First measurement of the Lambda(+)(c) -> p eta ' decay

JHEP03(2022)090 Published for SISSA by Springer Received:December 30, 2021 Accepted:February 25, 2022 Published:March 15, 2022 First measurement of the Λ+ c→pη0decay BELLE The BELLE collaboration S. X. Li,14 J. X. Cui,14 C. P. Shen,14 I. Adachi,20,16 H. Aihara,84 S. Al Said,79,39 D. M. Asner,3H. Atmacan,8T. Aushev,22 R. Ayad,79 V. Babu,9P. Behera,27 K. Belous,29 M. Bessner,19 V. Bhardwaj,24 B. Bhuyan,25 T. Bilka,5D. Bodrov,22,45 G. Bonvicini,88 J. Borah,25 A. Bozek,60 M. Bračko,49,36 P. Branchini,32 T. E. Browder,19 A. Budano,32 M. Campajola,31,56 D. Červenkov,5M.-C. Chang,13 P. Chang,59 V. Chekelian,50 A. Chen,58 B. G. Cheon,18 K. Chilikin,45 H. E. Cho,18 K. Cho,41 S.-J. Cho,90 S.-K. Choi,7Y. Choi,77 S. Choudhury,34 D. Cinabro,88 S. Cunliffe,9S. Das,48 N. Dash,27 G. De Nardo,31,56 G. De Pietro,32 R. Dhamija,26 F. Di Capua,31,56 Z. Doležal,5T. V. Dong,11 D. Epifanov,4,64 T. Ferber,9 D. Ferlewicz,51 B. G. Fulsom,66 R. Garg,67 V. Gaur,87 N. Gabyshev,4,64 A. Giri,26 P. Goldenzweig,37 B. Golob,46,36 E. Graziani,32 T. Gu,68 T. Hara,20,16 K. Hayasaka,62 H. Hayashii,57 W.-S. Hou,59 K. Inami,55 A. Ishikawa,20,16 M. Iwasaki,65 Y. Iwasaki,20 W. W. Jacobs,28 E.-J. Jang,17 S. Jia,14 Y. Jin,84 K. K. Joo,6J. Kahn,37 A. B. Kaliyar,80 K. H. Kang,38 Y. Kato,55 T. Kawasaki,40 H. Kichimi,20 C. Kiesling,50 C. H. Kim,18 D. Y. Kim,76 Y.-K. Kim,90 K. Kinoshita,8P. Kodyš,5T. Konno,40 A. Korobov,4,64 S. Korpar,49,36 E. Kovalenko,4,64 P. Križan,46,36 R. Kroeger,52 P. Krokovny,4,64 M. Kumar,48 R. Kumar,69 K. Kumara,88 Y.-J. Kwon,90 T. Lam,87 M. Laurenza,32,72 S. C. Lee,43 J. Li,43 L. K. Li,8Y. Li,14 L. Li Gioi,50 J. Libby,27 D. Liventsev,88,20 A. Martini,9M. Masuda,83,70 T. Matsuda,53 D. Matvienko,4,64,45 S. K. Maurya,25 F. Meier,10 M. Merola,31,56 K. Miyabayashi,57 R. Mizuk,45,22 R. Mussa,33 M. Nakao,20,16 D. Narwal,25 Z. Natkaniec,60 A. Natochii,19 L. Nayak,26 N. K. Nisar,3S. Nishida,20,16 K. Nishimura,19 K. Ogawa,62 S. Ogawa,81 H. Ono,61,62 P. Oskin,45 P. Pakhlov,45,54 G. Pakhlova,22,45 T. Pang,68 S. Pardi,31 S.-H. Park,20 S. Patra,24 T. K. Pedlar,47 R. Pestotnik,36 L. E. Piilonen,87 T. Podobnik,46,36 V. Popov,22 M. T. Prim,2M. Röhrken,9A. Rostomyan,9N. Rout,27 G. Russo,56 D. Sahoo,34 S. Sandilya,26 A. Sangal,8T. Sanuki,82 V. Savinov,68 G. Schnell,1,23 J. Schueler,19 C. Schwanda,30 A. J. Schwartz,8Y. Seino,62 K. Senyo,89 M. E. Sevior,51 M. Shapkin,29 C. Sharma,48 V. Shebalin,19 J.-G. Shiu,59 Open Access,c The Authors. Article funded by SCOAP3.https://doi.org/10.1007/JHEP03(2022)090 JHEP03(2022)090 B. Shwartz,4,64 F. Simon,50 E. Solovieva,45 S. Stanič,63 M. Starič,36 Z. S. Stottler,87 M. Sumihama,15,70 K. Sumisawa,20,16 T. Sumiyoshi,86 W. Sutcliffe,2 M. Takizawa,74,21,71 U. Tamponi,33 K. Tanida,35 F. Tenchini,9K. Trabelsi,44 M. Uchida,85 Y. Unno,18 K. Uno,62 S. Uno,20,16 P. Urquijo,51 S. E. Vahsen,19 R. Van Tonder,2G. Varner,19 A. Vinokurova,4,64 E. Waheed,20 D. Wang,12 E. Wang,68 M.