**Observation of a Significant Excess of**

**0**

_{}

_{}**0**

_{Events in}

_{Events in}

_{B}

_{B}_{Meson Decays}

_{Meson Decays}

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 K. Goetzen,7T. Held,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. Bruinsma,12

M. Chao,12D. Kirkby,12A. J. Lankford,12M. Mandelkern,12R. K. Mommsen,12W. Roethel,12D. P. Stoker,12 C. Buchanan,13B. L. Hartfiel,13B. C. Shen,14D. del Re,15H. K. Hadavand,15E. J. Hill,15D. B. MacFarlane,15 H. P. Paar,15Sh. Rahatlou,15V. Sharma,15J.W. Berryhill,16C. Campagnari,16B. Dahmes,16N. Kuznetsova,16 S. L. Levy,16O. Long,16A. Lu,16M. A. Mazur,16 J. D. Richman,16W. Verkerke,16T.W. Beck,17J. Beringer,17 A. M. Eisner,17C. A. Heusch,17W. S. Lockman,17T. Schalk,17R. E. Schmitz,17B. A. Schumm,17A. Seiden,17M. Turri,17

W. Walkowiak,17D. C. Williams,17M. G. Wilson,17J. Albert,18E. Chen,18G. P. Dubois-Felsmann,18A. Dvoretskii,18 D. G. Hitlin,18I. Narsky,18F. C. Porter,18A. Ryd,18A. Samuel,18S. Yang,18S. Jayatilleke,19G. Mancinelli,19

B. T. Meadows,19M. D. Sokoloff,19T. Abe,20F. Blanc,20P. Bloom,20S. Chen,20P. J. Clark,20W. T. Ford,20 U. Nauenberg,20A. Olivas,20P. Rankin,20J. Roy,20J. G. Smith,20W. C. van Hoek,20L. Zhang,20J. L. Harton,21T. Hu,21 A. Soffer,21W. H. Toki,21R. J. Wilson,21J. Zhang,21D. Altenburg,22T. Brandt,22J. Brose,22T. Colberg,22M. Dickopp,22

R. S. Dubitzky,22A. Hauke,22H. M. Lacker,22E. Maly,22R. Mu¨ller-Pfefferkorn,22R. Nogowski,22S. Otto,22
J. Schubert,22K. R. Schubert,22R. Schwierz,22B. Spaan,22L. Wilden,22D. Bernard,23G. R. Bonneaud,23F. Brochard,23
J. Cohen-Tanugi,23P. Grenier,23Ch. Thiebaux,23G. Vasileiadis,23M. Verderi,23A. Khan,24 D. Lavin,24F. Muheim,24
S. Playfer,24J. E. Swain,24M. Andreotti,25V. Azzolini,25D. Bettoni,25C. Bozzi,25R. Calabrese,25G. Cibinetto,25
E. Luppi,25M. Negrini,25L. Piemontese,25A. Sarti,25E. Treadwell,26F. Anulli,27,* R. Baldini-Ferroli,27_{M. Biasini,}27,_{*}

A. Calcaterra,27R. de Sangro,27D. Falciai,27G. Finocchiaro,27P. Patteri,27I. M. Peruzzi,27,* M. Piccolo,27
M. Pioppi,27,* A. Zallo,27_{A. Buzzo,}28_{R. Capra,}28_{R. Contri,}28_{G. Crosetti,}28_{M. Lo Vetere,}28_{M. Macri,}28_{M. R. Monge,}28

S. Passaggio,28C. Patrignani,28E. Robutti,28A. Santroni,28S. Tosi,28S. Bailey,29M. Morii,29E. Won,29W. Bhimji,30 D. A. Bowerman,30P. D. Dauncey,30U. Egede,30I. Eschrich,30J. R. Gaillard,30G.W. Morton,30J. A. Nash,30 P. Sanders,30G. P. Taylor,30G. J. Grenier,31S.-J. Lee,31U. Mallik,31J. Cochran,32H. B. Crawley,32J. Lamsa,32

W. T. Meyer,32S. Prell,32E. I. Rosenberg,32J. Yi,32M. Davier,33G. Grosdidier,33A. Ho¨cker,33S. Laplace,33 F. Le Diberder,33V. Lepeltier,33A. M. Lutz,33T. C. Petersen,33S. Plaszczynski,33M. H. Schune,33L. Tantot,33 G. Wormser,33V. Brigljevic´,34C. H. Cheng,34D. J. Lange,34D. M. Wright,34A. J. Bevan,35J. P. Coleman,35J. R. Fry,35

