Measurement of the Semileptonic Branching Ratio of B0 s to an Orbitally Excited D�� s State: BrðB0 s ! D� s1ð2536Þ�þ�XÞ V.M. Abazov,36 B. Abbott,76 M. Abolins,66 B. S. Acharya,29 M. Adams,52 T. Adams,50 E. Aguilo,6 S. H. Ahn,31 M. Ahsan,60 G.D. Alexeev,36 G. Alkhazov,40 A. Alton,65 G. Alverson,64 G. A. Alves,2 M. Anastasoaie,35 L. S. Ancu,35 T. Andeen,54 S. Anderson,46 B. Andrieu,17 M. S. Anzelc,54 Y. Arnoud,14 M. Arov,61 M. Arthaud,18 A. Askew,50 B. Åsman,41 A. C. S. Assis Jesus,3 O. Atramentov,50 C. Autermann,21 C. Avila,8 C. Ay,24 F. Badaud,13 A. Baden,62 L. Bagby,53 B. Baldin,51 D.V. Bandurin,60 S. Banerjee,29 P. Banerjee,29 E. Barberis,64 A.-F. Barfuss,15 P. Bargassa,81 P. Baringer,59 J. Barreto,2 J. F. Bartlett,51 U. Bassler,18 D. Bauer,44 S. Beale,6 A. Bean,59 M. Begalli,3 M. Begel,72 C. Belanger-Champagne,41 L. Bellantoni,51 A. Bellavance,51 J. A. Benitez,66 S. B. Beri,27 G. Bernardi,17 R. Bernhard,23 I. Bertram,43 M. Besançon,18 R. Beuselinck,44 V. A. Bezzubov,39 P. C. Bhat,51 V. Bhatnagar,27 C. Biscarat,20 G. Blazey,53 F. Blekman,44 S. Blessing,50 D. Bloch,19 K. Bloom,68 A. Boehnlein,51 D. Boline,63 T. A. Bolton,60 G. Borissov,43 T. Bose,78 A. Brandt,79 R. Brock,66 G. Brooijmans,71 A. Bross,51 D. Brown,82 N. J. Buchanan,50 D. Buchholz,54 M. Buehler,82 V. Buescher,22 S. Bunichev,38 S. Burdin,43 S. Burke,46 T. H. Burnett,83 C. P. Buszello,44 J.M. Butler,63 P. Calfayan,25 S. Calvet,16 J. Cammin,72 W. Carvalho,3 B. C. K. Casey,51 N.M. Cason,56 H. Castilla-Valdez,33 S. Chakrabarti,18 D. Chakraborty,53 K.M. Chan,56 K. Chan,6 A. Chandra,49 F. Charles,19,* E. Cheu,46 F. Chevallier,14 D.K. Cho,63 S. Choi,32 B. Choudhary,28 L. Christofek,78 T. Christoudias,44 S. Cihangir,51 D. Claes,68 Y. Coadou,6 M. Cooke,81 W. E. Cooper,51 M. Corcoran,81 F. Couderc,18 M.-C. Cousinou,15 S. Crépé-Renaudin,14 D. Cutts,78 M. Ćwiok,30 H. da Motta,2 A. Das,46 G. Davies,44 K. De,79 S. J. de Jong,35 E. De La Cruz-Burelo,65 C. De Oliveira Martins,3 J. D. Degenhardt,65 F. Déliot,18 M. Demarteau,51 R. Demina,72 D. Denisov,51 S. P. Denisov,39 S. Desai,51 H. T. Diehl,51 M. Diesburg,51 A. Dominguez,68 H. Dong,73 L. V. Dudko,38 L. Duflot,16 S. R. Dugad,29 D. Duggan,50 A. Duperrin,15 J. Dyer,66 A. Dyshkant,53 M. Eads,68 D. Edmunds,66 J. Ellison,49 V. D. Elvira,51 Y. Enari,78 S. Eno,62 P. Ermolov,38 H. Evans,55 A. Evdokimov,74 V.N. Evdokimov,39 A. V. Ferapontov,60 T. Ferbel,72 F. Fiedler,24 F. Filthaut,35 W. Fisher,51 H. E. Fisk,51 M. Ford,45 M. Fortner,53 H. Fox,23 S. Fu,51 S. Fuess,51 T. Gadfort,71 C. F. Galea,35 E. Gallas,51 E. Galyaev,56 C. Garcia,72 A. Garcia-Bellido,83 V. Gavrilov,37 P. Gay,13 W. Geist,19 D. Gelé,19 C. E. Gerber,52 Y. Gershtein,50 D. Gillberg,6 G. Ginther,72 N. Gollub,41 B. Gómez,8 A. Goussiou,56 P. D. Grannis,73 H. Greenlee,51 Z. D. Greenwood,61 E.M. Gregores,4 G. Grenier,20 Ph. Gris,13 J.