A Measurement of the Semileptonic Branching Fraction of the B_s Meson

Syracuse University

Syracuse University

SURFACE

SURFACE

Physics

College of Arts and Sciences

10-25-2011

A Measurement of the Semileptonic Branching Fraction of the B_s

A Measurement of the Semileptonic Branching Fraction of the B_s

Meson

Meson

Duncan Brown

Department of Physics, Syracuse University, Syracuse, NY

J. P. Lees

Laboratoire d'Annecy-le-Vieux de Physique des Particules

Follow this and additional works at: https://surface.syr.edu/phy

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Recommended Citation

Recommended Citation

Brown, Duncan and Lees, J. P., "A Measurement of the Semileptonic Branching Fraction of the B_s Meson" (2011). Physics. 244.

https://surface.syr.edu/phy/244

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arXiv:1110.5600v1 [hep-ex] 25 Oct 2011

BA_BAR-PUB-11/021 SLAC-PUB-14653

A Measurement of the Semileptonic Branching Fraction of the B

Meson

A. Palanoab_,3 _{M. Pappagallo}ab_,3 _{G. Eigen,}4 _{B. Stugu,}4 _{D. N. Brown,}5 _{L. T. Kerth,}5 _{Yu. G. Kolomensky,}5

G. Lynch,5_{H. Koch,}6 _{T. Schroeder,}6 _{D. J. Asgeirsson,}7_{C. Hearty,}7_{T. S. Mattison,}7 _{J. A. McKenna,}7 _{A. Khan,}8

V. E. Blinov,9 _{A. R. Buzykaev,}9 _{V. P. Druzhinin,}9 _{V. B. Golubev,}9 _{E. A. Kravchenko,}9 _{A. P. Onuchin,}9

S. I. Serednyakov,9 _{Yu. I. Skovpen,}9 _{E. P. Solodov,}9 _{K. Yu. Todyshev,}9 _{A. N. Yushkov,}9 _{M. Bondioli,}10

D. Kirkby,10_{A. J. Lankford,}10_{M. Mandelkern,}10 _{D. P. Stoker,}10 _{H. Atmacan,}11 _{J. W. Gary,}11_{F. Liu,}11_{O. Long,}11

G. M. Vitug,11 C. Campagnari,12T. M. Hong,12D. Kovalskyi,12J. D. Richman,12 C. A. West,12 A. M. Eisner,13 J. Kroseberg,13 _{W. S. Lockman,}13 _{A. J. Martinez,}13_{T. Schalk,}13 _{B. A. Schumm,}13 _{A. Seiden,}13 _{C. H. Cheng,}14

D. A. Doll,14 _{B. Echenard,}14_{K. T. Flood,}14 _{D. G. Hitlin,}14 _{P. Ongmongkolkul,}14 _{F. C. Porter,}14 _{A. Y. Rakitin,}14

R. Andreassen,15 _{M. S. Dubrovin,}15 _{Z. Huard,}15 _{B. T. Meadows,}15 _{M. D. Sokoloff,}15 _{L. Sun,}15_{P. C. Bloom,}16

W. T. Ford,16 _{A. Gaz,}16 _{M. Nagel,}16 _{U. Nauenberg,}16_{J. G. Smith,}16 _{S. R. Wagner,}16_{R. Ayad,}17, ∗ _{W. H. Toki,}17

B. Spaan,18 M. J. Kobel,19 K. R. Schubert,19R. Schwierz,19 D. Bernard,20M. Verderi,20P. J. Clark,21S. Playfer,21 D. Bettonia_,22 _{C. Bozzi}a_,22 _{R. Calabrese}ab_,22 _{G. Cibinetto}ab_,22 _{E. Fioravanti}ab_,22 _{I. Garzia}ab_,22 _{E. Luppi}ab_,22

M. Muneratoab_,22 _{M. Negrini}ab_,22 _{L. Piemontese}a_,22 _{V. Santoro,}22 _{R. Baldini-Ferroli,}23 _{A. Calcaterra,}23

R. de Sangro,23 _{G. Finocchiaro,}23 _{M. Nicolaci,}23 _{P. Patteri,}23 _{I. M. Peruzzi,}23, † _{M. Piccolo,}23_{M. Rama,}23

A. Zallo,23_{R. Contri}ab_,24 _{E. Guido}ab_,24 _{M. Lo Vetere}ab_,24 _{M. R. Monge}ab_,24 _{S. Passaggio}a_,24 _{C. Patrignani}ab_,24

E. Robuttia_,24 _{B. Bhuyan,}25 _{V. Prasad,}25_{C. L. Lee,}26_{M. Morii,}26 _{A. J. Edwards,}27_{A. Adametz,}28_{J. Marks,}28

U. Uwer,28 _{F. U. Bernlochner,}29 _{H. M. Lacker,}29_{T. Lueck,}29 _{P. D. Dauncey,}30 _{M. Tibbetts,}30 _{P. K. Behera,}31

U. Mallik,31_{C. Chen,}32 _{J. Cochran,}32_{W. T. Meyer,}32_{S. Prell,}32_{E. I. Rosenberg,}32_{A. E. Rubin,}32 _{A. V. Gritsan,}33

Z. J. Guo,33 _{N. Arnaud,}34 _{M. Davier,}34 _{D. Derkach,}34 _{G. Grosdidier,}34 _{F. Le Diberder,}34 _{A. M. Lutz,}34

B. Malaescu,34 _{P. Roudeau,}34 _{M. H. Schune,}34 _{A. Stocchi,}34 _{G. Wormser,}34 _{D. J. Lange,}35_{D. M. Wright,}35

I. Bingham,36 _{C. A. Chavez,}36 _{J. P. Coleman,}36_{J. R. Fry,}36 _{E. Gabathuler,}36 _{D. E. Hutchcroft,}36 _{D. J. Payne,}36

