Table Of Content

Abstract We have performed a search for the rare leptonic decay B+ µ+ν µ → with data collected at the Υ(4S) resonance by the BABAR experiment at the PEP-II storage ring. In a sample of 88.4 million BB pairs, we find no significant evidence for a signal and set an upper limit on the branching fraction (B+ µ+ν )< 6.6 10−6 at the 90% confidence level. 4 µ B → × 0 0 2 n a J 6 1 v 2 0 0 1 0 4 0 / x e - p e h : v i X r a 1 BABAR-PUB-03/39 SLAC-PUB-10296 Search for the Rare Leptonic Decay B+ → µ+ν µ B. Aubert,1 R. Barate,1 D. Boutigny,1 F. Couderc,1 J.-M. Gaillard,1 A. Hicheur,1 Y. Karyotakis,1 J. P. Lees,1 V. Tisserand,1 A. Zghiche,1 A. Palano,2 A. Pompili,2 J. C. Chen,3 N. D. Qi,3 G. Rong,3 P. Wang,3 Y. S. Zhu,3 G. Eigen,4 I. Ofte,4 B. Stugu,4 G. S. Abrams,5 A. W. Borgland,5 A. B. Breon,5 D. N. Brown,5 J. Button-Shafer,5 R. N. Cahn,5 E. Charles,5 C. T. Day,5 M. S. Gill,5 A. V. Gritsan,5 Y. Groysman,5 R. G. Jacobsen,5 R. W. Kadel,5 J. Kadyk,5 L. T. Kerth,5 Yu. G. Kolomensky,5 G. Kukartsev,5 C. LeClerc,5 M. E. Levi,5 G. Lynch,5 L. M. Mir,5 P. J. Oddone,5 T. J. Orimoto,5 M. Pripstein,5 N. A. Roe,5 M. T. Ronan,5 V. G. Shelkov,5 A. V. Telnov,5 4 W. A. Wenzel,5 K. Ford,6 T. J. Harrison,6 C. M. Hawkes,6 S. E. Morgan,6 A. T. Watson,6 N. K. Watson,6 0 M. Fritsch,7 K. Goetzen,7 T. Held,7 H. Koch,7 B. Lewandowski,7 M. Pelizaeus,7 K. Peters,7 H. Schmuecker,7 0 2 M. Steinke,7 J. T. Boyd,8 N. Chevalier,8 W. N. Cottingham,8 M. P. Kelly,8 T. E. Latham,8 C. Mackay,8 F. F. Wilson,8 K. Abe,9 T. Cuhadar-Donszelmann,9 C. Hearty,9 T. S. Mattison,9 J. A. McKenna,9 D. Thiessen,9 n a P. Kyberd,10 A. K. McKemey,10 L. Teodorescu,10 V. E. Blinov,11 A. D. Bukin,11 V. B. Golubev,11 J V. N. Ivanchenko,11 E. A. Kravchenko,11 A. P. Onuchin,11 S. I. Serednyakov,11 Yu. I. Skovpen,11 E. P. Solodov,11 6 A. N. Yushkov,11 D. Best,12 M. Bruinsma,12 M. Chao,12 I. Eschrich,12 D. Kirkby,12 A. J. Lankford,12 M. Mandelkern,12 R. K. Mommsen,12 W. Roethel,12 D. P. Stoker,12 C. Buchanan,13 B. L. Hartfiel,13 J. W. Gary,14 1 v J. Layter,14 B. C. Shen,14 K. Wang,14 D. del Re,15 H. K. Hadavand,15 E. J. Hill,15 D. B. MacFarlane,15 2 H. P. Paar,15 Sh. Rahatlou,15 V. Sharma,15 J. W. Berryhill,16 C. Campagnari,16 B. Dahmes,16 S. L. Levy,16 0 O. Long,16 A. Lu,16 M. A. Mazur,16 J. D. Richman,16 W. Verkerke,16 T. W. Beck,17 J. Beringer,17 A. M. Eisner,17 0 1 C. A. Heusch,17 W. S. Lockman,17 T. Schalk,17 R. E. Schmitz,17 B. A. Schumm,17 A. Seiden,17 P. Spradlin,17 0 W. Walkowiak,17 D. C. Williams,17 M. G. Wilson,17 J. Albert,18 E. Chen,18 G. P. Dubois-Felsmann,18 4 A. Dvoretskii,18 R. J. Erwin,18 D. G. Hitlin,18 I. Narsky,18 T. Piatenko,18 F. C. Porter,18 A. Ryd,18 A. Samuel,18 0 S. Yang,18 S. Jayatilleke,19 G. Mancinelli,19 B. T. Meadows,19 M. D. Sokoloff,19 T. Abe,20 F. Blanc,20 P. Bloom,20 / x S. Chen,20 P. J. Clark,20 W. T. Ford,20 U. Nauenberg,20 A. Olivas,20 P. Rankin,20 J. Roy,20 J. G. Smith,20 e - W. C. van Hoek,20 L. Zhang,20 J. L. Harton,21 T. Hu,21 A. Soffer,21 W. H. Toki,21 R. J. Wilson,21 J. Zhang,21 p D. Altenburg,22 T. Brandt,22 J. Brose,22 T. Colberg,22 M. Dickopp,22 E. Feltresi,22 A. Hauke,22 H. M. Lacker,22 e h E. Maly,22 R. Mu¨ller-Pfefferkorn,22 R. Nogowski,22 S. Otto,22 J. Schubert,22 K. R. Schubert,22 R. Schwierz,22 : B. Spaan,22 D. Bernard,23 G. R. Bonneaud,23 F. Brochard,23 P. Grenier,23 Ch. Thiebaux,23 G. Vasileiadis,23 v i M. Verderi,23 D. J. Bard,24 A. Khan,24 D. Lavin,24 F. Muheim,24 S. Playfer,24 M. Andreotti,25 V. Azzolini,25 X D. Bettoni,25 C. Bozzi,25 R. Calabrese,25 G. Cibinetto,25 E. Luppi,25 M. Negrini,25 L. Piemontese,25 A. Sarti,25 ar E. Treadwell,26 R. Baldini-Ferroli,27 A. Calcaterra,27 R. de Sangro,27 G. Finocchiaro,27 P. Patteri,27 M. Piccolo,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,28 C. Patrignani,28 E. Robutti,28 A. Santroni,28 S. Tosi,28 S. Bailey,29 M. Morii,29 E. Won,29 R. S. Dubitzky,30 U. Langenegger,30 W. Bhimji,31 D. A. Bowerman,31 P. D. Dauncey,31 U. Egede,31 J. R. Gaillard,31 G. W. Morton,31 J. A. Nash,31 G. P. Taylor,31 G. J. Grenier,32 S.