-Z. Wang,59 S. Watanuki,90 E. Won,42 X. Xu,75 B. D. Yabsley,78 W. Yan,73 H. Ye,9J. H. Yin,42 Y. Yusa,62 Y. Zhai,34 V. Zhilich4,64 and V. Zhukova45 1Department of Physics, University of the Basque Country UPV/EHU, 48080 Bilbao, Spain 2University of Bonn, 53115 Bonn, Germany 3Brookhaven National Laboratory, Upton, New York 11973, U.S.A. 4Budker Institute of Nuclear Physics SB RAS, Novosibirsk 630090, Russian Federation 5Faculty of Mathematics and Physics, Charles University, 121 16 Prague, The Czech Republic 6Chonnam National University, Gwangju 61186, South Korea 7Chung-Ang University, Seoul 06974, South Korea 8University of Cincinnati, Cincinnati, OH 45221, U.S.A. 9Deutsches Elektronen-Synchrotron, 22607 Hamburg, Germany 10Duke University, Durham, NC 27708, U.S.A. 11Institute of Theoretical and Applied Research (ITAR), Duy Tan University, Hanoi 100000, Vietnam 12University of Florida, Gainesville, FL 32611, U.S.A. 13Department of Physics, Fu Jen Catholic University, Taipei 24205, Taiwan 14Key Laboratory of Nuclear Physics and Ion-beam Application (MOE) and Institute of Modern Physics, Fudan University, Shanghai 200443, PR China 15Gifu University, Gifu 501-1193, Japan 16SOKENDAI (The Graduate University for Advanced Studies), Hayama 240-0193, Japan 17Gyeongsang National University, Jinju 52828, South Korea 18Department of Physics and Institute of Natural Sciences, Hanyang University, Seoul 04763, South Korea 19University of Hawaii, Honolulu, HI 96822, U.S.A. 20High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Japan 21J-PARC Branch, KEK Theory Center, High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Japan 22National Research University Higher School of Economics, Moscow 101000, Russian Federation 23IKERBASQUE, Basque Foundation for Science, 48013 Bilbao, Spain 24Indian Institute of Science Education and Research Mohali, SAS Nagar, 140306, India 25Indian Institute of Technology Guwahati, Assam 781039, India 26Indian Institute of Technology Hyderabad, Telangana 502285, India 27Indian Institute of Technology Madras, Chennai 600036, India 28Indiana University, Bloomington, IN 47408, U.S.A. 29Institute for High Energy Physics, Protvino 142281, Russian Federation 30Institute of High Energy Physics, Vienna 1050, Austria 31INFN — Sezione di Napoli, I-80126 Napoli, Italy 32INFN — Sezione di Roma Tre, I-00146 Roma, Italy 33INFN — Sezione di Torino, I-10125 Torino, Italy JHEP03(2022)090 34Iowa State University, Ames, Iowa 50011, U.S.A. 35Advanced Science Research Center, Japan Atomic Energy Agency, Naka 319-1195, Japan 36J. Stefan Institute, 1000 Ljubljana, Slovenia 37Institut für Experimentelle Teilchenphysik, Karlsruher Institut für Technologie, 76131 Karlsruhe, Germany 38Kavli Institute for the Physics and Mathematics of the Universe (WPI), University of Tokyo, Kashiwa 277-8583, Japan 39Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia 40Kitasato University, Sagamihara 252-0373, Japan 41Korea Institute of Science and Technology Information, Daejeon 34141, South Korea 42Korea University, Seoul 02841, South Korea 43Kyungpook National University, Daegu 41566, South Korea 44Université Paris-Saclay, CNRS/IN2P3, IJCLab, 91405 Orsay, France 45P.N. Lebedev Physical Institute of the Russian Academy of Sciences, Moscow 119991, Russian Federation 46Faculty of Mathematics and Physics, University of Ljubljana, 1000 Ljubljana, Slovenia 47Luther College, Decorah, IA 52101, U.S.A. 48Malaviya National Institute of Technology Jaipur, Jaipur 302017, India 49Faculty of Chemistry and Chemical Engineering, University of Maribor, 2000 Maribor, Slovenia 50Max-Planck-Institut für Physik, 80805 München, Germany 51School of Physics, University of Melbourne, Victoria 3010, Australia 52University of Mississippi, University, MS 38677, U.S.A. 53University of Miyazaki, Miyazaki 889-2192, Japan 54Moscow Physical Engineering Institute, Moscow 115409, Russian Federation 55Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan 56Università di Napoli Federico II, I-80126 Napoli, Italy 57Nara Women’s University, Nara 630-8506, Japan 58National Central University, Chung-li 32054, Taiwan 59Department of Physics, National Taiwan University, Taipei 10617, Taiwan 60H. Niewodniczanski Institute of Nuclear Physics, Krakow 31-342, Poland 61Nippon Dental University, Niigata 951-8580, Japan 62Niigata University, Niigata 950-2181, Japan 63University of Nova Gorica, 5000 Nova Gorica, Slovenia 64Novosibirsk State University, Novosibirsk 630090, Russian Federation 65Osaka City University, Osaka 558-8585, Japan 66Pacific Northwest National Laboratory, Richland, WA 99352, U.S.A. 67Panjab University, Chandigarh 160014, India 68University of Pittsburgh, Pittsburgh, PA 15260, U.S.A. 69Punjab Agricultural University, Ludhiana 141004, India 70Research Center for Nuclear Physics, Osaka University, Osaka 567-0047, Japan 71Meson Science Laboratory, Cluster for Pioneering Research, RIKEN, Saitama 351-0198, Japan 72Dipartimento di Matematica e Fisica, Università di Roma Tre, I-00146 Roma, Italy 73Department of Modern Physics and State Key Laboratory of Particle Detection and Electronics, University of Science and Technology of China, Hefei 230026, PR China JHEP03(2022)090 74Showa Pharmaceutical University, Tokyo 194-8543, Japan 75Soochow University, Suzhou 215006, China 76Soongsil University, Seoul 06978, South Korea 77Sungkyunkwan University, Suwon 16419, South Korea 78School of Physics, University of Sydney, New South Wales 2006, Australia 79Department of Physics, Faculty of Science, University of Tabuk, Tabuk 71451, Saudi Arabia 80Tata Institute of Fundamental Research, Mumbai 400005, India 81Toho University, Funabashi 274-8510, Japan 82Department of Physics, Tohoku University, Sendai 980-8578, Japan 83Earthquake Research Institute, University of Tokyo, Tokyo 113-0032, Japan 84Department of Physics, University of Tokyo, Tokyo 113-0033, Japan 85Tokyo Institute of Technology, Tokyo 152-8550, Japan 86Tokyo Metropolitan University, Tokyo 192-0397, Japan 87Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, U.S.A. 88Wayne State University, Detroit, MI 48202, U.S.A. 89Yamagata University, Yamagata 990-8560, Japan 90Yonsei University, Seoul 03722, South Korea E-mail: [email protected] Abstract: We present the first measurement of the branching fraction of the singly Cabibbo-suppressed (SCS) decay Λ+ c→pη0with η0→ηπ+π−, using a data sample corresponding to an integrated luminosity of 981 fb−1, collected by the Belle detector at the KEKB e+e−asymmetric-energy collider. A significant Λ+ c→pη0signal is observed for the first time with a signal significance of 5.4σ. The relative branching fraction with respect to the normalization mode Λ+ c→pK−π+is measured to be B(Λ+ c→pη0) B(Λ+ c→pK−π+)= (7.54 ±1.32 ±0.73) ×10−3, where the uncertainties are statistical and systematic, respectively. Using the world-average value of B(Λ+ c→pK−π+) = (6.28 ±0.32) ×10−2, we obtain B(Λ+ c→pη0) = (4.73 ±0.82 ±0.46 ±0.24) ×10−4, where the uncertainties are statistical, systematic, and from B(Λ+ c→pK−π+), respectively. Keywords: Branching fraction, e+-e−Experiments, Charm Physics, Particle and Resonance Production ArXiv ePrint: 2112.14276 JHEP03(2022)090 Contents 1 Introduction 1 2 The Belle detector and data sample 1 3 Selection criteria 2 4 Signal and background estimation 4 5 Systematic uncertainties 6 6 