C. Touramanis,36_{A. J. Bevan,}37 _{F. Di Lodovico,}37 _{R. Sacco,}37 _{M. Sigamani,}37 _{G. Cowan,}38 _{D. N. Brown,}39

C. L. Davis,39_{A. G. Denig,}40 _{M. Fritsch,}40 _{W. Gradl,}40 _{A. Hafner,}40 _{E. Prencipe,}40 _{K. E. Alwyn,}41 _{D. Bailey,}41

R. J. Barlow,41, ‡ _{G. Jackson,}41 _{G. D. Lafferty,}41 _{E. Behn,}42 _{R. Cenci,}42 _{B. Hamilton,}42 _{A. Jawahery,}42

D. A. Roberts,42 G. Simi,42 C. Dallapiccola,43 R. Cowan,44 D. Dujmic,44 G. Sciolla,44 D. Lindemann,45 P. M. Patel,45_{S. H. Robertson,}45 _{M. Schram,}45 _{P. Biassoni}ab_,46 _{N. Neri}ab_,46 _{F. Palombo}ab_,46 _{S. Stracka}ab_,46

L. Cremaldi,47 _{R. Godang,}47, § _{R. Kroeger,}47 _{P. Sonnek,}47 _{D. J. Summers,}47 _{X. Nguyen,}48 _{M. Simard,}48

P. Taras,48_{G. De Nardo}ab_,49 _{D. Monorchio}ab_,49 _{G. Onorato}ab_,49 _{C. Sciacca}ab_,49 _{G. Raven,}50_{H. L. Snoek,}50

C. P. Jessop,51_{K. J. Knoepfel,}51 _{J. M. LoSecco,}51_{W. F. Wang,}51 _{K. Honscheid,}52 _{R. Kass,}52 _{J. Brau,}53 _{R. Frey,}53

N. B. Sinev,53 _{D. Strom,}53 _{E. Torrence,}53_{E. Feltresi}ab_,54 _{N. Gagliardi}ab_,54 _{M. Margoni}ab_,54 _{M. Morandin}a_,54

M. Posoccoa_,54 _{M. Rotondo}a_,54 _{F. Simonetto}ab_,54 _{R. Stroili}ab_,54 _{S. Akar,}55 _{E. Ben-Haim,}55 _{M. Bomben,}55

G. R. Bonneaud,55 _{H. Briand,}55 _{G. Calderini,}55 _{J. Chauveau,}55 _{O. Hamon,}55 _{Ph. Leruste,}55 _{G. Marchiori,}55

J. Ocariz,55 _{S. Sitt,}55 _{M. Biasini}ab_,56 _{E. Manoni}ab_,56 _{S. Pacetti}ab_,56 _{A. Rossi}ab_,56 _{C. Angelini}ab_,57 _{G. Batignani}ab_,57

S. Bettariniab_,57_{M. Carpinelli}ab_,57, ¶ _{G. Casarosa}ab_,57_{A. Cervelli}ab_,57 _{F. Forti}ab_,57_{M. A. Giorgi}ab_,57 _{A. Lusiani}ac_,57

B. Oberhofab_,57 _{E. Paoloni}ab_,57 _{A. Perez}a_,57 _{G. Rizzo}ab_,57 _{J. J. Walsh}a_,57 _{D. Lopes Pegna,}58_{C. Lu,}58 _{J. Olsen,}58

A. J. S. Smith,58 _{A. V. Telnov,}58 _{F. Anulli}a_,59 _{G. Cavoto}a_,59 _{R. Faccini}ab_,59 _{F. Ferrarotto}a_,59 _{F. Ferroni}ab_,59

M. Gasperoab_,59 _{L. Li Gioi}a_,59 _{M. A. Mazzoni}a_,59 _{G. Piredda}a_,59 _{F. Renga}ab_,59 _{C. B¨unger,}60 _{O. Gr¨unberg,}60

T. Hartmann,60 _{T. Leddig,}60_{H. Schr¨oder,}60 _{R. Waldi,}60 _{T. Adye,}61 _{E. O. Olaiya,}61_{F. F. Wilson,}61_{S. Emery,}62

G. Hamel de Monchenault,62 _{G. Vasseur,}62 _{Ch. Y`eche,}62_{D. Aston,}63_{D. J. Bard,}63_{R. Bartoldus,}63_{C. Cartaro,}63

M. R. Convery,63_{J. Dorfan,}63 _{G. P. Dubois-Felsmann,}63 _{W. Dunwoodie,}63 _{M. Ebert,}63 _{R. C. Field,}63 _{M. Franco}

Sevilla,63 _{B. G. Fulsom,}63 _{A. M. Gabareen,}63 _{M. T. Graham,}63 _{P. Grenier,}63 _{C. Hast,}63 _{W. R. Innes,}63

M. H. Kelsey,63_{H. Kim,}63 _{P. Kim,}63_{M. L. Kocian,}63 _{D. W. G. S. Leith,}63 _{P. Lewis,}63_{B. Lindquist,}63 _{S. Luitz,}63

V. Luth,63 _{H. L. Lynch,}63_{D. B. MacFarlane,}63_{D. R. Muller,}63 _{H. Neal,}63 _{S. Nelson,}63 _{M. Perl,}63_{T. Pulliam,}63