E. Gabathuler,35R. Gamet,35M. Kay,35R. J. Parry,35D. J. Payne,35R. J. Sloane,35C. Touramanis,35J. J. Back,36 P. F. Harrison,36H.W. Shorthouse,36P. Strother,36P. B. Vidal,36C. L. Brown,37G. Cowan,37R. L. Flack,37 H. U. Flaecher,37S. George,37M. G. Green,37A. Kurup,37C. E. Marker,37T. R. McMahon,37S. Ricciardi,37 F. Salvatore,37G. Vaitsas,37M. A. Winter,37D. Brown,38C. L. Davis,38J. Allison,39R. J. Barlow,39A. C. Forti,39 P. A. Hart,39M. C. Hodgkinson,39F. Jackson,39G. D. Lafferty,39A. J. Lyon,39J. H. Weatherall,39J. C. Williams,39 A. Farbin,40A. Jawahery,40D. Kovalskyi,40C. K. Lae,40V. Lillard,40D. A. Roberts,40G. Blaylock,41C. Dallapiccola,41 K. T. Flood,41S. S. Hertzbach,41R. Kofler,41V. B. Koptchev,41T. B. Moore,41S. Saremi,41H. Staengle,41S. Willocq,41 R. Cowan,42G. Sciolla,42F. Taylor,42R. K. Yamamoto,42D. J. J. Mangeol,43P. M. Patel,43A. Lazzaro,44F. Palombo,44 J. M. Bauer,45L. Cremaldi,45V. Eschenburg,45R. Godang,45R. Kroeger,45J. Reidy,45D. A. Sanders,45D. J. Summers,45

H.W. Zhao,45S. Brunet,46D. Cote-Ahern,46C. Hast,46P. Taras,46H. Nicholson,47C. Cartaro,48N. Cavallo,48,† G. De Nardo,48F. Fabozzi,48,†C. Gatto,48L. Lista,48P. Paolucci,48D. Piccolo,48C. Sciacca,48M. A. Baak,49G. Raven,49

J. M. LoSecco,50T. A. Gabriel,51B. Brau,52K. K. Gan,52K. Honscheid,52D. Hufnagel,52H. Kagan,52R. Kass,52 T. Pulliam,52Q. K. Wong,52J. Brau,53R. Frey,53C. T. Potter,53N. B. Sinev,53D. Strom,53E. Torrence,53F. Colecchia,54

A. Dorigo,54F. Galeazzi,54M. Margoni,54M. Morandin,54M. Posocco,54M. Rotondo,54F. Simonetto,54R. Stroili,54 G. Tiozzo,54C. Voci,54M. Benayoun,55H. Briand,55J. Chauveau,55P. David,55Ch. de la Vaissie`re,55L. Del Buono,55 O. Hamon,55M. J. J. John,55Ph. Leruste,55J. Ocariz,55M. Pivk,55L. Roos,55J. Stark,55S. T’Jampens,55G. Therin,55

P. F. Manfredi,56V. Re,56P. K. Behera,57L. Gladney,57Q. H. Guo,57J. Panetta,57C. Angelini,58G. Batignani,58 S. Bettarini,58M. Bondioli,58F. Bucci,58G. Calderini,58M. Carpinelli,58V. Del Gamba,58F. Forti,58M. A. Giorgi,58

A. Lusiani,58G. Marchiori,58F. Martinez-Vidal,58M. Morganti,58,‡N. Neri,58E. Paoloni,58M. Rama,58G. Rizzo,58 F. Sandrelli,58J. Walsh,58M. Haire,59D. Judd,59K. Paick,59D. E. Wagoner,59N. Danielson,60P. Elmer,60C. Lu,60 V. Miftakov,60J. Olsen,60A. J. S. Smith,60H. A. Tanaka,60E.W. Varnes,60F. Bellini,61G. Cavoto,60,61R. Faccini,15,61