Conclusions 8 1 Introduction Hadronic decays of charmed baryons provide an ideal laboratory to understand the interplay of weak and strong interactions in the charm system [1–3]. Decays of charmed baryons receive sizable nonfactorizable contributions from W-exchange diagrams, which are subject to color and helicity suppression [4–7]. Therefore, the study of nonfactorizable contributions is critical to understand the dynamics of charmed baryon decays. To avoid theoretical difficulties in the factorization approach [4], one can use the SU(3)Fflavor symmetry to relate the amplitudes among different decays [5,8,9]. Other theoretical approaches provide calculations based on dynamical models [10–12]. For the singly Cabibbo-suppressed (SCS) decay Λ+ c→pη0, theoretical predictions on its branching fraction under different assumptions vary by more than an order of magnitude as listed in table 1. Currently, this decay has not yet been observed. In this study, based on an e+e−annihilation data sample of 981 fb−1collected by the Belle experiment, we measure the branching fraction of the signal mode Λ+ c→pη0 with respect to the normalization mode Λ+ c→pK−π+. Throughout this paper, chargeconjugate modes are implicitly included unless stated otherwise. The paper is organized as follows. Section 2introduces the Belle detector and data sample. Section 3discusses the event selection criteria. The signal and background estimations are presented in section 4. Sections 5and 6describe the systematic uncertainty and conclusion, respectively. 2 The Belle detector and data sample This measurement is based on a data sample corresponding to an integrated luminosity of 981 fb−1, collected with the Belle detector at the KEKB asymmetric-energy e+e−collider [14,15]. About 70% of the data were recorded at the Υ(4S)resonance, and the rest – 1 – JHEP03(2022)090 SU(3)Fsymmetry [5]SU(3)Fsymmetry [13] Constituent quark model [3] B(Λ+ c→pη0) 0.4−0.6 1.22+1.43 −0.87 0.04 −0.2 Table 1. Comparison of different theoretical predictions for B(Λ+ c→pη0)(in units of 10−3). were collected at other Υ(nS)(n= 1, 2, 3, or 5) states or at center-of-mass (CM) energies a few tens of MeV below the Υ(4S)or the Υ(nS)peaks. The Belle detector is a large-solid-angle magnetic spectrometer that consists of a silicon vertex detector (SVD), a 50-layer central drift chamber (CDC), an array of aerogel threshold Cherenkov counters (ACC), a barrel-like arrangement of time-of-flight scintillation counters (TOF), and an electromagnetic calorimeter comprised of CsI(Tl) crystals (ECL) located inside a superconducting solenoid coil that provides a 1.5 T magnetic field. An iron flux-return located outside of the coil is instrumented to detect K0 Lmesons and to identify muons. The detector is described in detail elsewhere [16,17]. The origin of the coordinate system is defined as the position of the nominal interaction point, and the axis aligning with the direction opposite the e+beam is defined as the zaxis. Monte Carlo (MC) simulated events are used to optimize the selection criteria, study backgrounds, and determine the signal reconstruction efficiency. Samples of simulated signal MC events are generated by EvtGen [18] and propagated through a detector simulation based on geant3 [19]. The e+e−→c¯cevents are simulated using pythia [20]; the decays Λ+ c→pK−π+and η0→ηπ+π−are generated with a phase space model. We take into account the effect of final-state radiation from charged particles by using the photos package [22]. Simulated samples of Υ(4S)→B+B−/B0¯ B0,Υ(5S)→B(∗) s¯ B(∗) s/B(∗)¯ B(∗)(π)/Υ(4S)γ,e+e−→q¯q (q=u, d, s, c)at √s= 10.52, 10.58, and 10.867 GeV, and Υ(1S, 2S, 3S)decays, normalized to the same integrated luminosity as real data, are used to develop the selection criteria and perform the background study [23]. 