Observation of a narrow meson state decaying to D-s(+)pi(0) at a mass of 2.32 GeV/c(2)

Observation of a Narrow Meson State Decaying to

D

_{at a Mass of}

₂

_:

_{32 GeV}

_=c

B. Aubert,1R. Barate,1D. Boutigny,1J.-M. Gaillard,1A. Hicheur,1Y. Karyotakis,1J. P. Lees,1P. Robbe,1V. Tisserand,1 A. Zghiche,1A. Palano,2A. Pompili,2J. C. Chen,3N. D. Qi,3G. Rong,3P. Wang,3Y. S. Zhu,3G. Eigen,4I. Ofte,4 B. Stugu,4G. S. Abrams,5A.W. Borgland,5A. B. Breon,5D. N. Brown,5J. Button-Shafer,5R. N. Cahn,5E. Charles,5

C. T. Day,5M. S. Gill,5A.V. Gritsan,5Y. Groysman,5R. G. Jacobsen,5R.W. Kadel,5J. Kadyk,5L. T. Kerth,5 Yu. G. Kolomensky,5J. F. Kral,5G. Kukartsev,5C. LeClerc,5M. E. Levi,5G. Lynch,5L. M. Mir,5P. J. Oddone,5 T. J. Orimoto,5M. Pripstein,5N. A. Roe,5A. Romosan,5M. T. Ronan,5V. G. Shelkov,5A.V. Telnov,5W. A. Wenzel,5

K. Ford,6T. J. Harrison,6C. M. Hawkes,6D. J. Knowles,6S. E. Morgan,6R. C. Penny,6A. T. Watson,6N. K. Watson,6 T. Deppermann,7K. Goetzen,7H. Koch,7B. Lewandowski,7M. Pelizaeus,7K. Peters,7H. Schmuecker,7M. Steinke,7

N. R. Barlow,8J. T. Boyd,8N. Chevalier,8W. N. Cottingham,8M. P. Kelly,8T. E. Latham,8C. Mackay,8F. F. Wilson,8 K. Abe,9T. Cuhadar-Donszelmann,9C. Hearty,9T. S. Mattison,9J. A. McKenna,9D. Thiessen,9P. Kyberd,10

A. K. McKemey,10V. E. Blinov,11A. D. Bukin,11V. B. Golubev,11V. N. Ivanchenko,11E. A. Kravchenko,11 A. P. Onuchin,11S. I. Serednyakov,11Yu. I. Skovpen,11E. P. Solodov,11A. N. Yushkov,11D. Best,12M. Chao,12D. Kirkby,12

A. J. Lankford,12M. Mandelkern,12S. McMahon,12R. K. Mommsen,12W. Roethel,12D. P. Stoker,12C. Buchanan,13 D. del Re,14H. K. Hadavand,14E. J. Hill,14D. B. MacFarlane,14H. P. Paar,14Sh. Rahatlou,14U. Schwanke,14V. Sharma,14 J.W. Berryhill,15C. Campagnari,15B. Dahmes,15N. Kuznetsova,15S. L. Levy,15O. Long,15A. Lu,15M. A. Mazur,15 J. D. Richman,15W.Verkerke,15T.W. Beck,16J. Beringer,16A. M. Eisner,16C. A. Heusch,16W. S. Lockman,16T. Schalk,16 R. E. Schmitz,16B. A. Schumm,16A. Seiden,16M. Turri,16W. Walkowiak,16D. C. Williams,16M. G. Wilson,16J. Albert,17 E. Chen,17G. P. Dubois-Felsmann,17A. Dvoretskii,17 D. G. Hitlin,17I. Narsky,17 F. C. Porter,17A. Ryd,17A. Samuel,17

S. Yang,17S. Jayatilleke,18G. Mancinelli,18B. T. Meadows,18M. D. Sokoloff,18T. Abe,19T. Barillari,19F. Blanc,19 P. Bloom,19P. J. Clark,19W. T. Ford,19U. Nauenberg,19A. Olivas,19P. Rankin,19J. Roy,19J. G. Smith,19W. C. van Hoek,19

L. Zhang,19J. L. Harton,20T. Hu,20A. Soffer,20W. H. Toki,20R. J. Wilson,20J. Zhang,20D. Altenburg,21T. Brandt,21 J. Brose,21T. Colberg,21M. Dickopp,21R. S. Dubitzky,21A. Hauke,21H. M. Lacker,21E. Maly,21