F. Ferrarotto,61F. Ferroni,61M. Gaspero,61M. A. Mazzoni,61S. Morganti,61M. Pierini,61G. Piredda,61 F. Safai Tehrani,61C. Voena,61S. Christ,62G. Wagner,62R. Waldi,62T. Adye,63N. De Groot,63B. Franek,63 N. I. Geddes,63G. P. Gopal,63E. O. Olaiya,63S. M. Xella,63R. Aleksan,64S. Emery,64A. Gaidot,64S. F. Ganzhur,64 P.-F. Giraud,64G. Hamel de Monchenault,64W. Kozanecki,64M. Langer,64M. Legendre,64G.W. London,64B. Mayer,64

G. Schott,64G. Vasseur,64Ch. Yeche,64M. Zito,64 M.V. Purohit,65A.W. Weidemann,65F. X. Yumiceva,65D. Aston,66 R. Bartoldus,66N. Berger,66A. M. Boyarski,66O. L. Buchmueller,66M. R. Convery,66D. P. Coupal,66D. Dong,66 J. Dorfan,66D. Dujmic,66W. Dunwoodie,66R. C. Field,66T. Glanzman,66S. J. Gowdy,66E. Grauges-Pous,66T. Hadig,66 V. Halyo,66T. Hryn’ova,66W. R. Innes,66C. P. Jessop,66M. H. Kelsey,66P. Kim,66M. L. Kocian,66U. Langenegger,66 D.W. G. S. Leith,66S. Luitz,66V. Luth,66H. L. Lynch,66H. Marsiske,66R. Messner,66D. R. Muller,66C. P. O’Grady,66 V. E. Ozcan,66A. Perazzo,66M. Perl,66S. Petrak,66B. N. Ratcliff,66S. H. Robertson,66A. Roodman,66A. A. Salnikov,66 R. H. Schindler,66J. Schwiening,66G. Simi,66A. Snyder,66A. Soha,66J. Stelzer,66D. Su,66M. K. Sullivan,66J. Va’vra,66 S. R. Wagner,66M. Weaver,66A. J. R. Weinstein,66W. J. Wisniewski,66D. H. Wright,66C. C. Young,66P. R. Burchat,67

A. J. Edwards,67T. I. Meyer,67B. A. Petersen,67C. Roat,67S. Ahmed,68M. S. Alam,68J. A. Ernst,68M. Saleem,68 F. R. Wappler,68W. Bugg,69M. Krishnamurthy,69S. M. Spanier,69R. Eckmann,70H. Kim,70J. L. Ritchie,70 R. F. Schwitters,70J. M. Izen,71I. Kitayama,71X. C. Lou,71S. Ye,71F. Bianchi,72M. Bona,72F. Gallo,72D. Gamba,72 C. Borean,73L. Bosisio,73G. Della Ricca,73S. Dittongo,73S. Grancagnolo,73L. Lanceri,73P. Poropat,73,xL. Vitale,73

G. Vuagnin,73R. S. Panvini,74Sw. Banerjee,75C. M. Brown,75D. Fortin,75 P. D. Jackson,75R. Kowalewski,75 J. M. Roney,75H. R. Band,76S. Dasu,76M. Datta,76A. M. Eichenbaum,76J. R. Johnson,76P. E. Kutter,76H. Li,76R. Liu,76

F. Di Lodovico,76A. Mihalyi,76A. K. Mohapatra,76Y. Pan,76R. Prepost,76S. J. Sekula,76 J. H. von Wimmersperg-Toeller,76J. Wu,76S. L. Wu,76Z. Yu,76and H. Neal77

(*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

*13*

_{University of California at Irvine, Irvine, California 92697, USA}

_{University of California at Los Angeles, Los Angeles, California 90024, USA}14* _{University of California at Riverside, Riverside, California 92521, USA}*
15

*16*

_{University of California at San Diego, La Jolla, California 92093, USA}

_{University of California at Santa Barbara, Santa Barbara, California 93106, USA}17* _{University of California at Santa Cruz, Institute for Particle Physics, Santa Cruz, California 95064, USA}*
18

_{California Institute of Technology, Pasadena, California 91125, USA}19* _{University of Cincinnati, Cincinnati, Ohio 45221, USA}*
20

*21*

_{University of Colorado, Boulder, Colorado 80309, USA}

_{Colorado State University, Fort Collins, Colorado 80523, USA}22* _{Technische Universita¨t Dresden, Institut fu¨r Kern- und Teilchenphysik, D-01062 Dresden, Germany}*
23

