reconstruction efficiency as a function of the cc mass, for (a) cc, (b) cc . Circular points are for c pK and squarepoints are for c pK0S. The lines show the result of the fit with a linear function.TABLE II. Summary of the systematic uncertainties in the B and B2 measurement (%).Source cc with c cc with c c3123 cc with 0c cc with 0cParticle ID 2.0 2.4 2.0 3.5 3.8Tracking 1.8 2.1 1.8 1.8 2.1Signal PDF 3.5 3.5 28.0 3.5 3.5Luminosity 1.4 1.4 1.4 1.4 1.4B 26.0 26.0 1.6 0.7 0.7xp 2.1 2.3 1.2 6.0 5.7Nic2645 4.3 4.3total 26.5 26.6 28.2 9.2 9.2KATO et al. PHYSICAL REVIEW D 89, 052003 (2014)052003-6with a 1 MeV=c2 step. The upper limit is in the range of4.125.0 fb for the cc and 2.526.5 fb for the cc .C. Search for the c 3055 and c 3123In this section, a search for the c 3055 and c 3123is described. Here, we require xp to be greater than 0.7. Inthe analysis by BABAR [6], they required pc K >2.9 GeV=c, which is similar to our xp cut as illustrated bythe pc K distribution, with the xp cut and2.9 GeV=c2 < Mc K < 3.2 GeV=c2 required asshown in Fig. 3(a). Figure 3(b) shows the Mc distribution, where contributions from the c2455and the c2520 baryons are clearly visible. We selectthe c2455 [c2520] region by requiringjMc mc j < 518 MeV=c2, where mc is thenominal mass of the c2455 or c2520.Figure 3(c) shows the Mc K distribution forthe c2455 signal region together with the sameplot for the c2455 sideband region, defined asjMc mc2455 15 MeV=c2j < 5 MeV=c2.Clear peaks corresponding to the c2980, c3055,and c3080 are seen. To obtain the statistical signifi-cance of the c3055, an UML fit is applied. PDFs forthe c components are represented by a Breit-Wignerline shape convolved with a Gaussian to account forthe invariant-mass resolution (res). Using the signal MCevents, we estimate res to vary from 1.2 to 1.8 MeV=c2,depending on the masses of the c states. The width andmean of the Breit-Wigner functions are treated as freep*(c+K-+)(GeV/c)01002003004005006002.5 3 3.5 4 4.5 5Events/(0.025 GeV/c)M(c++) (GeV/c2)01002003004005006007008002.42 2.44 2.46 2.48 2.5 2.52 2.54 2.56 2.58 2.6500Evens/(0.001GeV/c2 )(a) (b))2) (GeV/c+-K+cM(2.95 3 3.05 3.1 3.15 3.2 )2Events / ( 0.004 GeV/c020406080100120140 (c))2) (GeV/c+-K+cM(2.95 3 3.05 3.1 3.15 3.2 )2Events / ( 0.004 GeV/c020406080100120140160180200220(d)FIG. 3 (color online). (a) The pc K distribution from data. (b) TheMc distribution. The vertical lines show the selectedregions of the c2455 and c2520. (c) The Mc K distribution with c2455 selection. The dots with error barsshow the distribution for the c2455 selected region whereas the rectangles show the distribution for the c2455 sidebandregion. The solid line shows the fit result. The dashed, dotted, and dash-dotted lines show the contributions from the background,c3055, and c2980, or c3080, respectively. (d) The Mc K distribution with c2520 selection. The dots witherror bars show the distribution for c2520 selected region whereas the rectangles show the distribution for the c2520sideband region. The solid line shows the fit result. The dashed, dotted, and dash-dotted lines show the contributions from thebackground, c3123, and c3080, respectively.SEARCH FOR DOUBLY CHARMED BARYONS AND STUDY PHYSICAL REVIEW D 89, 052003 (2014)052003-7parameters. The background PDF, f1x, is modeled with athreshold function:f1x 1 expx x0=mx=x0abx=x0 1 if x > x0f1x 0 if x < x0; (1)where a, b, x0, and m are free parameters in the fit.The fit result is shown in Fig. 3(c). To estimate thestatistical significance of the c3055, we comparethe likelihood values for the fits with and without thec3055 component. The obtained 2 ln L0=L value is54.7. By taking into account the change of the numberof degrees of freedom (ndf) by the inclusion of thec3055 component, the statistical significance of thec3055 becomes 6.8. The 2=ndf of the fit withthe c3055 component, for the binning of Fig. 3(c), is54.8/61.Figure 3(d) shows the Mc K distribution forthe c2520 selected region together with the sameplot for the c2520 sideband region, defined asjMc mc2520 27 MeV=c2j < 12 MeV=c2.A clear peak corresponding to the c3080 is seen, whileno peak structure is seen in the mass near 3.123 GeV=c2.An UML fit is applied to extract the signal yield. Again,the c components are represented by a Breit-Wignerfunction convolved with a Gaussian. For the c3080component, the mass and width of the Breit-Wignerare treated as free parameters; while for the c3123component, the mass and width are fixed to the valuesobtained in Ref.