B. N. Ratcliff,63 _{A. Roodman,}63 _{A. A. Salnikov,}63 _{R. H. Schindler,}63 _{A. Snyder,}63 _{D. Su,}63 _{M. K. Sullivan,}63

A. K. Yarritu,63 _{C. C. Young,}63 _{V. Ziegler,}63_{W. Park,}64_{M. V. Purohit,}64_{R. M. White,}64 _{J. R. Wilson,}64

A. Randle-Conde,65 _{S. J. Sekula,}65 _{M. Bellis,}66_{J. F. Benitez,}66 _{P. R. Burchat,}66_{T. S. Miyashita,}66_{M. S. Alam,}67

J. A. Ernst,67 _{R. Gorodeisky,}68 _{N. Guttman,}68 _{D. R. Peimer,}68 _{A. Soffer,}68 _{P. Lund,}69 _{S. M. Spanier,}69

R. Eckmann,70 _{J. L. Ritchie,}70 _{A. M. Ruland,}70 _{C. J. Schilling,}70 _{R. F. Schwitters,}70 _{B. C. Wray,}70

J. M. Izen,71 _{X. C. Lou,}71 _{F. Bianchi}ab_,72 _{D. Gamba}ab_,72 _{L. Lanceri}ab_,73 _{L. Vitale}ab_,73 _{F. Martinez-Vidal,}74

A. Oyanguren,74 _{H. Ahmed,}75 _{J. Albert,}75 _{Sw. Banerjee,}75_{H. H. F. Choi,}75_{G. J. King,}75_{R. Kowalewski,}75

M. J. Lewczuk,75 _{I. M. Nugent,}75_{J. M. Roney,}75 _{R. J. Sobie,}75 _{N. Tasneem,}75 _{T. J. Gershon,}76 _{P. F. Harrison,}76

T. E. Latham,76 _{E. M. T. Puccio,}76 _{H. R. Band,}77 _{S. Dasu,}77 _{Y. Pan,}77 _{R. Prepost,}77 _{and S. L. Wu}77

(The BA_BAR _{Collaboration)}

1_{Laboratoire d’Annecy-le-Vieux de Physique des Particules (LAPP),}

Universit´e de Savoie, CNRS/IN2P3, F-74941 Annecy-Le-Vieux, France

2_{Universitat de Barcelona, Facultat de Fisica, Departament ECM, E-08028 Barcelona, Spain} 3_{INFN Sezione di Bari}a_{; Dipartimento di Fisica, Universit`a di Bari}b_{, I-70126 Bari, Italy}

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_{Ruhr Universit¨at Bochum, Institut f¨ur Experimentalphysik 1, D-44780 Bochum, Germany}

7_{University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1} 8_{Brunel University, Uxbridge, Middlesex UB8 3PH, United Kingdom}

9_{Budker Institute of Nuclear Physics, Novosibirsk 630090, Russia} 10_{University of California at Irvine, Irvine, California 92697, USA} 11_{University of California at Riverside, Riverside, California 92521, USA} 12_{University of California at Santa Barbara, Santa Barbara, California 93106, USA}

13_{University of California at Santa Cruz, Institute for Particle Physics, Santa Cruz, California 95064, USA} 14_{California Institute of Technology, Pasadena, California 91125, USA}

15_{University of Cincinnati, Cincinnati, Ohio 45221, USA} 16_{University of Colorado, Boulder, Colorado 80309, USA} 17_{Colorado State University, Fort Collins, Colorado 80523, USA}

18_{Technische Universit¨at Dortmund, Fakult¨at Physik, D-44221 Dortmund, Germany}

19_{Technische Universit¨at Dresden, Institut f¨ur Kern- und Teilchenphysik, D-01062 Dresden, Germany} 20_{Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France}

21_{University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom}

22_{INFN Sezione di Ferrara}a_{; Dipartimento di Fisica, Universit`a di Ferrara}b_{, I-44100 Ferrara, Italy} 23_{INFN Laboratori Nazionali di Frascati, I-00044 Frascati, Italy}

24_{INFN Sezione di Genova}a_{; Dipartimento di Fisica, Universit`a di Genova}b_{, I-16146 Genova, Italy} 25_{Indian Institute of Technology Guwahati, Guwahati, Assam, 781 039, India}

26_{Harvard University, Cambridge, Massachusetts 02138, USA} 27_{Harvey Mudd College, Claremont, California 91711}

28_{Universit¨at Heidelberg, Physikalisches Institut, Philosophenweg 12, D-69120 Heidelberg, Germany} 29_{Humboldt-Universit¨at zu Berlin, Institut f¨ur Physik, Newtonstr. 15, D-12489 Berlin, Germany}

30_{Imperial College London, London, SW7 2AZ, United Kingdom} 31_{University of Iowa, Iowa City, Iowa 52242, USA} 32_{Iowa State University, Ames, Iowa 50011-3160, USA} 33_{Johns Hopkins University, Baltimore, Maryland 21218, USA}

34_{Laboratoire de l’Acc´el´erateur Lin´eaire, IN2P3/CNRS et Universit´e Paris-Sud 11,}

Centre Scientifique d’Orsay, B. P. 34, F-91898 Orsay Cedex, France

35_{Lawrence Livermore National Laboratory, Livermore, California 94550, USA} 36_{University of Liverpool, Liverpool L69 7ZE, United Kingdom} 37_{Queen Mary, University of London, London, E1 4NS, United Kingdom}