_{Ecole Polytechnique, LLR, F-91128 Palaiseau, France}24_{University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom}

26* _{Florida A&M University, Tallahassee, Florida 32307, USA}*
27

*28*

_{Laboratori Nazionali di Frascati dell’INFN, I-00044 Frascati, Italy}

_{Universita` di Genova, Dipartimento di Fisica and INFN, I-16146 Genova, Italy}29* _{Harvard University, Cambridge, Massachusetts 02138, USA}*
30

_{Imperial College London, London, SW7 2BW, United Kingdom}31* _{University of Iowa, Iowa City, Iowa 52242, USA}*
32

*33*

_{Iowa State University, Ames, Iowa 50011-3160, USA}*34*

_{Laboratoire de l’Acce´le´rateur Line´aire, F-91898 Orsay, France}

_{Lawrence Livermore National Laboratory, Livermore, Californa 94550, USA}35* _{University of Liverpool, Liverpool L69 3BX, United Kingdom}*
36

_{Queen Mary, University of London, E1 4NS, United Kingdom}37* _{University of London, Royal Holloway and Bedford New College, Egham, Surrey TW20 0EX, United Kingdom}*
38

_{University of Louisville, Louisville, Kentucky 40292, USA}39* _{University of Manchester, Manchester M13 9PL, United Kingdom}*
40

*41*

_{University of Maryland, College Park, Maryland 20742, USA}

_{University of Massachusetts, Amherst, Massachusetts 01003, USA}42* _{Massachusetts Institute of Technology, Laboratory for Nuclear Science, Cambridge, Massachusetts 02139, USA}*
43

_{McGill University, Montre´al, Que´bec, Canada H3A 2T8}44* _{Universita` di Milano, Dipartimento di Fisica and INFN, I-20133 Milano, Italy}*
45

_{University of Mississippi, University, Mississippi 38677, USA}46* _{Universite´ de Montre´al, Laboratoire Rene´ J. A. Le´vesque, Montre´al, Que´bec, Canada H3C 3J7}*
47

_{Mount Holyoke College, South Hadley, Massachusetts 01075, USA}48_{Universita` di Napoli Federico II, Dipartimento di Scienze Fisiche and INFN, I-80126, Napoli, Italy}

49* _{NIKHEF, National Institute for Nuclear Physics and High Energy Physics, NL-1009 DB Amsterdam, The Netherlands}*
50

_{University of Notre Dame, Notre Dame, Indiana 46556, USA}51* _{Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA}*
52

*53*

_{Ohio State University, Columbus, Ohio 43210, USA}

_{University of Oregon, Eugene, Oregon 97403, USA}54* _{Universita` di Padova, Dipartimento di Fisica and INFN, I-35131 Padova, Italy}*
55

_{Universite´s Paris VI et VII, Lab de Physique Nucle´aire H. E., F-75252 Paris, France}56* _{Universita` di Pavia, Dipartimento di Elettronica and INFN, I-27100 Pavia, Italy}*
57

_{University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA}58* _{Universita` di Pisa, Dipartimento di Fisica, Scuola Normale Superiore and INFN, I-56127 Pisa, Italy}*
59

_{Prairie View A&M University, Prairie View, Texas 77446, USA}60_{Princeton University, Princeton, New Jersey 08544, USA}

61* _{Universita` di Roma La Sapienza, Dipartimento di Fisica and INFN, I-00185 Roma, Italy}*
62

_{Universita¨t Rostock, D-18051 Rostock, Germany}63* _{Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, United Kingdom}*
64

_{DSM/Dapnia, CEA/Saclay, F-91191 Gif-sur-Yvette, France}65* _{University of South Carolina, Columbia, South Carolina 29208, USA}*
66

_{Stanford Linear Accelerator Center, Stanford, California 94309, USA}67* _{Stanford University, Stanford, California 94305-4060, USA}*
68

*69*

_{State University of New York, Albany, New York 12222, USA}

_{University of Tennessee, Knoxville, Tennessee 37996, USA}70* _{University of Texas at Austin, Austin, Texas 78712, USA}*
71

_{University of Texas at Dallas, Richardson, Texas 75083, USA}72* _{Universita` di Torino, Dipartimento di Fisica Sperimentale and INFN, I-10125 Torino, Italy}*
73