[6]. The background shape, f2x, isassumed to be:f2x 1 expx x02=mx=x0abx=x0 1 cx=x02 1 if x > x0f2x 0 if x < x0; (2)where a, b, c, x0, and m are fit parameters. The 2=ndfof the fit with the c3123 component for the binningof Fig. 3(d) is 28.6=42. The yield of the c3123 is8 22 events, which is consistent with zero. Hence, a95% C.L. upper limit for the production cross section isevaluated with the method described in the previoussection. To directly compare with the BABAR result inRef. [6], the upper limit for the product of the crosssection and branching fraction ofc producedwith xp > 0.7condition,Bc ee c3123X Bc pKis evaluated. As in Ref. [6], we assume Bc3123 c2520K is equal to 1.To take the uncertainty of the c3123 mass andwidth from Ref. [6] into account, we perform a pseu-doexperiment test. The background and c3080 con-tributions are generated with statistics similar to dataand based on the fit result. The c3123 component isgenerated with mass and width changed by 1, corre-sponding to their measured uncertainties (3122.9 1.30.4 MeV=c2 for the mass and 4.4 3.4 1.7 MeV=c2for the width). The yield of c3123 is extracted byfitting pseudoexperiment data with the procedure usedfor data. The ratio of the generated and extracted yield isregarded as a systematic uncertainty. Because the errorof the width is relatively large, its systematic uncertaintycontribution is dominant (28%). All of the systematicerrors are summarized in the third column of Table II.The 95% C.L. upper limit on Bc is 0.17 fb. As in thecase of the cc cross section, the measurement in Ref. [6]does not introduce a factor of two for the Bc calcu-lation. Therefore, we should double our measurement,which results in 0.34 fb, when comparing with BABARsresult. The value is much smaller than that quoted inRef. [6] (1.6 0.6 0.2 fb).The systematic uncertainties of the masses and widthsof the c and stability of the statistical significance ofthe c3055 are studied by the following fittingconfigurations. The systematic uncertainties due to thesignal PDF are studied by varying res by 5%. Thesystematic uncertainties due to possible interferencebetween the c3055 and c3080 are studied byfitting the distribution with an additional phase parameterbetween the two Breit-Wigner amplitudes. The system-atic uncertainty due to the background shape is studiedby fitting the mass spectra with a second-order poly-nomial as a background PDF in the range of3.0053.200 GeV=c2. In none of these fitting configu-rations does the statistical significance of the c3055fall below 6.6. We apply cut conditions of xp > 0.6 andxp > 0.8 instead of xp > 0.7 and reextract the masses andwidths of the c states. The differences from the defaultcut condition are regarded as systematic uncertainties.The measured masses, widths, and yields of the three cstates are summarized in Table III. All of these mea-surements are consistent with previous Belle measure-ment [2] within 2.5 and with the BABAR measurement[6] within 2.0.TABLE III. The measured masses and widths of the three cstates. The first error is statistical and second is systematic.Particle Mass (MeV=c2) Width (MeV=c2) Yieldc2980 2974.9 1.5 2.1 14.8 2.5 4.1 244 39c3055 3058.1 1.0 2.1 9.7 3.4 3.3 199 46c3080 (c) 3077.9 0.4 0.7 3.2 1.3 1.3 185 31c3080 (c) 3076.9 0.3 0.2 2.4 0.9 1.6 210 30KATO et al. PHYSICAL REVIEW D 89, 052003 (2014)052003-8IV. FINAL STATE WITH 0cIn this section, the analysis of the final state with the 0cis described. The reconstruction of the 0c is presentedfirst, followed by the analysis of the c2645. Finally,a search for cc decaying into the 0c final state isdescribed.A. Reconstruction of 0cThe 0c is reconstructed in three decay modes: ,K, and pKK. The is reconstructed from itsdecay intop. The proton and tracks for candidates arefitted to a commonvertex.The fitting result is used to suppressmisreconstructed candidates and to perform the subsequentvertex fit for the or 0c K. The invariantmass of the candidate is required to bewithin 3 MeV=c2 ofthe nominal mass, which corresponds to approximately 3of the mass resolution. The selection based on their decayvertex information is also applied [48]. The is recon-structed from its decay into . The and tracks for candidates are fitted to a common vertex. The fitting result isused to clean up the candidates and in the commonvertexfit for the 0c . The closest distance of the and along the z-direction is required to be less than 3 