-J. Lee,32 U. Mallik,32 J. Cochran,33 H. B. Crawley,33 J. Lamsa,33 W. T. Meyer,33 S. Prell,33 E. I. Rosenberg,33 J. Yi,33 M. Davier,34 G. Grosdidier,34 A. Ho¨cker,34 S. Laplace,34 F. Le Diberder,34 V. Lepeltier,34 A. M. Lutz,34 T. C. Petersen,34 S. Plaszczynski,34 M. H. Schune,34 L. Tantot,34 G. Wormser,34 V. Brigljević,35 C. H. Cheng,35 D. J. Lange,35 M. C. Simani,35 D. M. Wright,35 A. J. Bevan,36 J. P. Coleman,36 J. R. Fry,36 E. Gabathuler,36 R. Gamet,36 M. Kay,36 R. J. Parry,36 D. J. Payne,36 R. J. Sloane,36 C. Touramanis,36 J. J. Back,37 P. F. Harrison,37 G. B. Mohanty,37 C. L. Brown,38 G. Cowan,38 R. L. Flack,38 H. U. Flaecher,38 S. George,38 M. G. Green,38 A. Kurup,38 C. E. Marker,38 T. R. McMahon,38 S. Ricciardi,38 F. Salvatore,38 G. Vaitsas,38 M. A. Winter,38 D. Brown,39 C. L. Davis,39 J. Allison,40 N. R. Barlow,40 R. J. Barlow,40 P. A. Hart,40 M. C. Hodgkinson,40 G. D. Lafferty,40 A. J. Lyon,40 J. C. Williams,40 A. Farbin,41 W. D. Hulsbergen,41 A. Jawahery,41 D. Kovalskyi,41 C. K. Lae,41 V. Lillard,41 D. A. Roberts,41 G. Blaylock,42 C. Dallapiccola,42 K. T. Flood,42 S. S. Hertzbach,42 R. Kofler,42 V. B. Koptchev,42 T. B. Moore,42 S. Saremi,42 H. Staengle,42 S. Willocq,42 R. Cowan,43 G. Sciolla,43 F. Taylor,43 R. K. Yamamoto,43 D. J. J. Mangeol,44 P. M. Patel,44 S. H. Robertson,44 A. Lazzaro,45 F. Palombo,45 2 J. M. Bauer,46 L. Cremaldi,46 V. Eschenburg,46 R. Godang,46 R. Kroeger,46 J. Reidy,46 D. A. Sanders,46 D. J. Summers,46 H. W. Zhao,46 S. Brunet,47 D. Cote-Ahern,47 P. Taras,47 H. Nicholson,48 C. Cartaro,49 N. Cavallo,49 G. De Nardo,49 F. Fabozzi,49,∗ C. Gatto,49 L. Lista,49 P. Paolucci,49 D. Piccolo,49 C. Sciacca,49 M. A. Baak,50 G. Raven,50 L. Wilden,50 C. P. Jessop,51 J. M. LoSecco,51 T. A. Gabriel,52 T. Allmendinger,53 B. Brau,53 K. K. Gan,53 K. Honscheid,53 D. Hufnagel,53 H. Kagan,53 R. Kass,53 T. Pulliam,53 R. Ter-Antonyan,53 Q. K. Wong,53 J. Brau,54 R. Frey,54 O. Igonkina,54 C. T. Potter,54 N. B. Sinev,54 D. Strom,54 E. Torrence,54 F. Colecchia,55 A. Dorigo,55 F. Galeazzi,55 M. Margoni,55 M. Morandin,55 M. Posocco,55 M. Rotondo,55 F. Simonetto,55 R. Stroili,55 G. Tiozzo,55 C. Voci,55 M. Benayoun,56 H. Briand,56 J. Chauveau,56 P. David,56 Ch. de la Vaissière,56 L. Del Buono,56 O. Hamon,56 M. J. J. John,56 Ph. Leruste,56 J. Ocariz,56 M. Pivk,56 L. Roos,56 S. T’Jampens,56 G. Therin,56 P. F. Manfredi,57 V. Re,57 P. K. Behera,58 L. Gladney,58 Q. H. Guo,58 J. Panetta,58 F. Anulli,27,59 M. Biasini,59 I. M. Peruzzi,27,59 M. Pioppi,59 C. Angelini,60 G. Batignani,60 S. Bettarini,60 M. Bondioli,60 F. Bucci,60 G. Calderini,60 M. Carpinelli,60 V. Del Gamba,60 F. Forti,60 M. A. Giorgi,60 A. Lusiani,60 G. Marchiori,60 F. Martinez-Vidal,60,† M. Morganti,60 N. Neri,60 E. Paoloni,60 M. Rama,60 G. Rizzo,60 F. Sandrelli,60 J. Walsh,60 M. Haire,61 D. Judd,61 K. Paick,61 D. E. Wagoner,61 N. Danielson,62 P. Elmer,62 C. Lu,62 V. Miftakov,62 J. Olsen,62 A. J. S. Smith,62 E. W. Varnes,62 F. Bellini,63 G. Cavoto,62,63 R. Faccini,63 F. Ferrarotto,63 F. Ferroni,63 M. Gaspero,63 M. A. Mazzoni,63 S. Morganti,63 M. Pierini,63 G. Piredda,63 F. Safai Tehrani,63 C. Voena,63 S. Christ,64 G. Wagner,64 R. Waldi,64 T. Adye,65 N. De Groot,65 B. Franek,65 N. I. Geddes,65 G. P. Gopal,65 E. O. Olaiya,65 S. M. Xella,65 R. Aleksan,66 S. Emery,66 A. Gaidot,66 S. F. Ganzhur,66 P.-F. Giraud,66 G. Hamel de Monchenault,66 W. Kozanecki,66 M. Langer,66 M. Legendre,66 G. W. London,66 B. Mayer,66 G. Schott,66 G. Vasseur,66 Ch. Yeche,66 M. Zito,66 M. V. Purohit,67 A. W. Weidemann,67 F. X. Yumiceva,67 D. Aston,68 R. Bartoldus,68 N. Berger,68 A. M. Boyarski,68 O. L. Buchmueller,68 M. R. Convery,68 M. Cristinziani,68 D. Dong,68 J. Dorfan,68 D. Dujmic,68 W. Dunwoodie,68 E. E. Elsen,68 R. C. Field,68 T. Glanzman,68 S. J. Gowdy,68 T. Hadig,68 V. Halyo,68 T. Hryn’ova,68 W. R. Innes,68 M. H. Kelsey,68 P. Kim,68 M. L. Kocian,68 D. W. G. S. Leith,68 J. Libby,68 S. Luitz,68 V. Luth,68 H. L. Lynch,68 H. Marsiske,68 R. Messner,68 D. R. Muller,68 C. P. O’Grady,68 V. E. Ozcan,68 A. Perazzo,68 M. Perl,68 S. Petrak,68 B. N. Ratcliff,68 A. Roodman,68 