Observation of Ξ(1620 ) 0 and evidence for Ξ(1690 ) 0 in Ξ + c → Ξ − π + π + decays
Belle collaboration, M. Sumihama, I. Adachi, J. K. Ahn, H. Aihara, S. Al Said, D. M. Asner, H. Atmacan, T. Aushev, R. Ayad, V. Babu, I. Badhrees, S. Bahinipati, A. M. Bakich, V. Bansal, C. Beleno, M. Berger, V. Bhardwaj, B. Bhuyan, T. Bilka, J. Biswal, G. Bonvicini, A. Bozek, M. Bracko, T. E. Browder, D. Cervenkov, V. Chekelian, A. Chen, B. G. Cheon, K. Chilikin, K. Cho, S.-K. Choi, Y. Choi, S. Choudhury, D. Cinabro, S. Cunliffe Czank, N. Dash, S. Di Carlo, Z. Dolezal, T. V. Dong, Z. Drasal, S. Eidelman, D. Epifanov, J. E. Fast, B. G. Fulsom, R. Garg, V. Gaur, N. Gabyshev, A. Garmash, M. Gelb, A. Giri, P. Goldenzweig, E. Guido, J. Haba, K. Hayasaka, H. Hayashii, S. Hirose, W.-S. Hou, K. Inami, G. Inguglia, A. Ishikawa, R. Itoh, M. Iwasaki, Y. Iwasaki, W. W. Jacobs, H. B. Jeon, S. Jia, Y. Jin, K. K. Joo, T. Julius, A. B. Kaliyar, K. H. Kang, G. Karyan, Y. Kato, C. Kiesling, D. Y. Kim, J. B. Kim, K. T. Kim, S. H. Kim, K. Kinoshita, P. Kodys, S. Korpar, D. Kotchetkov, P. Krizan, R. Kroeger, P. Krokovny, R. Kumar, A. Kuzmin, Y.-J. Kwon, J. S. Lange, I. S. Lee, S. C. Lee, L. K. Li, Y. B. Li, L. Li Gioi, J. Libby, D. Liventsev, M. Lubej, T. Luo, M. Masuda, et al. (86 additional authors not shown)
aa r X i v : . [ h e p - e x ] O c t Observation of
Ξ(1620) and evidence for Ξ(1690) in Ξ + c → Ξ − π + π + decays M. Sumihama,
13, 72
I. Adachi,
19, 15
J. K. Ahn, H. Aihara, S. Al Said,
81, 37
D. M. Asner, H. Atmacan, T. Aushev, R. Ayad, V. Babu, I. Badhrees,
81, 36
S. Bahinipati, A. M. Bakich, V. Bansal, C. Bele˜no, M. Berger, V. Bhardwaj, B. Bhuyan, T. Bilka, J. Biswal, G. Bonvicini, A. Bozek, M. Braˇcko,
T. E. Browder, D. ˇCervenkov, V. Chekelian, A. Chen, B. G. Cheon, K. Chilikin, K. Cho, S.-K. Choi, Y. Choi, S. Choudhury, D. Cinabro, S. Cunliffe, T. Czank, N. Dash, S. Di Carlo, Z. Doleˇzal, T. V. Dong,
19, 15
Z. Dr´asal, S. Eidelman,
4, 65, 44
D. Epifanov,
4, 65
J. E. Fast, B. G. Fulsom, R. Garg, V. Gaur, N. Gabyshev,
4, 65
A. Garmash,
4, 65
M. Gelb, A. Giri, P. Goldenzweig, E. Guido, J. Haba,
19, 15
K. Hayasaka, H. Hayashii, S. Hirose, W.-S. Hou, K. Inami, G. Inguglia, A. Ishikawa, R. Itoh,
19, 15
M. Iwasaki, Y. Iwasaki, W. W. Jacobs, H. B. Jeon, S. Jia, Y. Jin, K. K. Joo, T. Julius, A. B. Kaliyar, K. H. Kang, G. Karyan, Y. Kato, C. Kiesling, D. Y. Kim, J. B. Kim, K. T. Kim, S. H. Kim, K. Kinoshita, P. Kodyˇs, S. Korpar,
48, 34
D. Kotchetkov, P. Kriˇzan,
45, 34
R. Kroeger, P. Krokovny,
R. Kumar, A. Kuzmin,
4, 65
Y.-J. Kwon, J. S. Lange, I. S. Lee, S. C. Lee, L. K. Li, Y. B. Li, L. Li Gioi, J. Libby, D. Liventsev,
90, 19
M. Lubej, T. Luo, M. Masuda, T. Matsuda, D. Matvienko,
4, 65, 44
M. Merola,
31, 57
K. Miyabayashi, H. Miyata, R. Mizuk,
44, 53, 54
G. B. Mohanty, H. K. Moon, T. Mori, R. Mussa, E. Nakano, T. Nakano, M. Nakao,
19, 15
T. Nanut, K. J. Nath, Z. Natkaniec, M. Niiyama, N. K. Nisar, S. Nishida,
19, 15
H. Ono,
63, 64
P. Pakhlov,
44, 53
G. Pakhlova,
44, 54
B. Pal, S. Pardi, H. Park, S. Paul, T. K. Pedlar, R. Pestotnik, L. E. Piilonen, V. Popov,
44, 54
M. Ritter, G. Russo, D. Sahoo, S. Sandilya, L. Santelj, T. Sanuki, V. Savinov, O. Schneider, G. Schnell,
1, 21
C. Schwanda, Y. Seino, K. Senyo, M. E. Sevior, V. Shebalin,
4, 65
C. P. Shen, T.-A. Shibata, J.-G. Shiu, B. Shwartz,
4, 65
F. Simon,
49, 83
A. Sokolov, E. Solovieva,
44, 54
M. Stariˇc, J. F. Strube, T. Sumiyoshi, M. Takizawa,
75, 20, 73