-F. Grivaz,16 A. Grohsjean,25 S. Grüendahl,51 M.W. Grünewald,30 J. Guo,73 F. Guo,73 P. Gutierrez,76 G. Gutierrez,51 A. Haas,71 N. J. Hadley,62 P. Haefner,25 S. Hagopian,50 J. Haley,69 I. Hall,66 R. E. Hall,48 L. Han,7 P. Hansson,41 K. Harder,45 A. Harel,72 R. Harrington,64 J.M. Hauptman,58 R. Hauser,66 J. Hays,44 T. Hebbeker,21 D. Hedin,53 J. G. Hegeman,34 J.M. Heinmiller,52 A. P. Heinson,49 U. Heintz,63 C. Hensel,59 K. Herner,73 G. Hesketh,64 M.D. Hildreth,56 R. Hirosky,82 J. D. Hobbs,73 B. Hoeneisen,12 H. Hoeth,26 M. Hohlfeld,22 S. J. Hong,31 S. Hossain,76 P. Houben,34 Y. Hu,73 Z. Hubacek,10 V. Hynek,9 I. Iashvili,70 R. Illingworth,51 A. S. Ito,51 S. Jabeen,63 M. Jaffré,16 S. Jain,76 K. Jakobs,23 C. Jarvis,62 R. Jesik,44 K. Johns,46 C. Johnson,71 M. Johnson,51 A. Jonckheere,51 P. Jonsson,44 A. Juste,51 E. Kajfasz,15 A.M. Kalinin,36 J. R. Kalk,66 J.M. Kalk,61 S. Kappler,21 D. Karmanov,38 P. A. Kasper,51 I. Katsanos,71 D. Kau,50 R. Kaur,27 V. Kaushik,79 R. Kehoe,80 S. Kermiche,15 N. Khalatyan,51 A. Khanov,77 A. Kharchilava,70 Y.M. Kharzheev,36 D. Khatidze,71 T. J. Kim,31 M.H. Kirby,54 M. Kirsch,21 B. Klima,51 J.M. Kohli,27 J.-P. Konrath,23 V.M. Korablev,39 A.V. Kozelov,39 D. Krop,55 T. Kuhl,24 A. Kumar,70 S. Kunori,62 A. Kupco,11 T. Kurča,20 J. Kvita,9 F. Lacroix,13 D. Lam,56 S. Lammers,71 G. Landsberg,78 P. Lebrun,20 W.M. Lee,51 A. Leflat,38 F. Lehner,42 J. Lellouch,17 J. Leveque,46 J. Li,79 Q. Z. Li,51 L. Li,49 S.M. Lietti,5 J. G. R. Lima,53 D. Lincoln,51 J. Linnemann,66 V.V. Lipaev,39 R. Lipton,51 Y. Liu,7 Z. Liu,6 A. Lobodenko,40 M. Lokajicek,11 P. Love,43 H. J. Lubatti,83 R. Luna,3 A. L. Lyon,51 A.K. A. Maciel,2 D. Mackin,81 R. J. Madaras,47 P. Mättig,26 C. Magass,21 A. Magerkurth,65 P. K. Mal,56 H. B. Malbouisson,3 S. Malik,68 V. L. Malyshev,36 H. S. Mao,51 Y. Maravin,60 B. Martin,14 R. McCarthy,73 A. Melnitchouk,67 L. Mendoza,8 P. G. Mercadante,5 M. Merkin,38 K.W. Merritt,51 J. Meyer,22 A. Meyer,21 T. Millet,20 J. Mitrevski,71 J. Molina,3 R. K. Mommsen,45 N.K. Mondal,29 R.W. Moore,6 T. Moulik,59 G. S. Muanza,20 M. Mulders,51 M. Mulhearn,71 O. Mundal,22 L. Mundim,3 E. Nagy,15 M. Naimuddin,51 M. Narain,78 N. A. Naumann,35 H.A. Neal,65 J. P. Negret,8 P. Neustroev,40 H. Nilsen,23 H. Nogima,3 S. F. Novaes,5 T. Nunnemann,25 V. O’Dell,51 D. C. O’Neil,6 G. Obrant,40 C. Ochando,16 D. Onoprienko,60 N. Oshima,51 J. Osta,56 R. Otec,10 G. J. Otero y Garzón,51 M. Owen,45 P. Padley,81 M. Pangilinan,78 N. Parashar,57 S.-J. Park,72 S. K. Park,31 J. Parsons,71 R. Partridge,78 N. Parua,55 A. Patwa,74 G. Pawloski,81 B. Penning,23 M. Perfilov,38 K. Peters,45 Y. Peters,26 P. Pétroff,16 M. Petteni,44 R. Piegaia,1 J. Piper,66 M.-A. Pleier,22 P. L.M. Podesta-Lerma,33 V.M. Podstavkov,51 PRL 102, 051801 (2009) P HY S I CA L R EV I EW LE T T E R S week ending 6 FEBRUARY 2009 0031-9007=09=102(5)=051801(7) 051801-1 � 2009 The American Physical Society Y. Pogorelov,56 M.