A. A. Salnikov,68 R. H. Schindler,68 J. Schwiening,68 G. Simi,68 A. Snyder,68 A. Soha,68 J. Stelzer,68 D. Su,68 M. K. Sullivan,68 J. Va’vra,68 S. R. Wagner,68 M. Weaver,68 A. J. R. Weinstein,68 W. J. Wisniewski,68 D. H. Wright,68 C. C. Young,68 P. R. Burchat,69 A. J. Edwards,69 T. I. Meyer,69 B. A. Petersen,69 C. Roat,69 M. Ahmed,70 S. Ahmed,70 M. S. Alam,70 J. A. Ernst,70 M. A. Saeed,70 M. Saleem,70 F. R. Wappler,70 W. Bugg,71 M. Krishnamurthy,71 S. M. Spanier,71 R. Eckmann,72 H. Kim,72 J. L. Ritchie,72 A. Satpathy,72 R. F. Schwitters,72 J. M. Izen,73 I. Kitayama,73 X. C. Lou,73 S. Ye,73 F. Bianchi,74 M. Bona,74 F. Gallo,74 D. Gamba,74 C. Borean,75 L. Bosisio,75 F. Cossutti,75 G. Della Ricca,75 S. Dittongo,75 S. Grancagnolo,75 L.Lanceri,75 P.Poropat,75,‡ L. Vitale,75 G. Vuagnin,75 R. S.Panvini,76 Sw. Banerjee,77 C. M.Brown,77 D. Fortin,77 P. D. Jackson,77 R. Kowalewski,77 J. M. Roney,77 H. R. Band,78 S. Dasu,78 M. Datta,78 A. M. Eichenbaum,78 J. R. Johnson,78 P. E. Kutter,78 H. Li,78 R. Liu,78 F. Di Lodovico,78 A. Mihalyi,78 A. K. Mohapatra,78 Y. Pan,78 R. Prepost,78 S. J. Sekula,78 J. H. von Wimmersperg-Toeller,78 J. Wu,78 S. L. Wu,78 Z. Yu,78 and H. Neal79 (The BABAR Collaboration) 1Laboratoire de Physique des Particules, F-74941 Annecy-le-Vieux, France 2Università di Bari, Dipartimento di Fisica and INFN, I-70126 Bari, Italy 3Institute of High Energy Physics, Beijing 100039, China 4University of Bergen, Inst. of Physics, N-5007 Bergen, Norway 5Lawrence Berkeley National Laboratory and University of California, Berkeley, CA 94720, USA 6University of Birmingham, Birmingham, B15 2TT, United Kingdom 7Ruhr Universität Bochum, Institut fu¨r Experimentalphysik 1, D-44780 Bochum, Germany 8University of Bristol, Bristol BS8 1TL, United Kingdom 9University of British Columbia, Vancouver, BC, Canada V6T 1Z1 10Brunel University, Uxbridge, Middlesex UB8 3PH, United Kingdom 11Budker Institute of Nuclear Physics, Novosibirsk 630090, Russia 12University of California at Irvine, Irvine, CA 92697, USA 13University of California at Los Angeles, Los Angeles, CA 90024, USA 14University of California at Riverside, Riverside, CA 92521, USA 15University of California at San Diego, La Jolla, CA 92093, USA 16University of California at Santa Barbara, Santa Barbara, CA 93106, USA 17University of California at Santa Cruz, Institute for Particle Physics, Santa Cruz, CA 95064, USA 18California Institute of Technology, Pasadena, CA 91125, USA 3 19University of Cincinnati, Cincinnati, OH 45221, USA 20University of Colorado, Boulder, CO 80309, USA 21Colorado State University, Fort Collins, CO 80523, USA 22Technische Universität Dresden, Institut fu¨r Kern- und Teilchenphysik, D-01062 Dresden, Germany 23Ecole Polytechnique, LLR, F-91128 Palaiseau, France 24University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom 25Università di Ferrara, Dipartimento di Fisica and INFN, I-44100 Ferrara, Italy 26Florida A&M University, Tallahassee, FL 32307, USA 27Laboratori Nazionali di Frascati dell’INFN, I-00044 Frascati, Italy 28Università di Genova, Dipartimento di Fisica and INFN, I-16146 Genova, Italy 29Harvard University, Cambridge, MA 02138, USA 30Universität Heidelberg, Physikalisches Institut, Philosophenweg 12, D-69120 Heidelberg, Germany 31Imperial College London, London, SW7 2AZ, United Kingdom 32University of Iowa, Iowa City, IA 52242, USA 33Iowa State University, Ames, IA 50011-3160, USA 34Laboratoire de l’Accélérateur Linéaire, F-91898 Orsay, France 35Lawrence Livermore National Laboratory, Livermore, CA 94550, USA 36University of Liverpool, Liverpool L69 3BX, United Kingdom 37Queen Mary, University of London, E1 4NS, United Kingdom 38University of London, Royal Holloway and Bedford New College, Egham, Surrey TW20 0EX, United Kingdom 39University of Louisville, Louisville, KY 40292, USA 40University of Manchester, Manchester M13 9PL, United Kingdom 41University of Maryland, College Park, MD 20742, USA 42University of Massachusetts, Amherst, MA 01003, USA 43Massachusetts Institute of Technology, Laboratory