U. Tamponi, K. Tanida, N. Taniguchi, F. Tenchini, M. Uchida, T. Uglov,
44, 54
S. Uno,
19, 15
P. Urquijo, S. E. Vahsen, C. Van Hulse, G. Varner, V. Vorobyev,
4, 65, 44
A. Vossen, B. Wang, C. H. Wang, M.-Z. Wang, P. Wang, X. L. Wang, M. Watanabe, S. Watanuki, E. Widmann, E. Won, H. Ye, J. Yelton, C. Z. Yuan, Y. Yusa, S. Zakharov,
Z. P. Zhang, V. Zhilich,
4, 65
V. Zhukova,
44, 53 and V. Zhulanov
4, 65 (The Belle Collaboration) University of the Basque Country UPV/EHU, 48080 Bilbao Beihang University, Beijing 100191 Brookhaven National Laboratory, Upton, New York 11973 Budker Institute of Nuclear Physics SB RAS, Novosibirsk 630090 Faculty of Mathematics and Physics, Charles University, 121 16 Prague Chonnam National University, Kwangju 660-701 University of Cincinnati, Cincinnati, Ohio 45221 Deutsches Elektronen–Synchrotron, 22607 Hamburg Duke University, Durham, North Carolina 27708 University of Florida, Gainesville, Florida 32611 Key Laboratory of Nuclear Physics and Ion-beam Application (MOE)and Institute of Modern Physics, Fudan University, Shanghai 200443 Justus-Liebig-Universit¨at Gießen, 35392 Gießen Gifu University, Gifu 501-1193 II. Physikalisches Institut, Georg-August-Universit¨at G¨ottingen, 37073 G¨ottingen SOKENDAI (The Graduate University for Advanced Studies), Hayama 240-0193 Gyeongsang National University, Chinju 660-701 Hanyang University, Seoul 133-791 University of Hawaii, Honolulu, Hawaii 96822 High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801 J-PARC Branch, KEK Theory Center, High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801 IKERBASQUE, Basque Foundation for Science, 48013 Bilbao Indian Institute of Science Education and Research Mohali, SAS Nagar, 140306 Indian Institute of Technology Bhubaneswar, Satya Nagar 751007 Indian Institute of Technology Guwahati, Assam 781039 Indian Institute of Technology Hyderabad, Telangana 502285 Indian Institute of Technology Madras, Chennai 600036 Indiana University, Bloomington, Indiana 47408 Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 Institute of High Energy Physics, Vienna 1050 Institute for High Energy Physics, Protvino 142281 INFN - Sezione di Napoli, 80126 Napoli INFN - Sezione di Torino, 10125 Torino Advanced Science Research Center, Japan Atomic Energy Agency, Naka 319-1195 J. Stefan Institute, 1000 Ljubljana Institut f¨ur Experimentelle Teilchenphysik, Karlsruher Institut f¨ur Technologie, 76131 Karlsruhe King Abdulaziz City for Science and Technology, Riyadh 11442 Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah 21589 Korea Institute of Science and Technology Information, Daejeon 305-806 Korea University, Seoul 136-713 Kyoto University, Kyoto 606-8502 Kyungpook National University, Daegu 702-701 LAL, Univ. Paris-Sud, CNRS/IN2P3, Universit´e Paris-Saclay, Orsay ´Ecole Polytechnique F´ed´erale de Lausanne (EPFL), Lausanne 1015 P.N. Lebedev Physical Institute of the Russian Academy of Sciences, Moscow 119991 Faculty of Mathematics and Physics, University of Ljubljana, 1000 Ljubljana Ludwig Maximilians University, 80539 Munich Luther College, Decorah, Iowa 52101 University of Maribor, 2000 Maribor Max-Planck-Institut f¨ur Physik, 80805 M¨unchen School of Physics, University of Melbourne, Victoria 3010 University of Mississippi, University, Mississippi 38677 University of Miyazaki, Miyazaki 889-2192 Moscow Physical Engineering Institute, Moscow 115409 Moscow Institute of Physics and Technology, Moscow Region 141700 Graduate School of Science, Nagoya