-E. Pol,2 P. Polozov,37 B. G. Pope,66 A.V. Popov,39 C. Potter,6 W. L. Prado da Silva,3 H. B. Prosper,50 S. Protopopescu,74 J. Qian,65 A. Quadt,22 B. Quinn,67 A. Rakitine,43 M. S. Rangel,2 K. Ranjan,28 P. N. Ratoff,43 P. Renkel,80 S. Reucroft,64 P. Rich,45 J. Rieger,55 M. Rijssenbeek,73 I. Ripp-Baudot,19 F. Rizatdinova,77 S. Robinson,44 R. F. Rodrigues,3 M. Rominsky,76 C. Royon,18 P. Rubinov,51 R. Ruchti,56 G. Safronov,37 G. Sajot,14 A. Sánchez-Hernández,33 M. P. Sanders,17 A. Santoro,3 G. Savage,51 L. Sawyer,61 T. Scanlon,44 D. Schaile,25 R.D. Schamberger,73 Y. Scheglov,40 H. Schellman,54 T. Schliephake,26 C. Schwanenberger,45 A. Schwartzman,69 R. Schwienhorst,66 J. Sekaric,50 H. Severini,76 E. Shabalina,52 M. Shamim,60 V. Shary,18 A.A. Shchukin,39 R.K. Shivpuri,28 V. Siccardi,19 V. Simak,10 V. Sirotenko,51 P. Skubic,76 P. Slattery,72 D. Smirnov,56 J. Snow,75 G. R. Snow,68 S. Snyder,74 S. Söldner-Rembold,45 L. Sonnenschein,17 A. Sopczak,43 M. Sosebee,79 K. Soustruznik,9 B. Spurlock,79 J. Stark,14 J. Steele,61 V. Stolin,37 D.A. Stoyanova,39 J. Strandberg,65 S. Strandberg,41 M.A. Strang,70 M. Strauss,76 E. Strauss,73 R. Ströhmer,25 D. Strom,54 L. Stutte,51 S. Sumowidagdo,50 P. Svoisky,56 A. Sznajder,3 M. Talby,15 P. Tamburello,46 A. Tanasijczuk,1 W. Taylor,6 J. Temple,46 B. Tiller,25 F. Tissandier,13 M. Titov,18 V. V. Tokmenin,36 T. Toole,62 I. Torchiani,23 T. Trefzger,24 D. Tsybychev,73 B. Tuchming,18 C. Tully,69 P.M. Tuts,71 R. Unalan,66 S. Uvarov,40 L. Uvarov,40 S. Uzunyan,53 B. Vachon,6 P. J. van den Berg,34 R. Van Kooten,55 W.M. van Leeuwen,34 N. Varelas,52 E.W. Varnes,46 I. A. Vasilyev,39 M. Vaupel,26 P. Verdier,20 L. S. Vertogradov,36 M. Verzocchi,51 F. Villeneuve-Seguier,44 P. Vint,44 P. Vokac,10 E. Von Toerne,60 M. Voutilainen,68 R. Wagner,69 H. D. Wahl,50 L. Wang,62 M.H. L. S Wang,51 J. Warchol,56 G. Watts,83 M. Wayne,56 M. Weber,51 G. Weber,24 L. Welty-Rieger,55 A. Wenger,42 N. Wermes,22 M. Wetstein,62 A. White,79 D. Wicke,26 G.W. Wilson,59 S. J. Wimpenny,49 M. Wobisch,61 D. R. Wood,64 T. R. Wyatt,45 Y. Xie,78 S. Yacoob,54 R. Yamada,51 M. Yan,62 T. Yasuda,51 Y. A. Yatsunenko,36 K. Yip,74 H.D. Yoo,78 S.W. Youn,54 J. Yu,79 A. Zatserklyaniy,53 C. Zeitnitz,26 T. Zhao,83 B. Zhou,65 J. Zhu,73 M. Zielinski,72 D. Zieminska,55 A. Zieminski,55,* L. Zivkovic,71 V. Zutshi,53 and E.G. Zverev38 (D0 Collaboration) 1Universidad de Buenos Aires, Buenos Aires, Argentina 2LAFEX, Centro Brasileiro de Pesquisas Fı́sicas, Rio de Janeiro, Brazil 3Universidade do Estado do Rio de Janeiro, Rio de Janeiro, Brazil 4Universidade Federal do ABC, Santo André, Brazil 5Instituto de Fı́sica Teórica, Universidade Estadual Paulista, São Paulo, Brazil 6University of Alberta, Edmonton, Alberta, Canada, Simon Fraser University, Burnaby, British Columbia, Canada, York University, Toronto, Ontario, Canada, and McGill University, Montreal, Quebec, Canada 7University of Science and Technology of China, Hefei, People’s Republic of China 8Universidad de los Andes, Bogotá, Colombia 9Center for Particle Physics, Charles University, Prague, Czech Republic 10Czech Technical University, Prague, Czech Republic 11Center for Particle Physics, Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic 12Universidad San Francisco de Quito, Quito, Ecuador 13LPC, Univ Blaise Pascal, CNRS/IN2P3, Clermont, France 14LPSC, Université Joseph Fourier Grenoble 1, CNRS/IN2P3, Institut National Polytechnique de Grenoble, France 15CPPM, IN2P3/CNRS, Université