for Nuclear Science, Cambridge, MA 02139, USA 44McGill University, Montréal, QC, Canada H3A 2T8 45Università di Milano, Dipartimento di Fisica and INFN, I-20133 Milano, Italy 46University of Mississippi, University, MS 38677, USA 47Université de Montréal, Laboratoire René J. A. Lévesque, Montréal, QC, Canada H3C 3J7 48Mount Holyoke College, South Hadley, MA 01075, USA 49Università di Napoli Federico II, Dipartimento di Scienze Fisiche and INFN, I-80126, Napoli, Italy 50NIKHEF, National Institute for Nuclear Physics and High Energy Physics, NL-1009 DB Amsterdam, The Netherlands 51University of Notre Dame, Notre Dame, IN 46556, USA 52Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA 53Ohio State University, Columbus, OH 43210, USA 54University of Oregon, Eugene, OR 97403, USA 55Università di Padova, Dipartimento di Fisica and INFN, I-35131 Padova, Italy 56Universités Paris VI et VII, Lab de Physique Nucléaire H. E., F-75252 Paris, France 57Università di Pavia, Dipartimento di Elettronica and INFN, I-27100 Pavia, Italy 58University of Pennsylvania, Philadelphia, PA 19104, USA 59Università di Perugia, Dipartimento di Fisica and INFN, I-06100 Perugia, Italy 60Università di Pisa, Dipartimento di Fisica, Scuola Normale Superiore and INFN, I-56127 Pisa, Italy 61Prairie View A&M University, Prairie View, TX 77446, USA 62Princeton University, Princeton, NJ 08544, USA 63Università di Roma La Sapienza, Dipartimento di Fisica and INFN, I-00185 Roma, Italy 64Universität Rostock, D-18051 Rostock, Germany 65Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, United Kingdom 66DSM/Dapnia, CEA/Saclay, F-91191 Gif-sur-Yvette, France 67University of South Carolina, Columbia, SC 29208, USA 68Stanford Linear Accelerator Center, Stanford, CA 94309, USA 69Stanford University, Stanford, CA 94305-4060, USA 70State Univ. of New York, Albany, NY 12222, USA 71University of Tennessee, Knoxville, TN 37996, USA 72University of Texas at Austin, Austin, TX 78712, USA 73University of Texas at Dallas, Richardson, TX 75083, USA 74Università di Torino, Dipartimento di Fisica Sperimentale and INFN, I-10125 Torino, Italy 75Università di Trieste, Dipartimento di Fisica and INFN, I-34127 Trieste, Italy 76Vanderbilt University, Nashville, TN 37235, USA 77University of Victoria, Victoria, BC, Canada V8W 3P6 78University of Wisconsin, Madison, WI 53706, USA 79Yale University, New Haven, CT 06511, USA (Dated: February 7, 2008) 4 We have performed a search for the rare leptonic decay B+ → µ+νµ with data collected at the Υ(4S) resonance by the BABAR experiment at thePEP-II storage ring. In a sample of 88.4 million BB pairs, we find no significant evidence for a signal and set an upper limit on the branching fraction B(B+ →µ+νµ)<6.6×10−6 at the 90% confidencelevel. PACSnumbers: 13.25.Hw,12.15.Hh,11.30.Er ThestudyofthepurelyleptonicdecaysB+ ℓ+ν (ℓ The BABAR detector is optimized for the asymmet- ℓ → =e,µ,orτ)canprovidesensitivitytopoorlyconstrained ric collisions at PEP-II and is described in detail else- StandardModel(SM)parametersandalsoactasaprobe where [8]. Charged particle trajectories are measured ofnewphysics. IntheSM,thesedecaysproceedthrough with a 5-layer double-sided silicon vertex tracker (SVT) W-bosonannihilationwithabranchingfractiongivenby anda40-layerdriftchamber(DCH),whicharecontained inthe1.5Tmagneticfieldofasuperconductingsolenoid. G2m m2 m2 2 (B+ ℓ+ν )= F B ℓ 1 ℓ f2 V 2τ , A detector of internally reflected Cherenkov radiation B → ℓ 8π (cid:18) − m2B(cid:19) B| ub| B+ provides identification of charged kaons and pions. The energies of neutral particles are measured by an electro- where G is the Fermi coupling constant, m and m F ℓ B magnetic calorimeter (EMC) consisting of 6580 CsI(Tl) are the lepton and B meson masses, and τ is the B+ crystals. The flux return of the solenoid is instrumented B+ lifetime. The decay rate is sensitive to the prod- with resistive plate chambers to provide muon identifi- uct of the Cabibbo-Kobayashi-Maskawa (CKM) matrix cation (IFR). A Monte Carlo (MC) simulation