University, Nagoya 464-8602 Kobayashi-Maskawa Institute, Nagoya University, Nagoya 464-8602 Universit`a di Napoli Federico II, 80055 Napoli Nara Women’s University, Nara 630-8506 National Central University, Chung-li 32054 National United University, Miao Li 36003 Department of Physics, National Taiwan University, Taipei 10617 H. Niewodniczanski Institute of Nuclear Physics, Krakow 31-342 Nippon Dental University, Niigata 951-8580 Niigata University, Niigata 950-2181 Novosibirsk State University, Novosibirsk 630090 Osaka City University, Osaka 558-8585 Pacific Northwest National Laboratory, Richland, Washington 99352 Panjab University, Chandigarh 160014 Peking University, Beijing 100871 University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Punjab Agricultural University, Ludhiana 141004 Research Center for Nuclear Physics, Osaka University, Osaka 567-0047 Theoretical Research Division, Nishina Center, RIKEN, Saitama 351-0198 University of Science and Technology of China, Hefei 230026 Showa Pharmaceutical University, Tokyo 194-8543 Soongsil University, Seoul 156-743 University of South Carolina, Columbia, South Carolina 29208 Stefan Meyer Institute for Subatomic Physics, Vienna 1090 Sungkyunkwan University, Suwon 440-746 School of Physics, University of Sydney, New South Wales 2006 Department of Physics, Faculty of Science, University of Tabuk, Tabuk 71451 Tata Institute of Fundamental Research, Mumbai 400005 Excellence Cluster Universe, Technische Universit¨at M¨unchen, 85748 Garching Department of Physics, Technische Universit¨at M¨unchen, 85748 Garching Department of Physics, Tohoku University, Sendai 980-8578 Earthquake Research Institute, University of Tokyo, Tokyo 113-0032 Department of Physics, University of Tokyo, Tokyo 113-0033 Tokyo Institute of Technology, Tokyo 152-8550 Tokyo Metropolitan University, Tokyo 192-0397 Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 Wayne State University, Detroit, Michigan 48202 Yamagata University, Yamagata 990-8560 Yonsei University, Seoul 120-749
We report the first observation of the doubly-strange baryon Ξ(1620) in its decay to Ξ − π + viaΞ + c → Ξ − π + π + decays based on a 980 fb − data sample collected with the Belle detector at theKEKB asymmetric-energy e + e − collider. The mass and width are measured to be 1610.4 ± +5 . − . (syst) MeV /c and 59.9 ± +2 . − . (syst) MeV, respectively. We obtain 4.0 σ evidence of the Ξ(1690) with the same data sample. These results shed light on the structure ofhyperon resonances with strangeness S = − PACS numbers: 13.66.Bc, 14.20.Jn
The constituent quark model has been very success-ful in describing the ground state of the flavor SU(3)octet and decuplet baryons [1–3]. However, some ob-served excited states do not agree well with the theo-retical prediction. It is thus important to study suchunusual states, both to probe the limitation of the quarkmodels and to spot unrevealed aspects of the quantum-chromodynamics(QCD) description of the structure ofhadron resonances. Intriguingly, the Ξ resonances withstrangeness S = − S = − J P =