R. Mu¨ller-Pfefferkorn,21R. Nogowski,21S. Otto,21K. R. Schubert,21R. Schwierz,21B. Spaan,21L. Wilden,21 D. Bernard,22G. R. Bonneaud,22F. Brochard,22J. Cohen-Tanugi,22Ch. Thiebaux,22G. Vasileiadis,22M. Verderi,22 A. Khan,23D. Lavin,23F. Muheim,23S. Playfer,23J. E. Swain,23J. Tinslay,23M. Andreotti,24D. Bettoni,24C. Bozzi,24

R. Calabrese,24G. Cibinetto,24E. Luppi,24M. Negrini,24 L. Piemontese,24A. Sarti,24E. Treadwell,25F. Anulli,26,* R. Baldini-Ferroli,26A. Calcaterra,26R. de Sangro,26D. Falciai,26G. Finocchiaro,26P. Patteri,26I. M. Peruzzi,26,*

M. Piccolo,26A. Zallo,26A. Buzzo,27R. Contri,27G. Crosetti,27M. Lo Vetere,27M. Macri,27M. R. Monge,27 S. Passaggio,27F. C. Pastore,27C. Patrignani,27E. Robutti,27A. Santroni,27S. Tosi,27S. Bailey,28M. Morii,28 M. L. Aspinwall,29W. Bhimji,29D. A. Bowerman,29P. D. Dauncey,29U. Egede,29I. Eschrich,29G.W. Morton,29

J. A. Nash,29P. Sanders,29G. P. Taylor,29G. J. Grenier,30S.-J. Lee,30U. Mallik,30J. Cochran,31H. B. Crawley,31 J. Lamsa,31W. T. Meyer,31S. Prell,31E. I. Rosenberg,31J. Yi,31M. Davier,32G. Grosdidier,32A. Ho¨cker,32S. Laplace,32

F. Le Diberder,32V. Lepeltier,32A. M. Lutz,32T. C. Petersen,32S. Plaszczynski,32M. H. Schune,32L. Tantot,32 G. Wormser,32V. Brigljevic´,33C. H. Cheng,33D. J. Lange,33D. M. Wright,33A. J. Bevan,34J. P. Coleman,34J. R. Fry,34

E. Gabathuler,34R. Gamet,34M. Kay,34R. J. Parry,34D. J. Payne,34R. J. Sloane,34C. Touramanis,34J. J. Back,35 P. F. Harrison,35H.W. Shorthouse,35P. Strother,35P. B. Vidal,35C. L. Brown,36G. Cowan,36R. L. Flack,36 H. U. Flaecher,36S. George,36M. G. Green,36A. Kurup,36C. E. Marker,36T. R. McMahon,36S. Ricciardi,36 F. Salvatore,36G. Vaitsas,36M. A. Winter,36D. Brown,37C. L. Davis,37J. Allison,38R. J. Barlow,38A. C. Forti,38 P. A. Hart,38F. Jackson,38G. D. Lafferty,38A. J. Lyon,38J. H. Weatherall,38J. C. Williams,38A. Farbin,39A. Jawahery,39

D. Kovalskyi,39C. K. Lae,39V. Lillard,39D. A. Roberts,39G. Blaylock,40C. Dallapiccola,40K. T. Flood,40 S. S. Hertzbach,40R. Kofler,40V. B. Koptchev,40T. B. Moore,40S. Saremi,40H. Staengle,40S. Willocq,40R. Cowan,41 G. Sciolla,41F. Taylor,41R. K. Yamamoto,41D. J. J. Mangeol,42M. Milek,42P. M. Patel,42A. Lazzaro,43F. Palombo,43 J. M. Bauer,44L. Cremaldi,44V. Eschenburg,44R. Godang,44R. Kroeger,44J. Reidy,44D. A. Sanders,44D. J. Summers,44 H.W. Zhao,44C. Hast,45P. Taras,45H. Nicholson,46C. Cartaro,47N. Cavallo,47G. De Nardo,47F. Fabozzi,47,†C. Gatto,47

L. Lista,47P. Paolucci,47D. Piccolo,47C. Sciacca,47M. A. Baak,48G. Raven,48J. M. LoSecco,49T. A. Gabriel,50 B. Brau,51T. Pulliam,51J. Brau,52R. Frey,52C. T. Potter,52N. B. Sinev,52D. Strom,52E. Torrence,52F. Colecchia,53 A. Dorigo,53F. Galeazzi,53M. Margoni,53M. Morandin,53M. Posocco,53M. Rotondo,53F. Simonetto,53R. Stroili,53 G. Tiozzo,53C. Voci,53M. Benayoun,54H. Briand,54J. Chauveau,54P. David,54Ch. de la Vaissie`re,54L. Del Buono,54