_{Universita` di Trieste, Dipartimento di Fisica and INFN, I-34127 Trieste, Italy}74* _{Vanderbilt University, Nashville, Tennessee 37235, USA}*
75

_{University of Victoria, Victoria, British Columbia, Canada V8W 3P6}76* _{University of Wisconsin, Madison, Wisconsin 53706, USA}*
77

_{Yale University, New Haven, Connectiut 06511, USA}(Received 5 August 2003; published 11 December 2003)

We present a study of the decay*B*0_{!}* _{}*0

*0*

_{}_{based on a sample of}

_{124}

_{}

_{10}6

_{BB}_{pairs recorded by the}

*BABAR*detector at the PEP-II asymmetric-energy

*B*Factory at SLAC. We observe46133events, where the first error is statistical and the second is systematic, corresponding to a significance of4

*:*2 standard deviations including systematic uncertainties. We measure the branching fraction B

*B*0! 0

*0*

_{}_{ }

_{2}

_{:}_{1}

_{}

_{0}

_{:}_{6}

_{}

_{0}

_{:}_{3}

_{10}6

_{, averaged over}

*0*

_{B}_{and}

*0*

_{B}_{decays.}

The study of*B* meson decays into charmless hadronic
final states plays an important role in the understanding
of*CP* violation in the*B* system. In the standard model,

*CP* violation arises from a single complex phase in
the Cabibbo-Kobayashi-Maskawa (CKM) quark-mixing
matrix *V* [1]. Measurements of the time-dependent

*CP*-violating asymmetry in the *B*0 _{!}_{}_{}_{decay}
mode by the*BABAR*and Belle Collaborations [2] provide
information on the angleargV_{td}*V*_{tb}*=V*_{ud}*V*_{ub} of the
unitarity triangle. However, in contrast to the
theoreti-cally clean determination of the angle in*B*0 _{decays to}

charmonium plus neutral-kaon final states [3,4], the
ex-traction ofin*B*0! is complicated by the
inter-ference of amplitudes with different weak phases. The
difference betweeneff, derived from the measured*B*0 !

asymmetry, andmay be evaluated using isospin
relations between the amplitudes for the decays*B*0_{B}0_{ !}

,*B*0_{B}0_{ !}* _{}*0

*0*

_{}_{, and}

_{B}_{!}

*0*

_{}_{}_{[5].}

The primary contributions to the decay*B*0 _{!}* _{}*0

*0*

_{}_{are}

expected to come from the so-called color-suppressed
tree and gluonic penguin amplitudes [6]. The branching
fraction for *B*0_{!}* _{}*0

*0*

_{}_{has been calculated in various}

QCD models [7]. All models use as inputs the values of
the CKM angles, typically taken from unitarity-triangle
fits. The predictions forBB0 _{!}* _{}*0

*0*

_{}_{}

_{are in the range}

0:31:1 106. In particular, the prediction from the
QCD factorization model of Beneke *et al.* has a central
value of 0:3106. Alternatively, studies using
phe-nomenological fits to experimental data for charmless*B*

decays [8] find values in the range 1:6–2:5 106 _{for}

the*B*0_{!}* _{}*0

*0*

_{}_{branching fraction.}

In this Letter, we report the observation of a significant
excess of *B*0_{!}* _{}*0

*0*

_{}_{decays based on}

_{124}

_{}

_{1 }

106_{4S !}_{BB}_{pairs (on-resonance), collected with}

the *BABAR* detector. We also use approximately 12 fb1

of data recorded 40 MeV below the *BB* threshold
(off-resonance).

*BABAR*is a solenoidal detector optimized for the
asym-metric-energy beams at PEP-II and is described in detail
in Ref. [9]. Charged particle (track) momenta are
mea-sured with a 5-layer double-sided silicon vertex tracker
and a 40-layer drift chamber inside a 1.5-T
superconduct-ing solenoidal magnet. Neutral cluster (photon) positions
and energies are measured with an electromagnetic
calo-rimeter (EMC) consisting of 6580 CsI(Tl) crystals. The
photon energy resolution is * _{E}=E* f2:3=EGeV1

*=*4

_{}

1:9g%, and the angular resolution from the interaction
point is* _{}* 3:9o

*p*

_{=}

_{E}_{GeV}

_{}

_{. The photon energy scale is}

determined using symmetric 0 _{!}_{}_{decays. Charged}

hadrons are identified with a detector of internally
re-flected Cherenkov light and ionization in the tracking
detectors. The instrumented magnetic-flux return detects
neutral hadrons and identifies muons. High efficiency for
recording *BB* events in which one *B* decays with low
multiplicity is achieved with a two-level trigger with
complementary tracking-based and calorimetry-based
trigger decisions.