of the element V andtheB decayconstantf ,whichispro- | ub| B BABAR detector based on GEANT4 [9] was used to opti- portional to the wave function for zero separation be- mize the signal selection criteria and evaluate the signal tween the quarks. Currently, our best understanding of efficiency. f comes from lattice gauge calculations where the the- B The B+ µ+ν decay produces a mono-energetic oreticaluncertainty is roughly 15%[1]. This uncertainty µ → muon in the B rest frame with p m /2. Since is a significant limitation on the extraction of V from µ B td ≈ precisionB0B0 mixingmeasurements[2]. Obse|rvat|ionof the neutrino goes undetected, we assume that all re- B+ ℓ+ν could provide the first direct measurement maining particles are associated with the decay of the ℓ → other B in the event, which we denote the “companion” of f . B In this note, we present a search for the decay B+ B. Signal events are selected using the kinematic vari- µ+νµ (charge conjugation is implied throughoutthis p→a- ables ∆E =EB∗ −Eb∗ and energy-substitutedmass, mES per). This decay is highly suppressed due to the depen- = Eb∗2−p∗B2 where p∗B (EB∗) is the momentum (en- dVence=on0|.V00ub3|62[a3n]danmd2ℓf(hel=ici1ty98suMpperVes[s1io],nt)h.eASssMumpirneg- ebregapym) oefnetrhgey,reaclolnisntrtuhceteΥd(c4oSm)praenstiofnraBmea.ndWEeb∗reisqutihree ub B |dicti|onfortheB+ µ+νµ branchingfractionisroughly mES to be consistent with the B meson mass, and the 4 10−7. The cu→rrent best published limit, from the energy of the companion B to be consistent with Eb∗ re- CL×EO collaboration, is (B+ µ+ν ) < 2.1 10−5 at sulting in ∆E 0. B → µ × ≃ the 90% confidence level [4]. To reduce non-hadronic backgrounds,we select events The B+ ℓ+ν decay modes are also potentially sen- that containat leastfour chargedtracksandhavea nor- ℓ → sitive to physics beyond the SM. For example, in two- malizedsecondFox-Wolframmoment[10]lessthan0.98. Higgs-doubletmodelssuchastheMinimalSupersymmet- Muon candidates are required to penetrate at least 2.2 ric Standard Model (MSSM), these decays can proceed interaction lengths of material in the IFR, have a mea- at tree-level via an intermediate H±, providing a possi- sured penetration within 0.8 interaction lengths of that ble enhancement up to current experimental limits [5]. expected for a muon, and have an associated energy in Similarly, in R-parity violating extensions of the MSSM, theEMCconsistentwiththatofaminimum-ionizingpar- B+ ℓ+ν may be mediated by scalar supersymmetric ticle. The muon track must have at least 12 DCH hits, ℓ → particles [6, 7]. Hence, upper limits on the B+ ℓ+νℓ momentumtransversetothe beamaxisp⊥ >0.1 GeV/c, → branchingfractionsconstraintheR-parityviolatingcou- and a point of closest approach to the interaction point plings. that is within 10cm along the beam axis and less than The data used in this analysis were collected with the 1.5cmin the transverseplane. For eachmuon candidate BABAR detector at the PEP-II storage ring. The data withmomentumbetween2.25and2.95 GeV/cintheCM sample consists of an integrated luminosity of 81.4 fb−1 frame, we attempt to reconstruct the companion B. accumulated at the Υ(4S) resonance (“on-resonance”) The companion B is formed from all charged tracks and 9.6 fb−1 accumulated at a center-of-mass (CM) en- satisfying the above criteria regarding the distance of ergy about 40 MeV below the Υ(4S) resonance (“off- closestapproachtotheinteractionpoint. Italsoincludes resonance”). The on-resonance sample corresponds to all neutral calorimeter clusters with energy greater than 88.4 million BB pairs. 