38_{University of London, Royal Holloway and Bedford New College, Egham, Surrey TW20 0EX, United Kingdom} 39_{University of Louisville, Louisville, Kentucky 40292, USA}

40_{Johannes Gutenberg-Universit¨at Mainz, Institut f¨ur Kernphysik, D-55099 Mainz, Germany} 41_{University of Manchester, Manchester M13 9PL, United Kingdom}

42_{University of Maryland, College Park, Maryland 20742, USA} 43_{University of Massachusetts, Amherst, Massachusetts 01003, USA}

44_{Massachusetts Institute of Technology, Laboratory for Nuclear Science, Cambridge, Massachusetts 02139, USA} 45_{McGill University, Montr´eal, Qu´ebec, Canada H3A 2T8}

46_{INFN Sezione di Milano}a_{; Dipartimento di Fisica, Universit`a di Milano}b_{, I-20133 Milano, Italy} 47_{University of Mississippi, University, Mississippi 38677, USA}

48_{Universit´e de Montr´eal, Physique des Particules, Montr´eal, Qu´ebec, Canada H3C 3J7} 49_{INFN Sezione di Napoli}a_{; Dipartimento di Scienze Fisiche,}

Universit`a di Napoli Federico IIb_{, I-80126 Napoli, Italy}

50_{NIKHEF, National Institute for Nuclear Physics and High Energy Physics, NL-1009 DB Amsterdam, The Netherlands} 51_{University of Notre Dame, Notre Dame, Indiana 46556, USA}

52_{Ohio State University, Columbus, Ohio 43210, USA} 53_{University of Oregon, Eugene, Oregon 97403, USA}

54_{INFN Sezione di Padova}a_{; Dipartimento di Fisica, Universit`a di Padova}b_{, I-35131 Padova, Italy} 55_{Laboratoire de Physique Nucl´eaire et de Hautes Energies,}

IN2P3/CNRS, Universit´e Pierre et Marie Curie-Paris6, Universit´e Denis Diderot-Paris7, F-75252 Paris, France

56_{INFN Sezione di Perugia}a_{; Dipartimento di Fisica, Universit`a di Perugia}b_{, I-06100 Perugia, Italy} 57_{INFN Sezione di Pisa}a_{; Dipartimento di Fisica,}

Universit`a di Pisab_{; Scuola Normale Superiore di Pisa}c_{, I-56127 Pisa, Italy} 58_{Princeton University, Princeton, New Jersey 08544, USA}

59_{INFN Sezione di Roma}a_{; Dipartimento di Fisica,}

Universit`a di Roma La Sapienzab_{, I-00185 Roma, Italy} 60_{Universit¨at Rostock, D-18051 Rostock, Germany}

61_{Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, United Kingdom} 62_{CEA, Irfu, SPP, Centre de Saclay, F-91191 Gif-sur-Yvette, France}

63_{SLAC National Accelerator Laboratory, Stanford, California 94309 USA} 64_{University of South Carolina, Columbia, South Carolina 29208, USA}

65_{Southern Methodist University, Dallas, Texas 75275, USA} 66_{Stanford University, Stanford, California 94305-4060, USA} 67_{State University of New York, Albany, New York 12222, USA} 68_{Tel Aviv University, School of Physics and Astronomy, Tel Aviv, 69978, Israel}

69_{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_{INFN Sezione di Torino}a_{; Dipartimento di Fisica Sperimentale, Universit`a di Torino</sub>b<sub>, I-10125 Torino, Italy</sub> 73<sub>INFN Sezione di Trieste</sub>a<sub>; Dipartimento di Fisica, Universit`a di Trieste}b_{, I-34127 Trieste, Italy}

74_{IFIC, Universitat de Valencia-CSIC, E-46071 Valencia, Spain} 75_{University of Victoria, Victoria, British Columbia, Canada V8W 3P6} 76_{Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom}

77_{University of Wisconsin, Madison, Wisconsin 53706, USA}

We report a measurement of the inclusive semileptonic branching fraction of the Bsmeson using

data collected with the BA_BARdetector in the center-of-mass (CM) energy region above the Υ(4S) resonance. We use the inclusive yield of φ mesons and the φ yield in association with a high-momentum lepton to perform a simultaneous measurement of the semileptonic branching fraction and the production rate of Bs mesons relative to all B mesons as a function of CM energy. The

inclusive semileptonic branching fraction of the Bs meson is determined to be B(Bs → ℓνX) =

9.5+2.5_−2.0(stat)+1.1_−1.9(syst)%, where ℓ indicates the average of e and µ.

PACS numbers: 14.40.Nd 13.20.He

Semileptonic decays of heavy-flavored hadrons serve as a powerful probe of the electroweak and strong inter-actions and are essential to determinations of Cabibbo-Kobayashi-Maskawa (CKM) matrix elements (see, for example, “Determination of Vcb and Vub” in Ref. [1]).

The inclusive semileptonic branching fractions of the Bd

and Bu mesons are measured to high precision by

ex-periments operating at the Υ(4S) resonance, which de-cays almost exclusively to BB pairs (here and through-out this note, BB refers to BdBd and BuBu).

How-ever, lacking an analogous production mechanism, in-formation on branching fractions of the Bs meson

re-mains scarce nearly two decades after its first observa-tion [1]. Here we report a measurement of the inclusive semileptonic branching fraction of the Bs meson using

data collected with the BA_BAR detector at the PEP-II asymmetric-energy electron-positron collider, located at the SLAC National Accelerator Laboratory. The data

were collected in a scan of center-of-mass (CM) energies above the Υ(4S) resonance, including the region near the BsBs threshold. As φ mesons are particularly abundant

in Bs decays due to the CKM-favored Bs → Ds

transi-tion, the inclusive production rate of φ mesons and the rate of φ mesons produced in association with a high momentum electron or muon can be used to simultane-ously determine the Bs semileptonic branching fraction

and the Bs production fraction as a function of the CM

energy ECM.