mm. Werequire cos > 0.95, where is the angle between themomentum vector of the and the vector between the IPand the decay vertex. The 2 of the common-vertex fit ofthe is required to be less than 50. The invariant mass of a candidate is required to be within 4 MeV=c2 of thenominal mass, which corresponds to approximately 3 ofthemass resolution. The daughter particles of the0c are fittedto a common vertex. The 0c candidates are selected byrequiring invariant masses of the daughter particles withcommon vertex fit to be within 12, 7, and 7 MeV=c2 ofthe nominal 0c mass for the , K, and pKKdecay modes, respectively, which correspond to approxi-mately 1.5 of the mass resolution. The 2 value of thecommonvertex fit for the0c is required to be less than 50.Themass constraint fit to the 0c mass is performed.We optimize the selection criteria for xp in the 0csystem with the method described in Sec. III B, againassuming that the xp spectrum for the cc is the same as thatfor the c . We require 0.45 < xp < 1.0 independent of thecc mass and the 0c decay mode. The same cut is appliedfor the analysis of the c2645.B. Study of the c 2645Unlike the c study, the signal-to-background ratio ofthe 0c largely depends on the decay modes. Therefore, toimprove our sensitivity for the cc, we perform a simulta-neous fit to the mass spectra for the three 0c decays withfixed relative signal ratios. We use the relative yields of thec2645 0c measured for the 0c decay modes toestimate a relative signal yield of the cc. The relativesignal yields of the cc (Nicc) in a given 0c decay channelcan be written asNicc Nic2645iccic2645; (3)where Nic2645 is the c2645 yield, icc is the recon-struction efficiency of the cc, and ic2645 is thereconstruction efficiency of thec2645. Both efficienciesinclude the secondary branching fractions for p of63.9 0.5% and of 99.887 0.035%. Theindex i denotes the decay mode of the 0c.Figure 4 shows the M0c) distribution for each 0cdecay mode below the cc search region. Clear peakscorresponding to the c2645 are seen in all decaymodes. The bump structures near 2.68 GeV=c2 originatefrom the process c2790 c00 0c with a missing in the reconstruction. The simultaneous UML fit isapplied to extract the relative yields and the width of thec2645. The c2645 signal is represented by a)2) (GeV/c+0cM(2.6 2.62 2.64 2.66 2.68 2.7 2.72 2.74 )2Events / ( 0.0015 GeV/c050100150200250300350400 (a))2) (GeV/c+0cM(2.6 2.62 2.64 2.66 2.68 2.7 2.72 2.74 )2Events / ( 0.0015 GeV/c0100200300400500 (b))2) (GeV/c+0cM(2.6 2.62 2.64 2.66 2.68 2.7 2.72 2.74 )2Events / ( 0.0015 GeV/c050100150200250300350400 (c)FIG. 4 (color online). M0c distributions below the cc search region for (a) 0c , (b) 0c K, (c) 0c pKK.The solid lines show the fit result. The dashed, dotted, and dash-dotted lines show the contributions from background, c2645, andc2790, respectively.SEARCH FOR DOUBLY CHARMED BARYONS AND STUDY PHYSICAL REVIEW D 89, 052003 (2014)052003-9Breit-Wigner function convolved with a Gaussian whosewidth corresponds to the mass resolution res. The value ofres is 1.05 MeV=c2, independent of the decay modes ofthe 0c. The PDF of the c2790 reflection is modeledusing MC. f2x in Eq. (2) is used as the background PDFfor the 0c decay mode whereas f1x in Eq. (1) isused for the 0c K and 0c pKK decaymodes. The width and mass of the c2645 are con-strained to be the same for the three decay modes. The yieldof the c2645 is 1298 51, 1444 58, and 974 47for the , K, and pKK decay mode,respectively. The mass and width are obtained to be2645.4 0.1 MeV=c2 and 2.6 0.2 MeV=c2, respec-tively. The 2=ndf of the fit for the binning in Fig. 4 is296/276.To check the consistency of the width measurementbetween the 0c decay modes, we fit the three mass spectraseparately. The measured widths are found to be consistentbetween the three decay modes: 2.9 0.3 MeV=c2,2.60.3MeV=c2, and 2.50.3MeV=c2 for 0c,K, and pKK, respectively. The measured widthis found to be consistent for the three decay modes. Thesystematic uncertainty of the width measurement due to thefit procedure is studied with pseudoexperiment eventssamples: the c2645 component is generated accordingto the signal MC sample with the natural width of2.6 MeV=c2 by signal MC, while contributions from back-ground and c2790 reflection are generated based on thefit result with the real data. The statistics of the pseudoexperi-ment samples are the same as for those of data. The widthof the c2645 is extracted from simultaneous fits