P. F. Manfredi,55V. Re,55L. Gladney,56Q. H. Guo,56J. Panetta,56C. Angelini,57G. Batignani,57S. Bettarini,57 M. Bondioli,57F. Bucci,57G. Calderini,57M. Carpinelli,57F. Forti,57M. A. Giorgi,57A. Lusiani,57G. Marchiori,57

F. Martinez-Vidal,57,‡M. Morganti,57N. Neri,57E. Paoloni,57M. Rama,57G. Rizzo,57F. Sandrelli,57J. Walsh,57 M. Haire,58D. Judd,58K. Paick,58D. E. Wagoner,58N. Danielson,59P. Elmer,59C. Lu,59V. Miftakov,59J. Olsen,59 A. J. S. Smith,59E.W. Varnes,59F. Bellini,60G. Cavoto,59,60R. Faccini,14,60F. Ferrarotto,60F. Ferroni,60M. Gaspero,60

M. A. Mazzoni,60S. Morganti,60M. Pierini,60G. Piredda,60F. Safai Tehrani,60C. Voena,60S. Christ,61G. Wagner,61 R. Waldi,61T. Adye,62N. De Groot,62B. Franek,62N. I. Geddes,62G. P. Gopal,62E. O. Olaiya,62S. M. Xella,62 R. Aleksan,63S. Emery,63A. Gaidot,63S. F. Ganzhur,63P.-F. Giraud,63G. Hamel de Monchenault,63W. Kozanecki,63

M. Langer,63G.W. London,63B. Mayer,63G. Schott,63G. Vasseur,63Ch. Yeche,63M. Zito,63M.V. Purohit,64 A.W. Weidemann,64 F. X. Yumiceva,64 D. Aston,65J. Bartelt,65R. Bartoldus,65N. Berger,65A. M. Boyarski,65 O. L. Buchmueller,65M. R. Convery,65D. P. Coupal,65D. Dong,65J. Dorfan,65D. Dujmic,65W. Dunwoodie,65 R. C. Field,65T. Glanzman,65S. J. Gowdy,65E. Grauges-Pous,65T. Hadig,65V. Halyo,65T. Hryn’ova,65W. R. Innes,65

C. P. Jessop,65M. H. Kelsey,65P. Kim,65M. L. Kocian,65U. Langenegger,65D.W. G. S. Leith,65S. Luitz,65V. Luth,65 H. L. Lynch,65H. Marsiske,65S. Menke,65R. Messner,65D. R. Muller,65C. P. O’Grady,65V. E. Ozcan,65A. Perazzo,65

M. Perl,65S. Petrak,65B. N. Ratcliff,65S. H. Robertson,65A. Roodman,65A. A. Salnikov,65R. H. Schindler,65 J. Schwiening,65G. Simi,65A. Snyder,65A. Soha,65J. Stelzer,65D. Su,65M. K. Sullivan,65H. A. Tanaka,65J. Va’vra,65

S. R. Wagner,65M. Weaver,65A. J. R. Weinstein,65W. J. Wisniewski,65D. H. Wright,65C. C. Young,65P. R. Burchat,66 A. J. Edwards,66T. I. Meyer,66C. Roat,66S. Ahmed,67M. S. Alam,67J. A. Ernst,67M. Saleem,67F. R. Wappler,67

W. Bugg,68M. Krishnamurthy,68S. M. Spanier,68R. Eckmann,69H. Kim,69J. L. Ritchie,69R. F. Schwitters,69 J. M. Izen,70I. Kitayama,70X. C. Lou,70S. Ye,70F. Bianchi,71M. Bona,71F. Gallo,71D. Gamba,71C. Borean,72 L. Bosisio,72G. Della Ricca,72S. Dittongo,72S. Grancagnolo,72L. Lanceri,72P. Poropat,72,xL. Vitale,72G. Vuagnin,72

R. S. Panvini,73Sw. Banerjee,74C. M. Brown,74D. Fortin,74P. D. Jackson,74R. Kowalewski,74J. M. Roney,74 H. R. Band,75S. Dasu,75M. Datta,75A. M. Eichenbaum,75H. Hu,75J. R. Johnson,75P. E. Kutter,75H. Li,75R. Liu,75

F. Di Lodovico,75A. Mihalyi,75A. K. Mohapatra,75Y. Pan,75R. Prepost,75S. J. Sekula,75 J. H. von Wimmersperg-Toeller,75J. Wu,75S. L. Wu,75Z. Yu,75and H. Neal76

(BABAR Collaboration)

1_{Laboratoire de Physique des Particules, F-74941 Annecy-le-Vieux, France} 2_{Universita` di Bari, Dipartimento di Fisica and INFN, I-70126 Bari, Italy}