12 + nor a first orbitalexcitation with J P = − has been identified. Determina-tion of the mass of the first excited state is a vital test ofour understanding of the structure of Ξ resonances. Onecandidate for the first excited state is the Ξ(1690), whichhas a three-star rating on a four-star scale [1]. Anothercandidate is the Ξ(1620), with a one-star rating [1]. Ifthe − state is found, it will be the doubly-strange ana-logue to the Λ(1405) state, which has been postulatedas a candidate meson-baryon molecular state or a pen-taquark [4].Experimental evidence for the Ξ(1620) → Ξ π decaywas reported in K − p interactions in the 1970’s [5–7]. Themass and width measurements are consistent but havelarge statistical uncertainties. The most recent experi-ment, in 1981, has not seen this resonance [8]. There is alingering theoretical controversy about the interpretationof the Ξ(1620) and Ξ(1690) states [9–16], extending fromtheir assignment in the quark model to their existence.This would be addressed with new high-quality exper-imental results for the first excited state with S = − c → s quark transition are a good laboratory to probethese strange baryons.In this Letter, we study the decay Ξ + c → Ξ ∗ π + , Ξ ∗ → Ξ − π + based on a data sample collected with the Belledetector at the KEKB asymmetric-energy e + e − (3.5 on8 GeV) collider [17]. The charge conjugate mode is in-cluded throughout this Letter. The sample correspondsto an integrated luminosity of 980 fb − . The major partof the data was taken at the Υ(4 S ) resonance; in ad-dition, smaller integrated luminosity samples were col-lected off resonance and at the Υ(1 S ), Υ(2 S ), Υ(3 S ),and Υ(5 S ). We use a Monte Carlo simulation (MC) sam-ple to characterize the mass resolution, detector accep-tance, and invariant mass distribution in the availablephase space. The MC samples are generated with EVT-GEN [18], and the detector response is simulated withGEANT3 [19].The Belle detector is a large-solid-angle magnetic spec-trometer that consists of a silicon vertex detector (SVD),a 50-layer central drift chamber (CDC), an array of aero-gel threshold Cherenkov counters (ACC), a barrel-like ar-rangement of time-of-flight scintillation counters (TOF),and an electromagnetic calorimeter comprised of CsI(Tl)crystals (ECL); all these components are located insidea superconducting solenoid coil that provides a 1.5 Tmagnetic field. The detector is described in detail else-where [20]. Two inner detector configurations were used.A 2.0 cm radius beampipe and a 3-layer SVD was usedfor the first sample of 156 fb − , while a 1.5 cm radiusbeampipe, a 4-layer SVD and a small-cell inner CDCwere used to record the remaining 824 fb − [21].We reconstruct the Ξ + c via the Ξ + c → Ξ − π + π + , Ξ − → Λ π − , Λ → pπ − decay channel. Final-state charged par-ticles, p and π ± , are identified using the information fromthe tracking (SVD, CDC) and charged-hadron identifica-tion (CDC, ACC, TOF) systems combined into likelihoodratios L ( i : j ) = L i / ( L i + L j ), where i, j ∈ { p, K, π } .The π ± particles are selected by requiring the likelihoodratios L ( π : K ) > .
6; this has about 90% efficiency.The likelihood ratios L ( p : π ) > . L ( p : K ) > . pπ − pairs with about98% efficiency. The three-momentum of the Λ is com-bined with that of a π − track to reconstruct the helixtrajectory of the Ξ − candidate; this helix is extrapo-lated back toward the IP. A vertex fit is applied to theΞ − → Λ π − decay and the χ is required to be lessthan 50. We retain Ξ − candidates whose mass is within ± . /c ( ± σ ) of the nominal Ξ − mass. Then, wecombine the Ξ − with two π + candidates, where the pionwith the lower (higher) momentum is labeled π + L ( π + H ).The closest distance between the π + track and the nom-inal e + e − interaction point must satisfy | dz | < . | dr | < .