3 Selection criteria Selection criteria are optimized by maximizing a figure-of-merit /(a 2+√nB)[24], where  is the signal efficiency; ais the target signal significance expressed in standard deviations in a one-sided Gaussian test, selected to be 5; nBis the number of background events expected in a two-dimensional signal region of η0and Λ+ csignals, which is defined as (0.95, 0.965) GeV/c2in M(ηπ+π−)and (2.27, 2.31) GeV/c2in M(pη0). We reconstruct the decays Λ+ c→pη0and Λ+ c→pK−π+, with the η0decay reconstructed in the cascade ηπ+π−,η→γγ. Final-state charged tracks are identified as p,K, or πcandidates using information from the charged-hadron identification systems (ACC, TOF, CDC) combined into a likelihood ratio, R(h|h0) = L(h)/(L(h)+L(h0)), where hand h0are π,K, or pas appropriate [25]. Tracks having R(p|π)>0.9and R(p|K)>0.9are identified as proton candidates; charged kaon candidates are required to have R(K|p)>0.4 and R(K|π)>0.9; and charged pion candidates to have R(π|p)>0.4and R(π|K)>0.4. A likelihood ratio for electron identification, R(e), is formed from ACC, CDC, and ECL information [26], and is required to be less than 0.9 for all charged tracks to suppress elec- – 2 – JHEP03(2022)090 ] 2 ) [GeV/c - π + πηM( 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 1 2 Events/ 1 MeV/c 0 500 1000 1500 2000 2500 Figure 1. The invariant mass distribution of ηπ+π−. The region between two red lines is selected as the η0signal region, and the regions between two blue lines are the η0sidebands. trons. The identification efficiencies of p,K, and πare 82%, 70%, and 97%, respectively. The probabilities of misidentifying has h0,P(h→h0), are estimated to be 3% [P(p→π)], 7% [P(p→K)], 10% [P(K→π)], 2% [P(K→p)], 5% [P(π→K)], and 1% [P(π→p)]. For each charged track, the distance of the closest approach with respect to the interaction point along the zaxis and in the transverse x−yplane is required to be less than 2.0 cm and 0.1 cm, respectively. Each track must have at least one SVD hit in both the zdirection and the x−yplane. Photon candidates are selected from ECL clusters not associated with any charged tracks. The photon energy is required to be greater than 90 MeV in the barrel region (−0.63 <cosθ < 0.85) and greater than 120 MeV in the endcap regions (−0.91 <cosθ < −0.63 or 0.85 <cosθ < 0.98) of the ECL, where θis the polar angle relative to the positive zaxis. To reject neutral hadrons, the ratio of the energy deposited in the central 3×3 array of ECL crystals to the total energy deposited in the enclosing 5×5array of crystals is required to be at least 0.9 for each photon candidate. The ηcandidates are reconstructed via their decay to two photons. The γγ invariant mass is required to satisfy 0.45 < M(γγ)<0.65 GeV/c2, and then a mass-constrained fit is performed for ηcandidates to improve the momentum resolution. The corresponding χ2 value of the mass-constrained fit on η(χ2 η) is required to be less than 10. To further suppress background events, we remove ηcandidates in which either of the daughter photons can