3_{Institute of High Energy Physics, Beijing 100039, China} 4_{University of Bergen, Institute of Physics, N-5007 Bergen, Norway}

5_{Lawrence Berkeley National Laboratory and University of California, Berkeley, California 94720, USA} 6_{University of Birmingham, Birmingham, B15 2TT, United Kingdom}

7_{Ruhr Universita¨t Bochum, Institut fu¨r Experimentalphysik 1, D-44780 Bochum, Germany} 8_{University of Bristol, Bristol BS8 1TL, United Kingdom}

9_{University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1} 10_{Brunel University, Uxbridge, Middlesex UB8 3PH, United Kingdom}

11_{Budker Institute of Nuclear Physics, Novosibirsk 630090, Russia} 12_{University of California at Irvine, Irvine, California 92697, USA} 13_{University of California at Los Angeles, Los Angeles, California 90024, USA}

14_{University of California at San Diego, La Jolla, California 92093, USA} 15_{University of California at Santa Barbara, Santa Barbara, California 93106, USA}

16_{University of California at Santa Cruz, Institute for Particle Physics, Santa Cruz, California 95064, USA} 17_{California Institute of Technology, Pasadena, California 91125, USA}

18_{University of Cincinnati, Cincinnati, Ohio 45221, USA} 19_{University of Colorado, Boulder, Colorado 80309, USA} 20_{Colorado State University, Fort Collins, Colorado 80523, USA}

21_{Technische Universita¨t Dresden, Institut fu¨r Kern- und Teilchenphysik, D-01062 Dresden, Germany} 22_{Ecole Polytechnique, LLR, F-91128 Palaiseau, France}

23_{University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom}

24_{Universita` di Ferrara, Dipartimento di Fisica and INFN, I-44100 Ferrara, Italy} 25_{Florida A&M University, Tallahassee, Florida 32307, USA}

26_{Laboratori Nazionali di Frascati dell’INFN, I-00044 Frascati, Italy} 27_{Universita` di Genova, Dipartimento di Fisica and INFN, I-16146 Genova, Italy}

29_{Imperial College London, London, SW7 2BW, United Kingdom} 30_{University of Iowa, Iowa City, Iowa 52242, USA} 31_{Iowa State University, Ames, Iowa 50011-3160, USA} 32_{Laboratoire de l’Acce´le´rateur Line´aire, F-91898 Orsay, France} 33_{Lawrence Livermore National Laboratory, Livermore, California 94550, USA}

34_{University of Liverpool, Liverpool L69 3BX, United Kingdom} 35_{Queen Mary, University of London, E1 4NS, United Kingdom}

36_{University of London, Royal Holloway and Bedford New College, Egham, Surrey TW20 0EX, United Kingdom} 37_{University of Louisville, Louisville, Kentucky 40292, USA}

38_{University of Manchester, Manchester M13 9PL, United Kingdom} 39_{University of Maryland, College Park, Maryland 20742, USA} 40_{University of Massachusetts, Amherst, Massachusetts 01003, USA}

41_{Massachusetts Institute of Technology, Laboratory for Nuclear Science, Cambridge, Massachusetts 02139, USA} 42_{McGill University, Montre´al, Quebec, Canada H3A 2T8}

43_{Universita` di Milano, Dipartimento di Fisica and INFN, I-20133 Milano, Italy} 44_{University of Mississippi, University, Mississippi 38677, USA}

45_{Universite´ de Montre´al, Laboratoire Rene´ J. A. Le´vesque, Montre´al, Quebec, Canada H3C 3J7} 46_{Mount Holyoke College, South Hadley, Massachusetts 01075, USA}

47_{Universita` di Napoli Federico II, Dipartimento di Scienze Fisiche and INFN, I-80126, Napoli, Italy}

48_{NIKHEF, National Institute for Nuclear Physics and High Energy Physics, NL-1009 DB Amsterdam, The Netherlands} 49_{University of Notre Dame, Notre Dame, Indiana 46556, USA}

50_{Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA} 51_{The Ohio State University, Columbus, Ohio 43210, USA}

52_{University of Oregon, Eugene, Oregon 97403, USA}

53_{Universita` di Padova, Dipartimento di Fisica and INFN, I-35131 Padova, Italy} 54_{Universite´s Paris VI et VII, Laboratoire de Physique Nucle´aire H.E., F-75252 Paris, France}

55_{Universita` di Pavia, Dipartimento di Elettronica and INFN, I-27100 Pavia, Italy} 56_{University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA}