16 (0 .
13) cm inthe transverse plane for π + L ( π + H ) for both π + L and π + H . Avertex fit is applied to the Ξ + c → Ξ − π + π + decay. The χ is required to be less than 50. To purify the Ξ + c sam-ples, the scaled momentum x p = p CM / q s − m (Ξ + c ) is required to exceed 0.5, where p CM is the momen-tum of Ξ + c in the e + e − center-of-mass system, s is thesquared total center-of-mass energy, and m (Ξ + c ) is theΞ + c nominal mass. We retain Ξ + c candidates that sat-isfy | M (Ξ − π + π + ) − m (Ξ + c ) | < . c . The region30.0 MeV/ c < | M (Ξ − π + π + ) − m (Ξ + c ) | < c defines the sideband for estimation of the combinatorialbackground.The M (Ξ − π + L ) and M (Ξ − π + H ) distributions of the fi-nal sample are shown in Fig. 1(a). Peaks correspond-ing to Ξ(1530) , Ξ(1620) , and Ξ(1690) are observed inthe M (Ξ − π + L ) distribution. A reflection due to Ξ(1530) decays is seen around 2.2 GeV/ c in M (Ξ − π + H ). Thehatched histograms are the distributions of the Ξ + c side-band events, where only the Ξ(1530) is observed. TheDalitz plot of M (Ξ − π + L ) vs. M (Ξ − π + H ) is shown inFig. 1(b). The cluster of events due to the Ξ(1530) isseen. The region 4.3 − c ) in M (Ξ − π + H )contains the Ξ(1620) and Ξ(1690) signals. There arecurrently no known particles with a mass in the range of2.1 − c that would decay into Ξ π . Such mas-sive particles would decay predominantly into a three-particles final state such as Ξ ππ . The peaks around1.60 and 1.69 GeV/ c in M (Ξ − π + L ) are interpreted as theΞ(1620) and Ξ(1690) resonances. We see an unknownstructure in the range 1.8 − c in M (Ξ − π + ).These events are expected to be due to resonances suchas Ξ(1820) , Ξ(1950) , and Ξ(2030) .The correction of the event-reconstruction efficiencyis applied to the mass spectrum. To calculate this ef-ficiency, we generate MC events for the non-resonantthree-body decay Ξ + c → Ξ − π + π + with a uniform dis-tribution in phase space. The efficiency is the number ofevents surviving the selections divided by the total num-ber of generated events, and is measured as a function of M (Ξ − π + L ); the resulting efficiency is from 0.082 to 0.097and shows a nearly flat distribution in M (Ξ − π + L ). Themass distribution is divided by this efficiency and is nor-malized by the total number of events.We perform a binned maximum-likelihood fit to theefficiency-corrected M (Ξ − π + L ) distribution. The fit is ap-plied for the data samples in the signal region and the X - p + )(GeV/c ) E ve n t s / ( . G e V / c ) ( Xp L )(GeV/c ) M ( Xp H )( G e V / c ) FIG. 1: (a) The Ξ − π + L (solid) and Ξ − π + H (dashed) invariantmass distributions in the Ξ + c signal region, as well as the cor-responding distributions (hatched) in Ξ + c sideband region. (b)The Dalitz distribution for Ξ + c → Ξ − π + H π + L . (color online) sideband region simultaneously. The fitting range is re-stricted to (1.46, 1.76) GeV/ c to avoid inclusion of theunknown structure between 1.8 and 2.1 GeV/ c . The fit-ting function for the mass spectrum in the signal regionincludes resonances due to the Ξ(1530) , Ξ(1620) , andΞ(1690) , a non-resonant contribution, and the combi-natorial background. The fitting function for the massspectrum in the sideband region includes the Ξ(1530) signal and the combinatorial background. The shape ofthe fitting function for the combinatorial backgrounds iscommon for the mass spectra in the signal region and thesideband region, and is made by a function with a thresh-old: u a exp( ub ) + cu , where u = 1 − [(2 − M ) / (2 − d )] and M = M (Ξ − π + L ); a, b, c, and d are free parame-ters. We assume an S-wave non-resonant contribution,and generate the distribution from the MC simulationof Ξ + c → Ξ − π + π + decays with a uniform distributionin phase space. The Ξ(1620) signal is modeled withthe S-wave relativistic Breit-Wigner function. The inter-ference between Ξ(1620) and the S-wave non-resonantprocess is taken into account, and these are coherentlyadded. The