de la Méditerranée, Marseille, France 16LAL, Univ Paris-Sud, IN2P3/CNRS, Orsay, France 17LPNHE, IN2P3/CNRS, Universités Paris VI and VII, Paris, France 18DAPNIA/Service de Physique des Particules, CEA, Saclay, France 19IPHC, Université Louis Pasteur et Université de Haute Alsace, CNRS/IN2P3, Strasbourg, France 20IPNL, Université Lyon 1, CNRS/IN2P3, Villeurbanne, France and Université de Lyon, Lyon, France 21III. Physikalisches Institut A, RWTH Aachen, Aachen, Germany 22Physikalisches Institut, Universität Bonn, Bonn, Germany 23Physikalisches Institut, Universität Freiburg, Freiburg, Germany 24Institut für Physik, Universität Mainz, Mainz, Germany 25Ludwig-Maximilians-Universität München, München, Germany 26Fachbereich Physik, University of Wuppertal, Wuppertal, Germany 27Panjab University, Chandigarh, India 28Delhi University, Delhi, India 29Tata Institute of Fundamental Research, Mumbai, India PRL 102, 051801 (2009) P HY S I CA L R EV I EW LE T T E R S week ending 6 FEBRUARY 2009 051801-2 30University College Dublin, Dublin, Ireland 31Korea Detector Laboratory, Korea University, Seoul, Korea 32SungKyunKwan University, Suwon, Korea 33CINVESTAV, Mexico City, Mexico 34FOM-Institute NIKHEF and University of Amsterdam/NIKHEF, Amsterdam, The Netherlands 35Radboud University Nijmegen/NIKHEF, Nijmegen, The Netherlands 36Joint Institute for Nuclear Research, Dubna, Russia 37Institute for Theoretical and Experimental Physics, Moscow, Russia 38Moscow State University, Moscow, Russia 39Institute for High Energy Physics, Protvino, Russia 40Petersburg Nuclear Physics Institute, St. Petersburg, Russia 41Lund University, Lund, Sweden, Royal Institute of Technology and Stockholm University, Stockholm, Sweden, and Uppsala University, Uppsala, Sweden 42Physik Institut der Universität Zürich, Zürich, Switzerland 43Lancaster University, Lancaster, United Kingdom 44Imperial College, London, United Kingdom 45University of Manchester, Manchester, United Kingdom 46University of Arizona, Tucson, Arizona 85721, USA 47Lawrence Berkeley National Laboratory and University of California, Berkeley, California 94720, USA 48California State University, Fresno, California 93740, USA 49University of California, Riverside, California 92521, USA 50Florida State University, Tallahassee, Florida 32306, USA 51Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA 52University of Illinois at Chicago, Chicago, Illinois 60607, USA 53Northern Illinois University, DeKalb, Illinois 60115, USA 54Northwestern University, Evanston, Illinois 60208, USA 55Indiana University, Bloomington, Indiana 47405, USA 56University of Notre Dame, Notre Dame, Indiana 46556, USA 57Purdue University Calumet, Hammond, Indiana 46323, USA 58Iowa State University, Ames, Iowa 50011, USA 59University of Kansas, Lawrence, Kansas 66045, USA 60Kansas State University, Manhattan, Kansas 66506, USA 61Louisiana Tech University, Ruston, Louisiana 71272, USA 62University of Maryland, College Park, Maryland 20742, USA 63Boston University, Boston, Massachusetts 02215, USA 64Northeastern