57_{Universita` di Pisa, Dipartimento di Fisica, Scuola Normale Superiore and INFN, I-56127 Pisa, Italy} 58_{Prairie View A&M University, Prairie View, Texas 77446, USA}

59_{Princeton University, Princeton, New Jersey 08544, USA}

60_{Universita` di Roma La Sapienza, Dipartimento di Fisica and INFN, I-00185 Roma, Italy} 61_{Universita¨t Rostock, D-18051 Rostock, Germany}

62_{Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, United Kingdom} 63_{DAPNIA, Commissariat a` l’Energie Atomique/Saclay, F-91191 Gif-sur-Yvette, France}

64_{University of South Carolina, Columbia, South Carolina 29208, USA} 65_{Stanford Linear Accelerator Center, Stanford, California 94309, USA}

66_{Stanford University, Stanford, California 94305-4060, USA} 67_{State University of New York, Albany, New York 12222, USA} 68_{University of Tennessee, Knoxville, Tennessee 37996, USA}

69_{University of Texas at Austin, Austin, Texas 78712, USA} 70_{University of Texas at Dallas, Richardson, Texas 75083, USA}

71_{Universita` di Torino, Dipartimento di Fisica Sperimentale and INFN, I-10125 Torino, Italy</sub> 72<sub>Universita` di Trieste, Dipartimento di Fisica and INFN, I-34127 Trieste, Italy}

73_{Vanderbilt University, Nashville, Tennessee 37235, USA} 74_{University of Victoria, Victoria, British Columbia, Canada V8W 3P6}

75_{University of Wisconsin, Madison, Wisconsin 53706, USA} 76_{Yale University, New Haven, Connecticut 06511, USA}

(Received 11 April 2003; published 17 June 2003)

We have observed a narrow state near2:32 GeV=c2_{in the inclusiveD}

s0invariant mass distribution

from ee annihilation data at energies near 10.6 GeV. The observed width is consistent with the experimental resolution. The small intrinsic width and the quantum numbers of the final state indicate that the decay violates isospin conservation. The state has natural spin-parity and the low mass suggests aJP_{0assignment. The data sample corresponds to an integrated luminosity of91 fb}1_{recorded by}

the BABAR detector at the SLAC PEP-II asymmetric-energyee storage ring.

DOI: 10.1103/PhysRevLett.90.242001 PACS numbers: 14.40.Lb, 12.40.Yx, 13.25.Ft

We have found a narrow state decaying to Ds0 at a mass near 2:32 GeV=c2_{. This result is obtained} from a 91 fb1 _{data sample recorded both on and}

Experimental information on the spectrum of the cs meson states is limited. The 1_S

0 ground state, the D s meson, is well established, as is the3_S

1 ground state, the

D_s2112_{. Only two othercsstates have been observed} thus far [1]. TheD_s12536_{has been detected in itsDK} decay mode and analysis of theD decay angular distri-bution prefersJP _{1 [2]. TheD}

sJ2573 was discov-ered in its D0_{K decay mode and so has natural} spin-parity. The assignment JP_{2 is consistent with the} data, but is not established [3].

The spectroscopy ofcsstates is simple in the limit of large charm-quark mass [4,5]. In that limit, the total angular momentumjj~ ~ll~ssof the light quark, obtained by summing its orbital and spin angular momenta, is conserved. TheP-wave states, all of which have positive parity, then havej3=2orj1=2. Combined with the spin of the heavy quark, the former gives total angular momentumJ2andJ1, while the latter givesJ1

and J0. The JP_{2 and J}P _{1 members of the}

j3=2doublet are expected to have small width [6], and are identified with the D_sJ2573 _{and D}

s12536, re-spectively, although the latter may include a small ad-mixture of the j1=2, JP _{1 state. Theoretical} models typically predict masses between 2.4 and

2:6 GeV=c2 _{for the remaining two states [6 –8], both of} which should decay by kaon emission. They would be expected to have large widths [6,8] and hence should be difficult to detect.

The experimental and theoretical status of theP-wave csstates thus can be summarized by stating that experi-ment has provided good candidates for the two states that theory predicts should be readily observable, but has no candidates for the two states that should be difficult to observe because of their large predicted widths.

The BABAR detector is a general purpose, solenoidal, magnetic spectrometer, which is described in detail else-where [9]. The detector components employed in this analysis are discussed briefly here. Charged particles are detected and their momenta measured by a combina-tion of a cylindrical drift chamber (DCH) and a silicon vertex tracker (SVT), both operating within a 1.5-T sole-noidal magnetic field. A ring-imaging Cherenkov detec-tor (DIRC) is used for charged-particle identification. Electrons are identified and photons measured with a CsI electromagnetic calorimeter.