Candidate 0 _{mesons are reconstructed as pairs}

of photons, spatially separated in the EMC, with an
invariant mass within 3 of the 0 _{mass. The mass}

resolution is approximately 8 MeV=c2 _{for }

high-momentum 0 _{mesons. Photon candidates are required}

to be consistent with the expected lateral shower shape, not to be matched to a track, and to have a minimum energy of 30 MeV. To reduce the background from false

0_{candidates, the angle}_{}

between the photon
momen-tum vector in the 0 _{rest frame and the}* _{}*0

_{momentum}

vector in the laboratory frame is required to satisfy

jcosj*<*0:95.

*B* meson candidates are reconstructed by combining
two 0 candidates. Two kinematic variables, used to
isolate the*B*0 !00signal, take advantage of the
kine-matic constraints of *B* mesons produced at the 4S.
The first is the beam-energy-substituted mass_{} m_{ES}

s=2 **p**_{i}**p*** _{B}*2

*2*

_{=E}*i*

**p**2

*B*

q

, where p*s* is the total *e* *e*

center-of-mass (c.m.) energy.E_{i};**p*** _{i}*is the
four-momen-tum of the initial

*e*

*e*system and

**p**

*B*is the

*B*candidate momentum, both measured in the laboratory frame. The second variable is E

*EB*

*s*

p

*=2*, where *EB* is the *B*
candidate energy in the c.m. frame. TheEresolution for
signal is approximately80 MeV.

The primary source of background is*e* *e*!*qq*q
*u; d; s; c*events where a0 from each quark jet randomly
combine to mimic a*B* decay. The jetlike*qq*background
is suppressed by requiring that the angle *S* between
the sphericity [10] axes of the *B* candidate and of the
remaining tracks and photons in the event, in the c.m.
frame, satisfiesjcos* _{S}*j

*<*0:7. The other source of back-ground is

*B*!0

_{(}

_{}_{!}

*0*

_{}_{}_{) decays in which the}

charged pion is emitted nearly at rest in the*B*rest frame
so that the remaining two 0 _{mesons are kinematically}

consistent with a *B*0 _{!}* _{}*0

*0*

_{}_{decay. Energy resolution}

smearing causes some *B* !0 _{events to have} _{E}

above the kinematic limit of*mBm*. From simulation,
other *B*decays contribute no more than one background
event.

The number of signal*B*0 !00 candidates is
deter-mined in an extended, unbinned maximum-likelihood fit.
The variables used in the fit are*m*ES,E, and a

Fisher-discriminant*F*. The*F*discriminant is a linear
combina-tion of three variables, optimized to separate signal from

*qq* background. The first two variables are sums: *L*_{0}
P

*ipi* and *L*2

P

*ipi*cos2*i* where *pi* is the momentum
and* _{i}*is the angle with respect to the thrust axis of the

*B*

Fisher-discriminant optimization is performed using simulated events.

The data are divided into two samples: a signal sample
with candidates satisfying*m*ES*>*5:2 GeV*=c*2andjEj*<*

0:2 GeV, and a sideband sample with candidates from
on-resonance data with*m*_{ES}*>*5:2 GeV*=c*2_{and}_{0:2}_{<}_{jEj}_{<}

0:4 GeV (and well outside the triangular region in *m*_{ES}

andEpopulated by*B* !0_{decays) and candidates}

from off-resonance data with *m*_{ES}*>*5:2 GeV*=c*2 _{and}

jEj*<*0:4 GeV. The sideband sample contains only*qq*

background candidates and is used in the fit to improve
the statistical precision of the*F*distribution for*qq*events.
There are 4470 events in the signal sample and 3253
events in the sideband sample. The *m*ES, E, and *F*

distributions are shown in Fig. 1 for all data used in the
fit. The reconstruction efficiency for *B*0!00 is

17:72:7%, and for*B*!0 is 0:80:1%,
de-rived from simulation. The errors are due to a systematic
uncertainty in the efficiency for high-momentum 0

mesons to pass the selection criteria; statistical uncertain-ties are negligible.