30 MeV. Particleidentificationis appliedto the charged 5 tracks to identify electrons, muons, kaons and protons TABLEI: Theboundariesofthesignalboxandvariousside- and the resulting mass hypotheses are applied to these bandsdefined for this analysis. tracks to improve the ∆E resolution. Events with additional identified leptons from the companion B are dis- region ∆E (GeV) mES (GeV/c2) carded since they typically arise from semi-leptonic B signal box [-0.75, 0.50 ] > 5.27 blinding box [-1.30, 0.70 ] > 5.24 or charm decays and indicate the presence of additional fit sideband [-0.75, 0.50 ] [ 5.10, 5.24 ] neutrinos. ∆E sideband (bottom) [ -3.00, -1.30 ] > 5.10 Once the companion B is reconstructed, we calculate ∆E sideband (top) [ 0.70, 1.50 ] > 5.10 the muon momentum in the rest frame of the signal B. We assume the signal B travelsin the direction opposite that of the companion B momentum in the Υ(4S) rest were determined using an optimization procedure that frame with a momentum determined by the two-body finds the combination of cuts that maximizes the quan- decay Υ(4S) B+B−. For signal muons, the pµ distri- tity S/√S+B where S and B are the signal and back- → bution peaks at 2.64 GeV/c with an RMS of about 100 ground yields in the MC simulation respectively. The MeV/c. signal branching fraction was set to the SM expectation Backgrounds may arise from any process producing during the optimization procedure. In the MC simula- charged tracks in the momentum range of the signal tion, 24.5%of signaldecayspassing allprevious cuts fall muon. ThetwomostsignificantbackgroundsareB semi- within the signal box. After applying all selection cri- leptonic decays involving b uµν¯ transitions where the teria, the B+ µ+ν efficiency is determined from the → µ endpoint of the muon spectrum approaches that of the → simulation to be (2.24 0.07)%, where the uncertainty is signal, and non-resonantqq¯(“continuum”) events where ± due to MC statistics. a charged pion is mistakenly identified as a muon. The Inadditiontothesignalbox,wehavedefinedaslightly pionmisidentification ratehas been studied using a pion larger blinding box and three sideband regions. The control sample taken from e+e− τ+τ− events in the boundaries of these regions in the (∆E, m ) plane are → ES data. In the momentum and polar angle region rele- listed in Table I. The data within the blinding box were vant for B+ µ+ν , the misidentification probability µ kept hidden until the analysis was completed in order → is estimated to be 2%. In order for continuum events to to avoid the introduction of bias in the event-selection populate the signal region of ∆E and m , there must ES process. be significantmissingenergydue to particlesoutside the We estimate the background in the signal box assum- detectoracceptance,unreconstructedneutralhadrons,or ing that the m distribution is described by the AR- ES additional neutrinos. The muon momentum spectrum GUS function [11]. This assumption is consistent with of the background decreases with increasing momentum the observed distributions in the MC simulation as well so we apply an asymmetric cut about the signal peak, as the data in the ∆E sidebands. The single parame- 2.58<p <2.78 GeV/c. µ ter of the ARGUS function (ζ) is determined from an The continuum background is further suppressed us- unbinned maximum likelihood fit using the data in the ing event-shape variables. These events tend to produce fit sideband defined in Table I. The ARGUS shape (A) a jet-like event topology whereas BB events tend to be is extrapolated through the signal box and constrained spherical. We define a variable, θT∗, which is the angle to be zero at the endpoint, which is fixed at E∗ = 5.29 b between the muon candidate momentum and the thrust GeV/c2. Figure 1 shows the results of the fit. The ex- axis of the companion B in the CM frame. For contin- pected background is calculated using uum background, cosθ∗ peaks sharply near one while | T| 5.29 the distribution is nearly flat for signal decays. By re- A(m )dm oqfuitrhinegco|nctoisnθuT∗u|m<ba0c.k5g5r,owuendrewmhoilveeraetpapirnoixnigm5a4t%elyof9t8h%e Nbkg =Nfit× R555...212704A(mEESS)dmEESS ≡Nfit×RARGUS (1) R signal decays. We also use the polar angle of the miss- where N is the number of events contributing to the fit ingmomentumvectorinthelaboratoryframe,θν,todis- fit. The resultis Nbkg =5.0+−11..84 events. The uncertainty criminateagainstcontinuumbackgrounds. Incontinuum is determined by varying ζ by the 1σ uncertainty