The energy scan data correspond to an integrated lu-minosity of 4.25 fb−1 collected in 2008 in 5 MeV steps in the range 10.54 GeV ≤ ECM ≤ 11.2 GeV. In a

previ-ous study [2], we presented a measurement of the inclu-sive b quark production cross section Rb = σ(e+e− →

bb)/σ0_(e+_e− _{→ µ}+_µ−_{) in this energy range, using this}

same data sample (σ0 _{is the zeroth-order QED}

18.55 fb−1 of data collected in 2007 at the peak of the Υ(4S) resonance, and 7.89 fb−1 collected 40 MeV be-low the Υ(4S), to evaluate backgrounds from continuum (e+_e− _{→ qq, q = u, d, s, c quark production) and BB}

events. We choose below-resonance data for which de-tector conditions most closely resemble those of the scan, and on-resonance data corresponding to roughly twice the luminosity of the below-resonance sample. The sizes of these samples are sufficient to reduce the correspond-ing systematic uncertainties below those associated with irreducible sources.

The BA_BARdetector is described in detail elsewhere [3]. The tracking system is composed of a five-layer sili-con vertex tracker (SVT) and a 40-layer drift chamber (DCH) in a 1.5-Tesla axial magnetic field. The SVT provides a precise determination of the track parame-ters near the interaction point and standalone tracking for charged particle transverse momenta (pt) down to

50 MeV/c. The DCH provides a 98% efficient measure-ment of charged particles with pt > 500 MeV/c. The

pt resolution is σpt/pt = (0.13 · pt+ 0.45)%. Hadron and muon identification in BA_BAR_{is achieved by using a} likelihood-based algorithm exploiting specific ionization measured in the SVT and the DCH in combination with information from an instrumented magnetic-flux return and the Cherenkov angle obtained from the detector of internally reflected Cherenkov light. Electron identifica-tion is provided by a combinaidentifica-tion of tracking and in-formation from the CsI(Tl) electromagnetic calorimeter, which also serves to measure photon energies. For the evaluation of event reconstruction efficiencies across the scan range, simulated samples of e+_e− _{→ µ}+_µ−_,

con-tinuum, and e+_e− _{→ B}(∗)

q B(∗)q , q = u, d, s events,

cre-ated with the kk2f [4], JETSET [5], and EvtGen [6] event generators, respectively, are processed through a Geant4_{[7] simulation of the B}A_BARdetector.

For this measurement, we present the scan data as a function of ECM in bins of 15 MeV. In each bin we

measure the number of BB-like events (defined below), the number of such events containing a φ meson, and the number of events in which the φ meson is accompanied by a charged lepton candidate. The results are normalized to the number of e+_e− _{→ µ}+_µ− _{events in the same}

en-ergy bin so that the luminosity dependence in each bin is removed. These three measurements are used to extract the fractional number of BsBs events and the

semilep-tonic branching fraction B(Bs → ℓνX). The procedure

is described in detail below.

To suppress QED background, events are preselected with a multihadronic event filter optimized to select BB and BsBsevents. The filter requires a minimum number

of charged tracks in the event (3), a minimum total event energy (4.5 GeV), a well-identified primary vertex near the expected collision point, and a maximum value of the ratio of the second to zeroth Fox Wolfram moments [8] (R2 < 0.2) calculated in the CM frame using both

charged tracks and energy depositions in the calorimeter, where the latter are required not to be associated with a track.

A different preselection is used to identify muon pair events. Events passing this selection must have at least two tracks. The two highest momentum tracks are re-quired to be back-to-back in the CM frame to within 10 degrees, appear at large angles to the beam axis (| cos θCM| < 0.7486), and have an invariant mass greater

than 7.5 GeV/c2_{. In addition, we require that less than}

1 GeV be deposited in the electromagnetic calorimeter. This selection is 43% efficient for simulated µ+_µ− _events

while rejecting virtually all continuum events.

Candidate φ mesons are reconstructed in the φ → K+_K− _{decay mode, by forming pairs of oppositely}

charged tracks that are consistent with the kaon hy-pothesis. In each event, the φ candidate with the best-identified K±_{daughters is selected by assigning a weight}

to each K± _{based on the particle identification criteria.}

The φ candidate with the largest sum of kaon weights is selected. The invariant mass distribution of these can-didates is used to determine the φ yield in a given ECM

bin using a maximum likelihood fit. Events containing φ candidates and an electron or muon candidate with a CM momentum exceeding 900 MeV/c are used to determine the yield of events with both a φ and a lepton (φ-lepton events). The requirement on the lepton momentum sup-presses background from semileptonic charm decays.

Figure 1 shows, as an example, the K+_K− _invariant

mass distribution for (a) all φ candidates, and (b) φ-lepton candidates, in the energy bin 10.8275 < ECM <

10.8425 GeV. These mass distributions are fit to the func-tion f (M ; N, b, c) ≡ N V (mKK; mφ, Γφ, σ) + N c (1 + b mKK) s 1 − 2mK mKK 2 , (1) with mK the world-average mass value [1] of the K±.