topseudoexperiment samples, and its mean value is obtained tobe 2.75 0.03 MeV=c2, which is higher than the inputvalue by 0.15 MeV=c2. The difference is included as thesystematic uncertainty from the fit procedure. The systematicuncertainty due to the background shape is studied by fittingthe data with a second-order polynomial function forthe alternative background shape. The fit region is restrictedto 2.622.75 GeV=c2. The width is obtained to be2.9 0.2 MeV=c2, which is 0.3 MeV=c2 higher than thedefault measurement. This deviation is included as asystematic uncertainty. To check the systematic uncertaintydue to the mass resolution, we evaluate the ratio of theresolution of the 0c in the data and MC,datamc, where data isthe resolution of 0c for data and mc is that for MC. Anadditional cut of 2.64GeV=c201020304050607080 (a) (b)(c) (d)3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4M(c0+) (GeV/c2)Events/(0.002GeV/c2 )02550751001251501752002253.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 43.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4M(c0+) (GeV/c2)Events/(0.002GeV/c2 )0204060801001201401603.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4M(c0+) (GeV/c2)Events/(0.002GeV/c2 ) (GeV/c2) B (fb)2FIG. 6 (color online). (a)(c): M0c distribution in the cc search region for 0c (a) , (b) K, (c) pKK. Thevertical error bars are from data. The dashed histograms are from signal MC. (d): 95% C.L. upper limit of the B2 for cc as a function ofthe mass with a 1 MeV=c2 step. (b)0.23.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4Mass (GeV/c2)Efficiency00. 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4Mass (GeV/c2)EfficiencyFIG. 5. Reconstruction efficiencies for the cc as a function of the cc mass for (a) cc, (b) cc . Circles, square, and triangle points arefor 0c , K, and pKK, respectively. The lines are the result of the fit with a linear function.SEARCH FOR DOUBLY CHARMED BARYONS AND STUDY PHYSICAL REVIEW D 89, 052003 (2014)052003-11The 95% C.L. upper limit for the product of the cross section and branching fractions produced with 0.45 < xp < 1.0condition,B2 ee cc X Bcc 0c B0c Nsig2L cc1 NKccNcc NpKKccNcc ;is evaluated with the same method as in Sec. III B. Inaddition to the sources from the study with the c , twoothers are included here. The systematic uncertainty fromthe reconstruction efficiency is estimated to be 3% usingthe yield of the B K with and without the require-ment using decay vertex information. The systematicuncertainties related to Nic2645 are taken from theirstatistical errors. The systematic uncertainties are summa-rized in the fourth column in Table II. Figures 6 and 7(d)show B2 for the cc as a function of the mass with a1 MeV=c2 step. The 95% C.L. upper limit on B2 is 0.0760.35 fb for the cc and 0.0820.40 fb for the cc .V. CONCLUSIONWe have presented a search for doubly charmed baryonsand a study of the charmed strange baryons c3055,c3123 and c2645 using the full data sample(980 fb1) collected with the Belle detector. The searchfor doubly charmed baryons is an improved study of ourprevious work [2]. We use about two times statistics ofprevious work and several additional decay modes thatwere not studied in the previous work.We search for the cc in the c K and0c final states. The c is reconstructed from thepK and pK0S decay modes. We do not find anysignificant cc signal and set a 95% C.L. upper limit on0102030405060708090(a) (b)(c) (d)3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4M(c0++) (GeV/c2)Events/(0.002GeV/c2 )0501001502002503003.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 43.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4M(c0++) (GeV/c2)Events/(0.002GeV/c2 )0204060801001201403.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4M(c0++) (GeV/c2)Events/(0.002GeV/c2 ) (GeV/c2) B2 (fb)FIG. 7 (color online). (a)(c): M0c distribution in the cc search region for 0c (a) , (b) K, (c) pKK. Thevertical error bars are from data. The dashed histograms are from signal MC. (d): 95% C.L. upper limit of the B2 for cc as a functionof the mass with a 1 MeV=c2 step.KATO et al. PHYSICAL REVIEW D 89, 052003 (2014)052003-12ee cc X Bcc c K with thescaled momentum 0.5 < xp < 1.0: 4.125.0 fb for cc and2.526.5 fb for cc . We also search for the cc in the0c final state. The 0c is reconstructed from the, K, and pKK decay modes. We do notfind any significant cc signal and set a 95% C.L. upperlimit on ee cc X Bcc 0cB0c with the scaled momentum 0.45