Ξ(1530) and Ξ(1690) signals are modeledwith P- and S-wave relativistic Breit-Wigner functionsconvolved with a fixed Gaussian resolution function ofwidth 1.38 MeV/ c and 2.04 MeV/ c , respectively, as de-termined from the MC simulation. The width and massof Ξ(1530) and Ξ(1620) particles are floated in the fit.The mass and width of the Ξ(1690) are fixed in the fitto the values (1686 MeV/ c and 10 MeV, respectively)measured by the WA89 Collaboration [22]. Figure 2(a)shows the Ξ − π + L mass spectrum with the fitting result.The χ /ndf (where ndf is the number of degrees of free-dom) is 66/86. For the Ξ(1690) resonance, the fit isrepeated by fixing the yield to zero; the resulting differ-ence in log-likelihood with respect the nominal fit andthe change of the number of degrees of freedom are usedto obtain the signal significance. The statistical signifi-cance of the Ξ(1690) is 4.5 σ . To check the stability ofthe significance of the Ξ(1690) , various fit conditions aretried. When the P-wave-only relativistic Breit-Wignerwith fixed mass and width is used as the fitting function,the significance is 4.0 σ . When the S-wave-only relativis-tic Breit-Wigner with the floated mass and width is used,the significance is 4.6 σ . We take the minimum value of4.0 σ as the significance including the systematic uncer-tainty. The measured mass and width of Ξ(1530) are1533.4 ± c and 11.2 ± are 1610.4 ± c and 60.0 ± σ ) at 1600 MeV/ c is 1.6 MeV/ c asdetermined from the MC simulation. The width of theΞ(1620) is 59.9 MeV after incorporating this mass reso-lution. ) E v en t s / ( . G e V / c (a) ] [GeV/c + p - X M P u ll - - - - ) E v en t s / ( . G e V / c (b) ] [GeV/c + p - X M P u ll - - - - FIG. 2: (a) The Ξ − π + L invariant mass spectrum (pointswith error bars), together with the fit result (solid bluecurve) including the following components: Ξ(1530) signal(dashed red curve), Ξ(1690) signal (dot-dashed pink curve),Ξ(1620) signal and non-resonant contribution (dot-dashedblack curve), the combinatorial backgrounds (dotted blackcurve). The bottom plots show the normalized residuals(pulls) of the fits. (b) The fit without the interference betweenΞ(1620) and the S-wave non-resonant process. The dot-dashed black curve represents the S-wave non-resonant pro-cess and the dot-dashed green curve represents the Ξ(1620) .(color online) We itemize the systematic uncertainties on the massand width of the Ξ(1620) resonance in Table I. Themass scale and width is checked by comparing the re-constructed mass of the Ξ(1530) in the Ξ − π + channelwith the nominal mass. The differences of the mass andwidth are − . c and − . + c → Ξ ∗ π + , Ξ ∗ → Ξ − π + events and analyze these events by the same program asfor the real data; the mass scale is checked by comparingthe reconstructed mass of Ξ ∗ with the generated mass.Here, the difference of the mass is − . c and thedifference of the width is less than the statistical error.The systematic uncertainty due to the mass shape of theΞ(1620) is obtained by applying the fit with the P-wave relativistic Breit-Wigner function instead of the S-wavefunction. The systematic error due to the mass shapeof the Ξ(1690) is obtained by applying the fit with theP-wave relativistic Breit-Wigner function instead of theS-wave function, with floated mass and width. The nom-inal bin width of the mass spectrum is 3.0 MeV/ c . Wedetermine its systematic uncertainty by changing the binsize from 2.5 to 3.5 MeV/ c and refitting.All of the above sources are uncorrelated, so the totalsystematic uncertainty is calculated by summing them inquadrature. TABLE I: Systematic uncertainties for the mass and thewidth of Ξ(1620) .Source Mass(MeV/ c ) Width(MeV)Mass scale − . − . . . . . ± . ± . +5 . − . . − . We refit the data using a function that excludesthe interference between Ξ(1620) and the S-wave non-resonant process. Figure 2(b) shows the Ξ − π + L massspectrum with this hypothesis. The χ /ndf is 80/87,which is worse than for the nominal fit. Here, the