V (mKK; mφ, Γφ, σ) is a Voigt profile (the convolution of

a Breit-Wigner function 1/((mKK− mφ)2+ Γφ2/4) with

a Gaussian resolution function) normalized to unity, so that N is the number of events in the peak. We fix the mean (mφ) and Breit-Wigner width (Γφ) to the world

av-erage values of the φ mass and natural width [1], and the width of the Gaussian resolution (σ) by first performing all of the φ fits with the parameter left free, then fixing it to the weighted mean of all of the values obtained across the scan. The value in data determined by this method is σ = 1.6 ± 0.04(stat) MeV/c2_{. The combinatoric}

back-ground is modeled as the product of a linear term and a threshold cutoff function parameterized by the slope of the linear term (b) and a relative scaling (c).

To determine the φ and φ-lepton yields from B decays in each ECMbin, the contribution of continuum events is

) 2 ) (GeV/c -K + m(K 0.98 1 1.02 1.04 1.06 1.08 1.1 1.12 1.14 2 Events/2.5 MeV/c 0 50 100 150 200 250 (a) ) 2 ) (GeV/c -K + m(K 0.98 1 1.02 1.04 1.06 1.08 1.1 1.12 1.14 2 Events/2.5 MeV/c 0 5 10 15 20 25 30 35 (b)

FIG. 1. Invariant mass distribution of φ → K+_K−

candi-dates in the energy bin 10.8275 GeV ≤ ECM≤10.8425 GeV:

(a) inclusive φ candidates; (b) φ-lepton candidates. The back-ground shape is shown by the dashed curve and the total fit by the solid curve.

subtracted. This is achieved by using the data collected below the Υ(4S) described above. The event, φ, and φ-lepton yields are measured in this dataset following the same procedures described above. These yields are cor-rected for the energy dependence of the reconstruction efficiencies and are then subtracted from the scan yields in each ECM bin. This procedure neglects the different

energy dependence of a small component of the hadronic and dimuon cross sections, primarily due to the presence of initial state radiative (ISR) e+_e− _{→ γΥ(1S, 2S, 3S)}

and two photon e+_e− _{→ e}+_e−_γ∗_γ∗ _{→ e}+_e−_X

h events,

which do not scale according to 1/E2

CM. The effect of

these contributions is to introduce a small energy depen-dence on the amount to be subtracted from each bin. The average size of this effect is estimated to be less than 2% of the below-resonance event yield. The impact on the result is taken as a systematic uncertainty.

The normalized event, φ, and φ-lepton yields after the continuum subtraction are presented in Fig. 2. These three quantities, denoted Ch, Cφ and Cφℓ respectively,

can be expressed in terms of contributions from events

(GeV) CM E 10.6 10.7 10.8 10.9 11 11.1 11.2 /15 MeV µµ Events per 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 (a) (GeV) CM E 10.6 10.7 10.8 10.9 11 11.1 11.2 /15 MeV µµ yield per φ 0 5 10 15 20 25 -3 10 × (b) (GeV) CM E 10.6 10.7 10.8 10.9 11 11.1 11.2 /15 MeV µµ

-lepton yield per

φ 0 1 2 3 4 5 -3 10 × (c)

FIG. 2. Relative (a) event, (b) φ, and (c) φ-lepton yields, normalized to the µ+_µ−_{yields. Corrections for detector}

effi-ciency have not been applied. The dotted vertical line indi-cates the Bsproduction threshold.

containing B_u,d(∗) and Bs(∗) events, the cross section ratio

RB≡ P q={u,d,s}

σ(e+_e−_{→ B}

qBq)/σµ+_µ−, and the related reconstruction efficiencies, as follows:

Ch= RB[fsǫsh+ (1 − fs)ǫh] (2) Cφ= RBfsǫsφP (BsBs→ φX) +(1 − fs)ǫφP (BB → φX)  (3) Cφℓ= RBfsǫsφℓP (BsBs→ φℓX) +(1 − fs)ǫφℓP (BB → φℓX) (4)

(with energy dependence implicit in all terms here and elsewhere), where

fs≡

NBs NBu+ NBd+ NBs

(5) and ǫX(ǫsX) is the efficiency for a Bu,d (Bs) pair to

con-tribute to the event, φ or φ-lepton yield. The efficien-cies are estimated from simulation, while P (BB → φX)

and P (BB → φℓX), which are the probabilities that a φ or a φ-lepton combination is produced in an event with a BB pair, are measured using the Υ(4S) data sample described above. Specifically, we determine the φ and φ-lepton yields in the the Υ(4S) data. We then apply Eqs. (2), (3), and (4) with fs= 0 to extract ǫφP (BB → φX)

and ǫφℓP (BB → φℓX). Simulations are used to

extrapo-late the values of the efficiencies to other energies.

(GeV) CM E 10.6 10.7 10.8 10.9 11 11.1 11.2 s f -0.05 0 0.05 0.1 0.15 0.2

FIG. 3. Results for the fraction fs as a function of ECM.

The inner error bars show the statistical uncertainties and the outer error bars the statistical and systematic uncertain-ties added in quadrature. The dotted line denotes the Bs

threshold.

The remaining unknown quantities of interest are the probabilities P (BsBs→ φX) and P (BsBs→ φℓX) that

a BsBs pair will yield a φ or φ-lepton event. To

esti-mate P (BsBs→ φX) we use the current world averages

[1] of the inclusive branching fractions B(Bs → DsX),

B(Ds → φX), and B(D → φX). Here and in the

fol-lowing D refers to the sum of D± _{and D}0_{contributions.}

Also needed are estimates of the unmeasured branching fractions B(Bs→ ccφ) and B(Bs→ DDsX). The former

quantity accounts for direct Bs→ φ production, a

sub-stantial fraction of which arises from Bsto charmonium

decays. We use the central value from the simulation, 1.7%, which is roughly consistent with charmonium pro-duction in the B system. For the latter quantity we use a naive quark model prediction of 15% for b → ccs.