University, Boston, Massachusetts 02115, USA 65University of Michigan, Ann Arbor, Michigan 48109, USA 66Michigan State University, East Lansing, Michigan 48824, USA 67University of Mississippi, University, Mississippi 38677, USA 68University of Nebraska, Lincoln, Nebraska 68588, USA 69Princeton University, Princeton, New Jersey 08544, USA 70State University of New York, Buffalo, New York 14260, USA 71Columbia University, New York, New York 10027, USA 72University of Rochester, Rochester, New York 14627, USA 73State University of New York, Stony Brook, New York 11794, USA 74Brookhaven National Laboratory, Upton, New York 11973, USA 75Langston University, Langston, Oklahoma 73050, USA 76University of Oklahoma, Norman, Oklahoma 73019, USA 77Oklahoma State University, Stillwater, Oklahoma 74078, USA 78Brown University, Providence, Rhode Island 02912, USA 79University of Texas, Arlington, Texas 76019, USA 80Southern Methodist University, Dallas, Texas 75275, USA 81Rice University, Houston, Texas 77005, USA 82University of Virginia, Charlottesville, Virginia 22901, USA 83University of Washington, Seattle, Washington 98195, USA (Received 26 December 2007; published 3 February 2009) In a data sample of approximately 1:3 fb�1 collected with the D0 detector between 2002 and 2006, the orbitally excited charm state D� s1ð2536Þ has been observed with a measured mass of 2535:7� 0:6ðstatÞ � 0:5ðsystÞ MeV=c2 via the decay mode B0 s ! D� s1ð2536Þ�þ��X. A first measurement is made of the branching ratio product Brð �b ! D� s1ð2536Þ�þ��XÞ� BrðD� s1 ! D��K0 SÞ. Assuming that D� s1ð2536Þ PRL 102, 051801 (2009) P HY S I CA L R EV I EW LE T T E R S week ending 6 FEBRUARY 2009 051801-3 production in semileptonic decay is entirely from B0 s , an extraction of the semileptonic branching ratio BrðB0 s ! D� s1ð2536Þ�þ��XÞ is made. DOI: 10.1103/PhysRevLett.102.051801 PACS numbers: 13.25.Hw, 14.40.Lb Semileptonic B0 s decays into orbitally excited P-wave strange-charm mesons (D�� s ) are expected to make up a significant fraction of B0 s semileptonic decays and are therefore important when comparing inclusive and ex- clusive decay rates, extracting CKM matrix elements, and using semileptonic decays in B0 s mixing analyses. For B meson semileptonic decays to heavier excited charm states, more of the available phase space is near zero recoil, increasing the importance of corrections in heavy-quark effective theory (HQET) [1], effectively tested here. D�� s mesons (also denotedDsJ) are composed of a heavy charm quark and a lighter strange quark in an L ¼ 1 state of orbital momentum. In the heavy-quark limit, the spin sQ of the heavy quark and the total angular momentum, jq ¼ sq þ L of the light degrees of freedom (quark and gluons), are separately conserved and the latter has possible values of jq ¼ 1 2 or 3 2 . The surprisingly light masses of the jq ¼ 1 2 states: D� s0ð2317Þ and Ds1ð2460Þ [2], plus the observation of new DsJ states [3], deepens the need for a better under- standing of these D�� s systems since they may be quark molecular