The objective of this analysis is to investigate the inclusively producedD_s0 _{mass spectrum by combining} charged particles corresponding to the decay D_s !

KK [10] with0 _{candidates reconstructed from a} pair of photons. Events of interest are required to have a ratio of the second to the zeroth Fox-Wolfram moment [11] less than 0.9. In addition, they must contain at least three reconstructed tracks yielding a net charge of 1

and at least two photons each of which must have energy greater than 100 MeV. Charged-kaon candidates are se-lected based on the Cherenkov-photon information from

the DIRC together with the measured energy loss in the SVT and DCH.

AKKcandidate pair is combined with a third track that fails the kaon criteria (and so is treated as a pion) in a geometrical fit to a common vertex. An acceptable KK candidate must have a fit probability greater than 0.1% and a trajectory consistent with originating from theee luminous region. Background fromD0_!

KK, which is evident from the correspondingKK mass distribution, is removed by requiring that theKK mass be less than1:84 GeV=c2_.

A candidate0is formed by constraining a photon pair to emanate from the intersection of theKK candi-date trajectory and the beam envelope, performing a one-constraint fit to the 0 mass, and requiring a fit probability greater than 1%. A given event may yield several acceptable 0 _{candidates. We retain only those} candidates for which neither photon belongs to another acceptable0 _candidate.

Finally, to reduce combinatorial background from the continuum and eliminate background from B-meson de-cay, eachKK0_{candidate must have a momentum}

p in the ee center-of-mass frame greater than

2:5 GeV=c.

The upper histogram in Fig. 1(a) shows theKK mass distribution for all candidates. Clear peaks corre-sponding to D and Ds mesons are seen. To reduce the

FIG. 1. (a) The distribution ofKK mass for all candi-date events. Additional selection criteria, described in the text, have been used to produce the lower histogram. (b) The two-photon mass distribution fromDs0candidate events.Ds and

0_{signal and sideband regions are shaded. (c) TheD}

s0mass

distribution for candidates in theDs signal (top histogram) and

KKsideband regions (shaded histogram) of (a). (d) The

Dsmass distribution for signalDs candidates and a photon

pair from the0 _{signal region of (b) (top histogram) and the}

background further, only those candidates with KK mass within 10 MeV=c2 _{of the 1020 mass or with}

K mass within50 MeV=c2 _{of theK892mass are} retained; these densely populated regions in the D_s Dalitz plot do not overlap. The decay products of the vector particles 1020 and K892 exhibit the ex-pected cos2

h behavior required by conservation of an-gular momentum, where h is the helicity angle. The signal-to-background ratio is further improved by requir-ingjcoshj>0:5. The lower histogram of Fig. 1(a) shows the net effect of these additional selection criteria. The Ds signal [1:955< mKK<1:979 GeV=c2] and sideband [1:912< mKK<1:934 GeV=c2 and

1:998< mKK<2:020 GeV=c2] regions are shaded. TheDs signal peak, consisting of approximately 80 000 events, is centered at a mass of 1967:20 0:03MeV=c2_{(statistical error only).}

Figure 1(b) shows the mass distribution for all two-photon combinations associated with theD_s candidates in the signal region of Fig. 1(a). The 0 _{signal [122<}

m<148 MeV=c2_{] and sideband [90< m<}

110 MeV=c2 _{and 160< m<180 MeV=c}2_{] regions} are shaded. Candidates in the Ds signal region of Fig. 1(a) are combined with the mass-constrained 0 candidates to yield the mass distribution of Fig. 1(c). A clear, narrow signal at a mass near2:32 GeV=c2 is seen. The shaded histogram represents the events in theDs !

KK mass sidebands combined with the0 candi-dates. In Fig. 1(d) the mass distributions result from the combination of theD_s candidates with the photon pairs from the0 _{signal and sideband regions of Fig. 1(b) (the} sideband distribution is again shaded). In this case, all photon pairs in the signal region of Fig. 1(b) are used. In Figs. 1(c) and 1(d) the2:32 GeV=c2 _{signal is absent from} the sideband distributions, indicating quite clearly that the peak is associated with theDs0 system. No other signal in the region up to2:7 GeV=c2 _{is evident in these} plots, except for a small Ds2112!Ds0 signal in Fig. 1(c).

In order to improve mass resolution, the nominal Ds mass [1] has been used to calculate theDs energy for the distributions of Fig. 1(d), for theDs signal distribution of Fig. 1(c), and for all subsequent mass distributions in-volvingDs candidates.