The inclusive φ yield in Bsdecays can be expressed as:

P (Bs→ φX) = B(Bs→ D(∗)s X) B(Ds→ φX)

+ B(Bs→ ccφ)

+ B(Bs→ DDsX) B(D → φX),

(6)

from which we determine

P (BsBs→ φX) = 2P (Bs→ φX) − P (Bs→ φX)2. (7)

The unknown quantities in Eqs. (2) and (3) are fs and

the common normalization RB. The ratio fscan be

de-termined as a function of ECM by eliminating RB

be-tween the two equations. The result is presented in Fig. 3. The ratio fs peaks around the Υ(5S) mass. The

to-tal excess below the BsBsthreshold and deficit above 11

GeV are consistent with zero within 1.5 and 1.3 standard deviations, respectively.

Using Eq. (4), a χ2 _{is constructed from the measured}

and expected values of P (BsBs→ φℓX) across the entire

scan. The χ2_{is minimized with respect to B(B}

s→ ℓνX).

The following processes contribute to Cφℓ from BsBs

events: primary leptons originating from a Bs

tonic decay, secondary leptons resulting from semilep-tonic decays of charmed mesons, and π±_{or K}±

misiden-tified as e± _{or µ}±_{. The contribution from primary}

lep-tons arises from events where one or both Bs mesons

decay semileptonically, and we determine the φ-lepton efficiency for each case (denoted ǫs

φℓ for one semileptonic

decay and ǫs

φℓℓ for two). It is found that ǫsφℓranges from

8.5% − 10% and ǫs φℓℓ is about 10%. l X) (%) → s Br(B 0 10 20 30 40 50 2_χ 10 15 20 25 30 35 40 45 50 55

FIG. 4. χ2 _{formed from the measured and expected yields,}

as described in the text, as a function of the semileptonic branching fraction. Note that since we express the branching fraction as the average of the e and µ channels, the physical bound is 50%.

For the secondary lepton contribution, we consider events with up to two leptons coming from D±_{, D}0 _or

D±

s decays. The selection efficiency in this case is

esti-mated as the product of the φ reconstruction efficiency in BsBsevents in which neither Bsdecays semileptonically

but a lepton candidate is identified (referred to below as ǫD

φ), and a lepton detection efficiency determined from

simulation (ǫD

ℓ ). It is found that ǫDφ lies in the range

15% − 16.5%, and ǫD

ℓ in the range 8% − 9.5% per lepton.

The contribution from hadrons that are misidentified as leptons is estimated from simulation to be 3.3% of the φ-lepton candidates in BsBs events.

For the expected and measured φ yields, we find:

ǫs φℓP (BsBs→ φℓX)Primary= (2ǫs_φℓ− ǫs_φℓℓ)B(Ds→ φX) [−2 + B(Ds→ φX)] [B(Bs→ ℓνX)]2 + B(Bs→ ℓνX)ǫsφℓ h B(Ds→ φX) + [1 − B(Ds→ φX)]P (Bs→ φX) i , (8) ǫsφℓP (BsBs→ φℓX)Secondary= 2ǫDℓǫDφ ( h B(Ds→ ℓνφ) + B(Ds→ ℓνX)B(Ds→ φX) − B(Ds→ ℓνφ)B(Ds→ φX) i [B(Bs→ ℓνX)]2 +hP (Bs→ φX)(B(Ds→ ℓνφ) − B(Ds→ ℓνX)) − B(Ds→ ℓνφ) − B(Bs→ DsX)B(Ds→ ℓνφ) − B(Bs→ DsX)B(Ds→ ℓνX)B(Ds→ φX) + B(Bs→ DsX)B(Ds→ ℓνφ)B(Ds→ φX) + (B(Ds→ φX) − 2) X i∈u,d,s B(Bs→ D(∗)−s Di(X))B(Di→ ℓνX) i B(Bs→ ℓνX) + B(Bs→ DsX)P (Bs→ φX) [B(Ds→ ℓνX) − B(Ds→ ℓνφ)] + B(Bs→ DsX)B(Ds→ ℓνφ) + [B(Bs→ φX) + B(Ds→ φX) − P (Bs→ φX)B(Ds→ φX)] × X i∈u,d,s B(Bs→ D(∗)−s Di(X))B(Di→ ℓνX) ) , (9) ǫsφℓP (BsBs→ φℓX)Expected= ǫφℓs × 0.591 × B(Bs→ ℓνX) − (2ǫsφℓ− ǫsφℓℓ) × 0.289 × (B(Bs→ ℓνX))2 +ǫDφǫDℓ 0.1375 − 0.2721 × B(Bs→ ℓνX) + 0.1339 × (B(Bs→ ℓνX))2 , (10) ǫsφℓP (BsBs→ φℓX)Measured= (1 − 0.033)  Cφℓ fsǫsh+ (1 − fs)ǫh fsCh −(1 − fs)ǫφℓP (BB → φℓX) fs  , (11)

where that Eq. (10) is the sum of Eqs. (8) and (9) after substituting the values of known quantities. The first line in Eq. (10) expresses the contribution from primary leptons and the second that from secondary leptons.