states, a new and very different arrangement of quarks. In our decay of interest, the jq ¼ 3 2 angular momentum can combine with the heavy-quark spin to form the JP ¼ 1þ (Ds1) state which must decay through a D wave to conserve jq ¼ 3 2 . The D� s1ð2536Þ is expected to decay dominantly into a D� and K meson to conserve angular momentum. In this Letter we present the first measurement of semi- leptonic B0 s decay into the narrow D� s1ð2536Þ state. This state is just above the D�K0 S mass threshold and has been observed previously [4]. Events compatible with the de- cay chain �b ! D� s1ð2536Þ�þ��X, D � s1ð2536Þ ! D��K0 S; D�� ! �D0��, K0 S ! �þ��, �D0 ! Kþ�� are recon- structed. Charge conjugate modes and reactions are always implied in this Letter. Assuming that D� s1ð2536Þ production in a semileptonic decay is entirely from B0 s , the branching ratio BrðB0 s ! D� s1ð2536Þ�þ��XÞ can be determined by normalizing to the known value of the branching fraction Brð �b ! D���þ��XÞ ¼ ð2:75� 0:19Þ% [5] to avoid uncertainties in the b-quark production rate. This semileptonic branch- ing ratio includes any decay channel or sequence of chan- nels resulting in a D� and a lepton (muon in our case), and all b hadrons, and therefore includes the relative produc- tion of each b hadron species starting from a �b quark. Since the final state of interest, D� s1ð2536Þ ! D��K0 S, is recon- structed from aD� and aK0 S, the selection is broken up into two sections: one to reconstruct the D� with an associated muon, coming dominantly from B meson decays resulting in a number of candidates, ND��, and then the addition and subsequent formation of a vertex of a K0 S with the D� and muon, resulting in NDs1 candidates. To find the branching ratio, the following formula is used: fð �b ! B0 sÞBrðB0 s ! D� s1� þ��XÞBrðD� s1 ! D��K0 SÞ ¼ Brð �b ! D���þ��XÞ NDs1 ND�� �ð �b ! D��Þ �ðB0 s ! Ds1� ! D��Þ 1 �K0 S : (1) The input fð �b ! B0 sÞ [5] is the fraction of decays where a b quark will hadronize to a B0 s hadron. �K0 S is the effi- ciency in the signal decay channel to reconstruct and make a vertex with a K0 S to form a Ds1ð2536Þ, given that aD� and a muon have already been reconstructed. Later we will identify the ratio of efficiencies as R gen D� ¼ �ðB0 s ! Ds1� ! D��Þ=�ð �b ! D��Þ. The D0 detector [6] and following analysis [7] are described in more detail elsewhere. The main elements relevant to this analysis are the silicon microstrip tracker (SMT), central fiber tracker (CFT), and muon detector systems. This measurement uses a large data sample, correspond- ing to approximately 1:3 fb�1 of integrated luminosity collected by the D0 detector between April 2002 and March 2006. Events were reconstructed using the standard D0 software suite. To avoid lifetime biases compared to the MC simulation, the small fraction of events were removed that entered the sample only via triggers that included requirements on impact parameters of tracks. To evaluate signal mass resolution and efficiencies, Monte Carlo (MC) simulated samples were generated for