For candidates in the signal sample the probabilities
P*ixx~j*;*~i* used in the maximum-likelihood fit are
the product of probability density functions (PDFs)
for the variables *xx~j* fmES*;*E; Fg, given the set of

parameters *~i*. The likelihood function is given by a
product over all *j*1to *N* candidates and a sum over

the*i* fB0 _{!}* _{}*0

*0*

_{}

_{; B}_{!}

*0*

_{}_{}

_{; qqg}_{hypotheses:}

L exp

X

3

*i*1

*ni*

Y*N*
*j*1

X3
*i*1

*niPixx~j*;*~i*

*:* (1)

The coefficients*ni*are the numbers of*B*0!00signal,

*B*!0 _{background, and} _{qq}_{background events in}

the sample. The number of*B*!0_{events is fixed in}

the fit to the expected value based on the measured
branching fraction BB _{!}* _{}_{}*0

_{ 11:0}

_{}

_{2:7 }

106 [11]. For candidates in the sideband sample, the
likelihood function includes only the PDF for the *F*

variable, and only the component for*qq* background. A
simultaneous fit to both signal sample and sideband
sample data is performed. Monte Carlo simulations are
used to verify that the likelihood fit is unbiased.

The PDFs are determined from data and simulation.
The *m*_{ES}andEvariables are correlated for both*B*0_{!}

0* _{}*0

_{and}

_{B}_{!}

*0*

_{}_{}_{, so a two-dimensional PDF }

de-rived from a smoothed, simulated distribution is used.
The *m*_{ES} distribution for *qq* events is modeled as a
threshold function [12] whose shape parameter is
deter-mined from data with jcos*S*j *>*0:9. TheE
distribu-tion for *qq*events is modeled as a quadratic polynomial
with parameters determined from data with *m*ES*<*

5:26 GeV*=c*2_{.}

The PDF for the*F*variable is modeled as a parametric
step function (PSF) for*B*0!00,*B*!0, and*qq*

events. A PSF is a binned distribution (as in a histogram),
whose parameters are the heights of each bin. Since
the parent distribution of*F* is not known, any functional
form (such as a multiple Gaussian) assumed for the
PDF will suffer from a systematic uncertainty due to
the choice of function. By binning the data, the
PSF substantially reduces this uncertainty. The PSF
is normalized to one, so that the number of free
parame-ters is the number of bins minus one. For both*B*0_{!}* _{}*0

*0*

_{}and *B* !0_{, the} _{F}_{PSF parameters are taken}

from a sample of 3:2104 _{fully} _{reconstructed}

*B*0 !*Dn*n1;2;3 events in data. The *F* PSF
has ten bins, with bin limits chosen so that each
bin contains approximately 10% of the *B*0 !*Dn*

events. Simulation is used to verify that the same
distribution can be used for both *B*0 _{!}* _{}*0

*0*

_{}_{and}

*B*!0_{. For}_{qq}_{background, the}_{F}_{PSF parameters}

are free parameters in the fit; these parameters are determined from data in both the signal and sideband samples.

All event-selection requirements, PDF parameters, and maximum-likelihood fit conditions were determined be-fore fitting the data.

The result of the fit is *n _{B}*0

_{!}0

*0 4613 events,*

_{}corresponding to a branching fraction of BB0_{!}

00 2:10:6 106. *B*0 and *B*0 decays are not
separated, so the branching fraction is measured for the
average of*B*0 _{and}* _{B}*0

_{. The}

_{m}ES,E, and*F* distributions

for events that pass a requirement on the signal
probabil-ity ratio are shown in Fig. 2. This requirement is
opti-mized to maximize the ratio *S=*p*S* *B*, where*S* is the
number of signal events and *B* is the number of
back-ground events in the plot. The significance of the event
yield is evaluated from the square root of the change in

2 l nL between the nominal fit and a separate fit in which the signal yield is fixed to zero, and is found to be4:7with statistical errors only.