The D_s0 _{mass distribution for pD} s0>

3:5 GeV=c is shown in Fig. 2(a). Similar distributions produced for p values ranging from 2.5 to 4:5 GeV=c show the same prominent peak at the same mass value. The fit function drawn on Fig. 2(a) comprises a Gaussian function describing the 2:32 GeV=c2 _{signal and a} third-order polynomial background distribution function. The fit yields 1267 53 candidates in the signal Gaussian with mass2316:8 0:4MeV=c2_{and standard deviation} 8:6 0:4MeV=c2 _{(statistical errors only). The} system-atic uncertainty in the mass is conservatively estimated to be less than 3 MeV=c2. The broad peak in Fig. 2(a)

centered at 2:16 GeV=c2 _{is due to random D}

s2112 combinations whereD_s2112_!D

s.

The signal, which we labelD_sJ2317_{, is observed in} both the and K0_{K decay modes of the D}

s. In addition, a sample of D_s !KK0 _{decays is} se-lected by adding 0 _{candidates (refit to the KK} vertex) to each KK candidate. The purity of this Ds sample is enhanced by requiring a0 fit probability of at least 10% and selecting theK ,K0_{,, ormass} regions for the relevant two-body subsystems. Each re-sulting Ds candidate is combined with a second 0 candidate with lab momentum greater than 300 MeV=c. A clear D_sJ2317 _{signal is observed as shown in} Fig. 2(b). A Gaussian fit yields 273 33 events with a mean of 2317:6 1:3MeV=c2 and width 8:8 1:1MeV=c2_{(statistical errors only). The mean and width} are consistent with the values obtained for the D_s !

KK decay mode. The mass distribution of the D_s !KK0 _{sample (not shown) peaks at} 1967:4 0:2MeV=c2_{(statistical error only).}

We use a Monte Carlo simulation to investigate the possibility that the D_sJ2317 _{signal could be due to} reflection from other charmed states. This simulation includes ee!ccc events and all known charm states and decays. The generated events were processed by a detailed detector simulation and subjected to the same reconstruction and event-selection procedure as that FIG. 2 (color online). TheDs0mass distribution for (a) the

decayDs !KKand (b) the decayDs !KK0.

used for the data. No peak is found in the2:32 GeV=c2

D_s0 _{signal region. In addition, no signal peak is} pro-duced when the K and identities are deliberately exchanged.

Mass resolution estimates for theKK0 _system are obtained directly from the data using a fit to the mass distribution Ds !KK0. The measured width from this mode is consistent with that of the D_sJ2317_{signal. A simulation of theD}

sJ2317decay to KK0 _{yields a similar mass resolution after} event reconstruction and selection criteria have been satisfied. We conclude that the intrinsic width of the D_sJ2317_{is small (}

&10 MeV). The cosh distribution of the DsJ2317

_{decay with}

respect to its direction in theeecenter-of-mass frame has been investigated. The efficiency-corrected distribu-tion is consistent with being flat, as expected for a spin-zero particle, or for a particle of higher spin that is produced unpolarized.

We have also performed a search for the decay D_sJ2317_!D

s. Shown in Fig. 3(a) is the Ds mass distribution obtained by combining aDs candidate in the signal region of Fig. 1(a) with a photon with an energy of at least 150 MeV that does not belong to a combination in the signal region of Fig. 1(b). The require-ment that the p of the Ds system be greater than

3:5 GeV=c is also imposed. There is a clear Ds2112 signal, but no indication ofD_sJ2317 _production.

The Ds mass distribution for pDs>

3:5 GeV=c, excluding any photon that belongs to the0 signal region of Fig. 1(b), is shown as the upper histogram of Fig. 3(b). No signal is observed near2:32 GeV=c2_{. The} shaded histogram corresponds to the subset of combina-tions for which either D_s combination lies in the Ds2112 region, defined as 2:096< mDs<

2:128 GeV=c2_{. Again, no D}

sJ2317 signal is evident, thus demonstrating the absence of aDs2112 decay mode at the present level of statistics.

The Ds0 mass distribution, excluding any photon that belongs to any0 _{candidate, is shown as the upper} histogram of Fig. 3(c). The shaded histogram corresponds to the subset of combinations in which theDsmass falls in the Ds2112 region. No signal is observed near

2:32 GeV=c2 in either case. A small peak, however, is visible near a mass of 2:46 GeV=c2_{. This mass} corre-sponds to the overlap region of the D_s2112_!D

s and D_sJ2317_!D

s0 signal bands that, because of the small widths of both theD_s2112_{and D}

sJ2317

mesons, produces a narrow peak in the D_s0 _mass distribution that survives aD_s2112 _selection.