be combined with other photons in the event to form π0→γγ candidates satisfying |M(γγ)−mπ0|<12 MeV/c2, where mπ0is the nominal π0mass [21]. With this veto, we reject 42% of the background, while retaining 83% of the signal. The η0candidates are reconstructed by combining two opposite-charge πtracks with an ηcandidate. The invariant mass distribution of ηπ+π−from data is shown in figure 1. Candidates η0are retained if 0.950 < M(ηπ+π−)<0.965 GeV/c2, corresponding to an efficiency of 96%. The η0sidebands are defined as 0.915 to 0.930 GeV/c2and 0.980 to 0.995 GeV/c2, which are the regions between two blue lines in the M(ηπ+π−)distribution. Candidates for Λ+ c→pK−π+and Λ+ c→pη0decays are reconstructed by combining p,K−,π+candidates, and p,η0candidates, respectively. A vertex fit is performed with the three charged tracks to suppress combinatorial background events. The resulting fit – 3 – JHEP03(2022)090 2 Events / 0.002 GeV/c 0 50 100 150 200 250 300 350 3 10× ] 2 ) [GeV/c + π - M(pK 2.24 2.26 2.28 2.3 2.32 2.34 Pull -4 -2 0 2 4 2 Events / 0.005 GeV/c 0 50 100 150 200 250 ] 2 ') [GeV/cηM(p 2.15 2.2 2.25 2.3 2.35 2.4 Pull -2 0 2 Figure 2. Fits to the invariant mass distributions of the pK−π+(Left) and pη0combinations (Right). Black dots with error bars represent the data; red solid lines represent the total fitted result; blue dashed lines represent the signal shape; magenta dot-dashed lines represent the background shape; and the green histogram is from normalized η0sidebands. quality is labeled χ2 vtx. For Λ+ c→pK−π+, the χ2 vtx is required to be less than 40, while for Λ+ c→pη0,χ2 vtx <15 is required. For both χ2 vtx requirements, the efficiency is larger than 98%. A scaled momentum of xp>0.53 is required to suppress background, especially from B-meson decays, where xp=p∗/pE2 cm/4c2−M2c2,Ecm is the CM energy, and p∗and M are the momentum and invariant mass, respectively, of the Λ+ ccandidates in the CM frame. After the preliminary selection, about 0.8% of the Λ+ c→pK−π+events and 13.3% of the Λ+ c→pη0events have two or more Λ+ ccandidates. We choose the best ηcandidate according to the smallest value of χ2 η; the rate of events having multiple Λ+ c→pη0candidates with this criterion is 1.6%. For such multi-candidate events, we choose a single Λ+ c candidate randomly. This best-candidate selection, based on the MC simulation, identifies the correct candidate 65% of the time. The pη0mass distribution for wrong-combination simulated signal events is found to be smooth. We keep all candidates of Λ+ c→pK−π+in multiple-candidate events as the multiplicity is negligible. 4 Signal and background estimation With the above selection criteria applied, the invariant mass distributions of normalization and signal modes are shown in figure 2. From a study of generic MC samples [23], no known peaking background processes contribute to mass distributions in the Λ+ csignal region. To extract the number of signal events, we perform an unbinned maximum-likelihood fit to the M(pK−π+)or M(pη0)distribution. The likelihood function is defined in terms of a signal PDF (FS) and a background PDF (FB) as L=e−(nS+nB) N! N Y i [nSFS(Mi) + nBFB(Mi)] ,(4.1) where Nis the total number of observed events; nSand nBare the numbers of signal events and background events, respectively; Mis pK−π+or pη0invariant mass; and idenotes the – 4 – JHEP03(2022)090 event index. The fit is performed to candidate events surviving the