from ± events, the missing momentum is often due to particles thefit. IntheMCsimulation(scaledtotheon-resonance thatwereoutsidethedetectoracceptance. Therefore,we luminosity), we find 5.7 0.5 background events in the ± require cosθν < 0.88 so that the missing momentum is signal box, in agreement with the data extrapolation. | | directed into the detector’s fiducial volume. The simulation indicates that the backgroundis primar- We selectB+ µ+ν signalcandidates with simulta- ilycontinuum,consistingof57%light-quark(uu,dd,ss), µ → neousrequirementson∆E andm ,thusforminga“sig- 23% cc, and 20% BB events. ES nalbox”defined by 0.75< ∆E < 0.5 GeV andm > By using the ARGUS function to describe the back- ES − 5.27 GeV/c2. The dimensions of the signal box, as well groundm distribution,weexpecttounderestimatethe ES as the above requirements on p , cosθ∗ and cosθ , contributionofbackgroundsthatpeakwithintheblinded µ | T| | ν| 6 15 V150 2c Ge V/ 0.2 100 e 10 s/ G e 1 ntri 50 0 E 0. 0 es/ 5 -4 -2 ∆E (GeV0) 2 i tr 150 En 2V/c e G100 0 5 0 5.1 5.15 5.2 5.25 5.3 0.0 m (GeV/c2) es/ 50 ES ntri E 0 5.1 5.15 5.2 5.25 5.3 FIG. 1: Results of the ARGUS fit to the on-resonance data m (GeV/c2) satisfying−0.75<∆E <0.5GeV. Thetwodashedlinesindi- ES catethelowerboundariesoftheblindedregionandsignalbox at 5.24 GeV/c2 and5.27 GeV/c2,respectively. Thefitisper- FIG.2: Thedistributionsof∆E andmES ofthecompanion formedonlyontheregion5.10<mES <5.24 GeV/c2 andex- B in the B+ → D0π+ control sample after all previous cuts havebeenapplied. Thepointsaretheon-resonancedatawhile trapolated intothesignal region toestimate thebackground. thehistogramistheMCsimulationnormalizedtothenumber The histogram shows the sum of all simulated background of reconstructed B+ →D0π+ decays. sources normalized to theon-resonance luminosity. regionofm . Thesimulationindicatesthatonlytherel- systematicerrorof1%isincludedduetotherequirement ES ativelysmallcomponentofbackgroundduetoBBevents that the event contain at least four charged tracks. exhibits a mildly peaking m distribution. When the The companion B reconstruction efficiency has been ES background extrapolation is applied to the simulation, studied using a control sample of fully reconstructed the resulting background estimate is 5.2 0.5 events, in B+ D0π+ and B+ D∗0π+ events. These are also agreement with the 5.7 events actually fo±und in the sig- two-→body decays in wh→ich the π+ momentum spectrum nalbox. Althoughneglectingpeakingbackgroundscould is similar to that of the µ+ in signalevents. Once recon- enhance an apparent signal, here the result would be a structed, the pion can be treated as if it were the signal more conservative upper limit. muonandtheD(∗)0 decayproductscanberemovedfrom We have evaluated the systematic uncertainty in the theeventtosimulatetheunobservedneutrino. Thenthe signal efficiency which includes the muon candidate se- companion B is reconstructed in the control sample as lection (particle identification and tracking efficiency) as it would be for signal. We then compare the efficien- wellasthe reconstructionefficiency ofthe companionB. cies for each of our companion B selection cuts in the Themuonidentificationefficiencyhasbeenstudiedusing B+ D(∗)0π+ dataandMCsimulation. Figure2shows → a muon control sample taken from e+e− e+e− µ+µ− acomparisonofon-resonancedataandsimulationforthe events in the data. The identification effi→ciency is mea- ∆E and mES distributions in the B+ D0π+ control → sured in the control sample in bins of momentum, polar sample. Weexpecttheresolutionobservedinthecontrol angle, and charge, and the results are incorporated into sample to represent that of B+ µ+νµ signal events. → the nominal MC simulation. Due to changing detector Wefindthattheefficiencyafterallselectioncutsislower conditions, the muon detection efficiency is not stable inthedatabyafactorof0.94 0.04wheretheuncertainty ± in time so the simulated events are luminosity-weighted is due to the statistics of the data and MC control sam- with the correct efficiencies for