signal and background. The standard D0 simulation and event reconstruction chain was used. Events were gener- ated with the PYTHIA generator [8] and decay chains of heavy hadrons were simulated with the EVTGEN decay package [9]. The detector response was modeled by GEANT [10]. Two background MC samples were also gen- erated: a c �c sample, and an inclusive b-quark sample containing all b hadron species with forced semileptonic decays to a muon. In both cases, all events containing both a D� and a muon were retained. B mesons were first selected using their semileptonic decays, B ! D���þX. At this point in the selection, the PRL 102, 051801 (2009) P HY S I CA L R EV I EW LE T T E R S week ending 6 FEBRUARY 2009 051801-4 http://dx.doi.org/10.1103/PhysRevLett.102.051801 D� þ� sample is dominated by B0 d ! D���þ��X de- cays. For this analysis, muons were required to have hits in more than one muon layer, to have an associated track in the central tracking system, and to have transverse mo- mentum p � T > 2 GeV=c, pseudorapidity j��j< 2, and to- tal momentum p� > 3 GeV=c. Two oppositely charged tracks with pT > 0:7 GeV=c and j�j< 2 were required to form a common �D0 vertex which were then combined with a muon candidate to form a common decay point following the procedure described in Ref. [11]. For each �D0�þ candidate, an additional soft pion was searched for with charge opposite to the charge of the muon and pT > 0:18 GeV=c. The K� and �þ from the decay of the D0 were both required to have more than five CFT hits. To reduce the contribution from prompt c �c production, a requirement was made on the transverse decay length, Lxy, significance of the D�� vertex of Lxy=�ðLxyÞ> 1. After these cuts, the total number of D� candidates in the mass difference, MðD�Þ �MðD0Þ, peak of Fig. 1 is ND�� ¼ 87 506� 496 (stat). D� s1ð2536Þ candidates were formed by combining a D� candidate with a K0 S. D � candidates were first selected by requiring the mass differenceMðD�Þ �MðD0Þ to be in the range 0:142–0:149 GeV=c2. The two tracks from the decay of the K0 S were required to have opposite charge and to have more than five hits in the CFT detector. The pT of the K0 S was required to be greater than 1 GeV=c to reduce the contribution of background K0 S mesons from fragmenta- tion. A vertex was then formed using the reconstructed K0 S and the D� candidate of the event. The decay length of the K0 S was required to be greater than 0.5 cm. To compute the D� s1ð2536Þ invariant mass, a mass constraint was applied using the known D�� mass [5] instead of the measured invariant mass of the K�� system. Finally, the invariant mass of the reconstructed D� s1ð2536Þ and muon was re- quired to be less than the mass of the B0 s meson [5]. The signal model employed for the fit to the D�K0 S invariant mass spectrum was a relativistic Breit-Wigner convoluted with a Gaussian function, with the reso- nance width fixed to the value 1:03� 0:05ðstatÞ � 0:12ðsystÞ MeV=c2 measured by the BABAR Collabora- tion [12] and a Gaussian width determined to be 2:8 