mea-sured mass and width of the Ξ(1620) are 1601.2 ± c and 63.6 ± particle is observedin its decay to Ξ − π + via Ξ + c → Ξ − π + π + decays. Thenumber of Ξ(1620) events is two orders of magnitudelarger than in previous experiments. The measured massand width of the Ξ(1620) are consistent with the resultsof previous measurements within the large uncertaintiesof the latter and are much more precise. The width ofthe Ξ(1620) is somewhat larger than that of the otherΞ ∗ particles [1].The constituent quark models have predicted the firstexcited states of Ξ around 1800 MeV/ c [3]; therefore, itis difficult to explain the structure of the Ξ(1620) andΞ(1690) in this context. Instead, it implies that thesestates are candidates of a new class of exotic hadrons.We observe in the low-mass region two states with a massdifference of about 80 MeV/ c : the Ξ(1620) is stronglycoupled to Ξ π and the Ξ(1690) to Σ K . The situationis similar to the two poles of the Λ(1405) [4] and sug-gests the possibility of two poles in the S = − S = − S = − and Ξ(1690) particles are found in thedecay of Ξ + c while their signals are not seen in the side-band events of Fig.1(a). These results offer a clue forunderstanding the quark structure of these exotic states.The result indicates that the hadronic decays of charmedbaryons via charm-to-strange quark transitions are po-tentially a promising system for further studies of strangebaryons [16].We thank the KEKB group for the excellent operationof the accelerator; the KEK cryogenics group for the ef-ficient operation of the solenoid; and the KEK computergroup, and the Pacific Northwest National Laboratory(PNNL) Environmental Molecular Sciences Laboratory(EMSL) computing group for strong computing support;and the National Institute of Informatics, and ScienceInformation NETwork 5 (SINET5) for valuable networksupport. We acknowledge support from the Ministryof Education, Culture, Sports, Science, and Technology(MEXT) of Japan, the Japan Society for the Promotionof Science (JSPS), and the Tau-Lepton Physics ResearchCenter of Nagoya University; the Australian ResearchCouncil including grants DP180102629, DP170102389,DP170102204, DP150103061, FT130100303; AustrianScience Fund under Grant No. P 26794-N20; the Na-tional Natural Science Foundation of China under Con-tracts No. 11435013, No. 11475187, No. 11521505,No. 11575017, No. 11675166, No. 11705209; Key Re-search Program of Frontier Sciences, Chinese Academyof Sciences (CAS), Grant No. QYZDJ-SSW-SLH011;the CAS Center for Excellence in Particle Physics(CCEPP); the Shanghai Pujiang Program under GrantNo. 18PJ1401000; the Ministry of Education, Youthand Sports of the Czech Republic under ContractNo. LTT17020; the Carl Zeiss Foundation, the DeutscheForschungsgemeinschaft, the Excellence Cluster Uni-verse, and the VolkswagenStiftung; the Department ofScience and Technology of India; the Istituto Nazionaledi Fisica Nucleare of Italy; National Research Founda-tion (NRF) of Korea Grants No. 2015H1A2A1033649,No. 2016R1D1A1B01010135, No. 2016K1A3A7A09005603, No. 2016R1D1A1B02012900, No. 2018R1A2B3003643, No. 2018R1A6A1A06024970, No. 2018R1D1A1B07047294; Radiation Science Research Institute, For-eign Large-size Research Facility Application Support-ing project, the Global Science Experimental Data HubCenter of the Korea Institute of Science and Technol-ogy Information and KREONET/GLORIAD; the PolishMinistry of Science and Higher Education and the Na-tional Science Center; the Grant of the Russian Feder- ation Government, Agreement No. 14.W03.31.0026; theSlovenian Research Agency; Ikerbasque, Basque Founda-tion for Science, Basque Government (No. IT956-16) andMinistry of Economy and Competitiveness (MINECO)(Juan de la Cierva), Spain; the Swiss National ScienceFoundation; the Ministry of Education and the Ministryof Science and Technology of Taiwan; and the UnitedStates Department of Energy and the National ScienceFoundation. 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