The number of signal events is stable when the*qq m*ES

and E PDF parameters, or *nB*_{!}* _{}*0, are allowed to

vary in the fit. A validation of the maximum-likelihood
fit is made by performing a simpler event-counting
analy-sis, based on the number of events satisfying tighter*m*_{ES},

E, and *F* requirements. The event-counting analysis
finds136events with an efficiency of 31%relative to
the maximum-likelihood fit. This agrees well with the
fitted result, and has a statistical significance of2:7.

This result is consistent with our previous limit for
this decay [13] based on 88106 _{BB}_{pairs. The data}

FIG. 2. The distributions of (a)*m*ES, (b)*E*, and (c) Fisher discriminant*F*for candidates in the signal data sample that satisfy an
optimized requirement on the signal probability, based on all variables except the one being plotted. The fraction of signal events
included in the plots is 24%, 42%, and 74% for*m*ES,*E*, and*F*, respectively. The PDF projections are shown as a dashed line for*qq*
background, a dotted line for*B*!0_{, and a dash-dotted line for}* _{B}*0

_{!}

*0*

_{}*0*

_{}_{signal. The solid line shows the sum of all PDF}projections. The PDF projections are scaled by the expected fraction of events passing the probability-ratio requirement. The ratio

described in Ref. [13] were reanalyzed with improved EMC energy calibration and tracking alignment. More events are observed in this data sample after the reanal-ysis, consistent with the improved understanding of the detector.

Systematic uncertainties on the event yield are
eval-uated by varying the fixed parameters and refitting the
data, and are summarized in Table I. The shape parameter
for the threshold function describing the*m*_{ES}distribution
for*qq*events is varied to account for the statistical error
from the fit to the sample with jcos* _{S}*j

*>*0:9 and the extrapolation fromjcos

*j*

_{S}*>*0:9tojcos

*j*

_{S}*<*0:7. The

*qq*Epolynomial parameters are varied by their
statis-tical errors. The number of *B* !0 _{background}

events is varied according to the uncertainties on the

*B*!0 _{branching fraction and reconstruction }

effi-ciency. Finally, the uncertainty in the mean of the E

distribution for*B*0_{!}* _{}*0

*0*

_{}_{is evaluated from a study of}

*B*!0 events that have a high-momentum 0.
Extrapolating from the uncertainty in the mean E for
this sample, we vary the mean of E by 12 MeV to
evaluate the systematic uncertainty on the signal yield.
The effect of these uncertainties on the significance of the
event yield is evaluated by choosing the variation that
reduces the signal in all four systematic effects, and then
refitting the data. The significance of the signal yield after
accounting for systematic uncertainties is 4:2. The
change in2 l nLas a function of the signal event yield
is shown in Fig. 2(d).

In summary, we observe 46133 *B*0_{!}* _{}*0

*0*

_{}events with a significance of 4:2 standard deviations
including systematic uncertainties. We measure a
branch-ing fraction BB0 _{!}* _{}*0

*0*

_{}_{ 2:1}

_{}

_{0:6}

_{}

_{0:3 }

_{10}6

_{,}

where the first error is statistical and the second is
system-atic. The branching fraction is an average for*B*0 and*B*0

decays. The systematic uncertainties from PDF variations
and efficiency have been combined in quadrature. This
result is consistent with, and supersedes, our previous
limit for this decay [13]; it is also consistent with other
prior limits [14]. The *B*0_{!}* _{}*0

*0*

_{}_{branching fraction is}

larger than some of the theoretical predictions, including the predictions of the QCD factorization model.

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

in-stitutions wish to thank SLAC for its support and kind hospitality. This work is supported by DOE and NSF (U.S.A.), 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.

†_{Also with Universita` della Basilicata, Potenza, Italy.}
‡_{Also with IFIC, Instituto de Fı´sica Corpuscular, }

CSIC-Universidad de Valencia, Valencia, Spain. x

Deceased.

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TABLE I. A summary of systematic uncertainties listed as
the change in the fitted event yield,*n _{B}*0

_{!}

*0*

_{}*0, for different parameter variations.*

_{}Parameter *nB*0_{!}* _{}*0

*0*

_{}*qq m*ESshape parameter 2*:*0

*qqE*quadratic polynomial 0*:*9
1*:*0

*nB*_{!}* _{}_{}*0 0

*:*9

*B*0_{!}* _{}*0

*0*

_{}_{}

_{E}_{mean}0

*:*6