selection criteria; nS and nBare free parameters in the fit. For the Λ+ c→pK−π+channel, we extract the Λ+ csignal yields by fitting the M(pK−π+)distribution. The signal PDF is a sum of two Gaussian functions with a common mean, and the background PDF is a second-order polynomial. All parameters of FSand FBare floated. The fit result is shown in figure 2(Left), along with the pull distribution. The fitted signal yield is Nnorm = 1472190 ±5726, where the uncertainty is statistical. From the MC simulation, the mass resolution for Λ+ c→pK−π+is 8 MeV/c2. For the Λ+ c→pη0channel, we first check the M(pπ+π−η)distribution from normalized η0sidebands, as shown in figure 2(Right). The distribution from the normalized η0sidebands is smoothly falling, indicating a negligible contribution from Λ+ c→pπ+π−ηdecays. We subsequently fit the M(pη0)distribution to extract the Λ+ csignal yield. A sum of a Gaussian function and a Crystal Ball (CB) function [27] is used as the signal PDF, and a second-order polynomial as the background PDF. The Gaussian and CB functions are fixed to have a common mean. All other parameters are floated in the fit. The fit result, along with the pull distribution, is shown in figure 2(Right). A clear Λ+ csignal is observed in the M(pη0)distribution. The fitted signal yield is Nsig = 294±52, where the uncertainty is statistical. The mass resolution for Λ+ c→pη0is 13 MeV/c2from the MC simulation, which is the half-width at half maximum. The statistical significance of the Λ+ csignal is 6.3σ, calculated from the difference of the logarithmic likelihoods, −2ln(L0/Lmax) = 59.2, where L0and Lmax are the maximized likelihoods without and with a signal component, respectively [28]. The significance takes into account the difference in the number of degrees of freedom in the two fits (∆ndf=7). Since the largest systematic uncertainty is due to the fit, as described in section 5, alternative fits to the M(pη0)spectrum under different fit conditions are performed and the Λ+ csignal significance is larger than 5.4σin all cases. To measure the branching fraction, we must divide these extracted signal yields by their reconstruction efficiencies. Since Λ+ c→pη0is a two-body decay and η0→ηπ+π− is well modeled by the phase space [29], we estimate the reconstruction efficiency directly from the simulated events by the ratio nsel/ngen, where nsel and ngen are the numbers of true signal events surviving the selection criteria and generated events, respectively. The signal mode reconstruction efficiency is determined to be sig = (2.22 ±0.02)%. However, the reconstruction efficiency for the decay Λ+ c→pK−π+can vary across the three-body phase space, as visualized in a Dalitz plot [30] with polarization neglected. To take this into account, we correct the reconstruction efficiency according to the Dalitz plot from data as follows. Figure 3shows the Dalitz distribution of M2(pK−)versus M2(K−π+) in the Λ+ c→pK−π+signal region from data which is defined as 2.274 <M(pK−π+)< 2.298 GeV/c2. The number of background events has been subtracted using the normalized Λ+ csidebands defined as (2.260, 2.272) GeV/c2and (2.300, 2.312) GeV/c2. The effect of the variation of the kinematic boundaries with M(pK−π+)is neglected. We divide the Dalitz plot for the data into 120×120 bins, with a bin size of 0.027 GeV2/c4for M2(pK−) and 0.016 GeV2/c4for M2(K−π+). The corrected reconstruction efficiency is determined – 5 –