each run period. Aver- ples. Most of this discrepancy is due to the requirement aged over the momentum and polar angle distributions on mES. The signal efficiency obtained from the simula- of muons from B+ µ+ν , we estimate that the muon tionisthereforecorrectedbythisfactorandasystematic µ → identification efficiency for this data sample is 61% with error of 4.3% is applied. A summary of the systematic asystematicuncertaintyof4.2%. Thetrackingefficiency uncertainties in the signal efficiency is given in Table II. of the muon candidate was evaluated from the fraction We estimate the overall signal selection efficiency to be of tracks reconstructed in the SVT that are also found 2.09 0.06 (stat) 0.13 (syst) %. ± ± in the DCH. We find that the tracking efficiency is over- Intheon-resonancedatawefind11eventsinthesignal estimated in the simulation by 0.8%, which is applied box where 5.0+1.8 background events are expected. The −1.4 as a correction to the signal efficiency. The associated distributionofthedatainthe(∆E,m )planeisshown ES systematic error is 2%. An additional tracking efficiency in Figure 3. The 90% CL upper limit on the number 7 at the 90% confidence level. TABLE II: Contributions to the systematic uncertainty on thesignal efficiency. We are grateful for the excellent luminosity and ma- chine conditions providedby our PEP-II colleagues,and source correction uncertainty for the substantial dedicated effort from the computing trackingefficiency organizations that support BABAR. The collaborating muon 0.992 2.0% companion B - 1.0% institutions wish to thank SLAC for its support and muon identification - 4.2% kind hospitality. This work is supported by DOE and companion B reconstruction 0.94 4.3% NSF(USA),NSERC(Canada),IHEP(China),CEAand total 0.932 6.4% CNRS-IN2P3 (France), BMBF and DFG (Germany), INFN (Italy), FOM (The Netherlands), NFR (Norway), 2 MIST (Russia), and PPARC (United Kingdom). Indi- vidualshavereceivedsupportfromtheA.P.SloanFoun- dation, Research Corporation,and Alexander von Hum- boldt Foundation. 0 ) V e G ( E ∆ -2 ∗ Also with Universitàdella Basilicata, Potenza, Italy † Also with IFIC, Instituto de F´ısica Corpuscular, CSIC- Universidad deValencia, Valencia, Spain ‡ Deceased -4 [1] S. M. Ryan,Nucl. Phys. Proc. Suppl. 106, 86 (2002). 5.1 5.15 5.2 5.25 5.3 [2] M. Battaglia et al.,arXiv:hep-ph/0304132. m (GeV/c2) [3] Particle Data Group, K. Hagiwara et al. Phys. Rev. D ES 66, 010001 (2002). [4] CLEO Collaboration, M. Artuso et al., Phys. Rev. Lett. FIG. 3: The distribution of ∆E vs mES in the on-resonance 75, 785 (1995). dataafter all selection criteria havebeen applied. Thesignal [5] W. S. Hou,Phys.Rev. D Brief Reports 48, 2342 (1993). box is represented by the solid lines while the dashed lines [6] S.BaekandY.G.Kim,Phys.Rev. D60,077701(1999). indicate theregion used to estimate thebackground. [7] A.G.AkeroydandS.Recksiegel,Phys.Lett. B541,121 (2002). [8] BABAR Collaboration, A. Palano et al., Nucl. Instrum. of signal events observed is n =12.1 events while the Methods A 479, 1 (2002). UL probability of a background fluctuation yielding the ob- [9] S. Agostinelli et al. [GEANT4 Collaboration], Nucl. In- strum. Methods A 506, 250 (2003). servednumber of events ormore is about4%. We set an [10] G. C. Fox and S. Wolfram, Phys. Rev. Lett. 41, 1581 upper limit on the B+ µ+ν branching fraction using → µ (1978). (B+ µ+ν )<n /SwhereSisthesensitivityofthe µ UL [11] The ARGUSfunction is defined by B → experiment which is the product of the signal efficiency and the number of B± mesons in the sample. Assum- innugmebqeuraolfpBro±dumcetisoonnsoifnBt0heanodn-Bre+soinnanΥc(e4Sda)tdaescaamysp,ltehies A(mES)∝mESq1−mES2/Eb∗2 exp[−ζ(1−mES2/Eb∗2)]. estimatedtobe88.4millionwithanuncertaintyof1.1%. Systematic uncertainties are included in the upper limit ARGUS Collaboration, H. Albrecht et al., Phys. Lett. B followingtheprescriptiongiveninreference[12]. Wefind 241, 278 (1990). [12] R.D.CousinsandV.L.Highland,Nucl. Instrum. Meth- (B+ µ+ν )<6.6 10−6 ods A 320, 331 (1992). µ B → ×