If the peak in theDs0mass distribution of Fig. 3(c) were due to the production of a narrow state with mass near 2:46 GeV=c2 _{decaying to D}

s21120, the kine-matics are such that a peak would be produced in the Ds0 mass distribution at a mass near 2:32 GeV=c2. Such a Ds0 mass peak, however, would have a root

mean square of 15 MeV=c2_{, which is significantly} larger than that obtained for the D_sJ2317 _{signal. In} addition, Monte Carlo studies indicate that if the apparent signal at 2:46 GeV=c2 were due to a state that decays entirely toDs21120, it would produce only one-sixth the signal we observe at2:32 GeV=c2.

Although we rule out the decay of a state of mass

2:46 GeV=c2 _{as the sole source of theD}

s0 mass peak corresponding to the D_sJ2317_{, such a state may be} produced in addition to the D_sJ2317_{. However, the} complexity of the overlapping kinematics of the D_s2112_!D

sandDsJ2317!Ds0 decays re-quires more detailed study, currently underway, in order to arrive at a definitive conclusion.

The decay of any cs state to Ds0 violates isospin conservation, thus guaranteeing a small width. It is pos-sible that the decay proceeds via 0 _{mixing, as} discussed by Cho and Wise [12]. For a parity-conserving decay only a spin-parity assignment in the natural JP seriesf0;1;2;. . .gis allowed. The low mass compared to those of the Ds12536 and the DsJ2573

_favors FIG. 3. The mass distribution for (a) Ds and (b) Ds

after excluding photons from the signal region of Fig. 1(b). (c) TheDs0mass distribution. The lower histograms of (b)

and (c) correspond toDsmasses that fall in theDs2112

JP_{0. In this case, decay to D}

s is excluded. However, decay of the D_sJ2317 _{to D}

s2112 is allowed and might compete with decay by pion emission. The shaded mass distribution of Fig. 3(b) suggests that this mode is absent, at least at the present level of statis-tics. This may simply indicate that decay by pion emission is favored over radiative decay.

Further studies are underway. If, however, the tentative JP_{0 assignment is confirmed, the low mass, small} width, and decay mode of the D_sJ2317 _{are quite} different from those predicted by potential models [6 –8]. In summary, in 91 fb1 of data collected by the BABAR experiment we have observed a narrow state in the inclusiveDs0 mass distribution near2:32 GeV=c2. We find no evidence for the decay of this state toDs,

D_s2112_{, or D}

s. Since a css meson of this mass contradicts current models of charm meson spectroscopy [6 –8], either these models need modification or the ob-served state is of a different type altogether, such as a four-quark state.

We are grateful for the excellent luminosity and ma-chine conditions provided by our PEP-II colleagues, and for the substantial dedicated effort from the computing organizations that support BABAR. The collaborating institutions thank SLAC for its support and kind hospi-tality. This work is supported by DOE and NSF (USA), NSERC (Canada), IHEP (China), CEA and CNRS-IN2P3 (France), BMBF and DFG (Germany), INFN (Italy), FOM (The Netherlands), NFR (Norway), MIST (Russia), and PPARC (United Kingdom). Individuals

have received support from the A. P. Sloan Foundation, Research Corporation, and Alexander von Humboldt Foundation.

*Also with Universita` di Perugia, Perugia, Italy. †<sub>Also with Universita` della Basilicata, Potenza, Italy.</sub> ‡_{Also with IFIC, Instituto de Fı´sica Corpuscular,}

CSIC-Universidad de Valencia, Valencia, Spain. x

Deceased.

[1] Particle Data Group, K. Hagiwaraet al., Phys. Rev. D66, 010001 (2002).

[2] CLEO Collaboration, J. P. Alexanderet al., Phys. Lett. B 303, 377 (1993).

[3] CLEO Collaboration, Y. Kubotaet al., Phys. Rev. Lett. 72, 1972 (1994).

[4] A. De Rujula, H. Georgi, and S. L. Glashow, Phys. Rev. Lett.37, 785 (1976).

[5] N. Isgur and M. B. Wise, Phys. Rev. Lett.66, 1130 (1991). [6] S. Godfrey and R. Kokoski, Phys. Rev. D43, 1679 (1991). [7] S. Godfrey and N. Isgur, Phys. Rev. D32, 189 (1985). [8] M. Di Pierro and E. Eichten, Phys. Rev. D64, 114004

(2001).

[9] BABAR Collaboration, B. Aubert et al., Nucl. Instrum. Methods Phys. Res., Sect. A479, 1 (2002).

[10] The inclusion of charge-conjugate configurations is im-plied throughout this Letter.

[11] G. C. Fox and S. Wolfram, Phys. Rev. Lett. 41, 1581 (1978).