The expression in Eq. (10) for the expected value of ǫs

φℓP (BsBs → φℓX) is quadratic in the unknown

B(Bs → ℓνX), and so a χ2 formed from the deviation

of the expected from the measured values (Eqs. (10) and (11)), summed over all bins above the BsBs threshold,

is quartic in this unknown. Minimizing the χ2 _with

re-spect to the Bs semileptonic branching fraction we find

B(Bs → ℓνX) = 9.5+2.5−2.0%. Figure 4 shows the

depen-dence of χ2 _{on B(B}

s→ ℓνX).

Systematic uncertainties are summarized in Table I and include the contributions described below.

• Uncertainties for branching fractions, which are ei-ther taken from Ref. [1] when known, or assumed to be 50% for B(Bs→ ccφ) and B(Bs→ DDsX).

These are separately listed in Table I, as is B(Bs→

DsX), which contributes a very large uncertainty

compared to the other branching fractions. • Requirements used in the event preselection,

in-cluding the lepton momentum requirement. The uncertainty due to the lepton momentum require-ment dominates in this group, and reflects the de-pendence of the result on the decay model used to

simulate Bssemileptonic decays.

• Fixed parameters used in the fits to mKK,

includ-ing mφ, Γφ, σ.

• The parameterization of the background and ab-sence of a term in the fit corresponding to threshold contributions from light scalars.

• Uncertainties in particle identification (PID) effi-ciencies and hadron misidentification probabilities. • The determination of P (BB → φX) and P (BB → φℓX) in Υ(4S) data (these quantities are deter-mined to 1% and 1.8% relative uncertainty). • Sensitivity of efficiencies to differences in branching

fractions implemented in simulation compared to their measured values.

• Uncertainties in the continuum-subtracted number of events due to ISR and two photon events, which do not follow a 1/E2

CM energy dependence.

• A correction made to the continuum subtraction of the number of BB-like events due to an over-subtraction found in simulation studies. The size of this correction is about 1% of the amount to be subtracted; we use ±100% of this correction as a systematic uncertainty.

• Possible bias in the χ2 _{minimization technique at}

low statistics. Firstly, evaluating the behavior of the χ2 _{function for many pseudo-data samples}

TABLE I. Relative multiplicative and additive systematic un-certainties for the measurement of B(Bs→ℓνX).

Multiplicative Systematics Relative

Uncertainty (%) B(Bs→Ds(∗)X) +8.72/−13.58

B(Bs→ccφ) (Unmeasured) ±3.20

B(Bs→DDsX) (Unmeasured) +1.12/−1.16

Other Branching Fractions +0.52/−0.54 Event and Lepton Selection +1.99/−2.85 Fixed Fit Parameters +0.49/−0.15 Background Parameterization ±0.93 PID and Lepton Fake Rate ±3.21 P (Bu,dBu,d→φ) +1.47/−1.69

Simulation Branching Fractions ±2.59 ISR and 2γ Background +1.57/−7.14 Correction to Event Subtraction +1.88/−4.59 Technique bias +0.39/−10.00 Total Multiplicative (+10.87/−19.92)% Additive Systematics Uncertainty (×10−3₎

Other Branching Fractions +0.56/−0.64 P (Bu,dBu,d→φℓν) +4.30/−3.90

Total Additive (+4.34/−3.95) × 10−3

Total Systematic (+11.20/−19.34)×10−3

a small bias at low statistics. Secondly, it was found that the analysis performed in high statistics sim-ulation tends to overestimate B(Bs→ ℓνX) by an

amount corresponding to half the statistical error reported.

To determine whether the uncertainties from these sources scale with the result or not, each was evaluated in a simulation sample with a higher semileptonic branching fraction and compared with the result in the normal sim-ulation sample. It was found that the uncertainty from the determination of P (BB → φℓX) in Υ(4S) data does not scale with the branching fraction, nor does the uncer-tainty contributed by several of the input branching frac-tions. These are thus separated in Table I. The remain-ing uncertainties are found to scale with B(Bs→ ℓνX)

and thus to be multiplicative.

Our final result for the inclusive semileptonic branch-ing fraction is 9.5+2.5+1.1−2.0−1.9%, which is the average of the

branching fractions to e and µ.

In conclusion, we performed a simultaneous

measure-ment of the Bs semileptonic branching fraction and its

production rates in the CM energy region from 10.56 GeV to 11.20 GeV. The semileptonic branching fraction is con-sistent with theoretical calculations [9]. Our measure-ment of the Bs production rates are consistent with the

predictions of coupled channel models [10], in which Bs

production peaks near the Υ(5S) and is vanishingly small elsewhere.

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 comput-ing organizations that support BA_BAR. The collaborat-ing institutions wish to thank SLAC for its support and kind hospitality. This work is supported by DOE and NSF (USA), NSERC (Canada), CEA and CNRS-IN2P3 (France), BMBF and DFG (Germany), INFN (Italy), FOM (The Netherlands), NFR (Norway), MES (Russia), MICIIN (Spain), STFC (United Kingdom). Individuals have received support from the Marie Curie EIF (Euro-pean Union), the A. P. Sloan Foundation (USA) and the Binational Science Foundation (USA-Israel).

∗ _{Now at Temple University, Philadelphia, Pennsylvania}

19122, USA

† _{Also with Universit`a di Perugia, Dipartimento di Fisica,}

Perugia, Italy

‡ _{Now at the University of Huddersfield, Huddersfield HD1}

3DH, UK

§ _{Now at University of South Alabama, Mobile, Alabama}

36688, USA

¶ _{Also with Universit`a di Sassari, Sassari, Italy}

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