MeV=c2 from MC simulation of the signal. The MC width value was scaled up by a factor of 1:10� 0:10 to account for differences between data and MC resolution estimates. The unbinned likelihood fit used an exponential function plus a first-order polynomial to model the back- ground with a threshold cutoff ofMðD�Þ þMðK0 SÞ. The fit, shown in Fig. 2, gives a central value for the mass peak of 2535:7� 0:7ðstatÞ MeV=c2, a yield of NDs1 ¼ 45:9� 9:1ðstatÞ events, and a significance of 6:1� for the back- ground to fluctuate up to or above the observed number of signal events. The efficiencies used in Eq. (1) are estimated using the MC simulation, after implementing suitable correction factors to ensure proper modeling of the underlying b-hadron pT spectrum, as well as trigger effects. An event-by-event weight, applied as a function of the gener- ated pT of the Bs, was determined by comparing the generated pTðBÞ in MC with the pT distribution of fully reconstructed Bþ ! J=cKþ candidates in data collected primarily with a dimuon trigger [13]. Most events for this analysis were recorded using single muon triggers, and an additional weight was applied as a function of pTð�Þ to further improve the simulation of trigger effects. Reweighted MC events were used in the determination of efficiencies described below, and indicated uncertainties are due to MC statistics. Using the MC sample of inclusive �b ! D��X events, specific major decay modes were identified. Efficiencies for each of these decay modes to pass the D�� selection, including the efficiency to reconstruct the soft pion from the D�, were then determined. The predicted fraction Fi of each channel contributing to the D�� sample before fur- )2 (GeV/c0 SInvariant Mass of D* K 2.5 2.52 2.54 2.56 2.58 2.6 2.62 2.64 2 E ve n ts /2 .5 M eV /c 10 20 30 D , 1.3 fb-1 Data Background (2536) signals1D Fit function FIG. 2 (color online). Invariant mass of D�K0 S with an asso- ciated muon. Shown is the result of the fit of the D�K0 S mass with the function described in the text. )2) (GeV/c0)-M(DM(D* 0.135 0.14 0.145 0.15 0.155 0.16 0.165 0.17 2 C an d id at es /0 .3 5 M eV /c 5000 10 000 15 000 D , 1.3 fb-1 Data Background D*signal Fit function FIG. 1 (color online). The mass difference MðD�Þ �MðD0Þ for events with 1:8 1, the fraction of ND�� from c �c production was estimated to be ð3:9�2:5Þ%. A check using a prompt c �c MC sample results in a consistent estimate. The value of ND�� was corrected downward accordingly. The contribution from c �c production to NDs1 where one charm quark hadronizes directly to a Ds1ð2536Þ and the other decays directly to a muon was estimated to be negligible using relative production ratios and spin- counting arguments [15]. Systematic uncertainties for the branching ratio product are summarized in Table I and discussed below. The un- certainty in the normalizing branching ratio [5] Brð �b ! D��XÞ was taken as a systematic uncertainty. For deter- miningND��, the signal and background model parameters were varied in a correlated fashion and a systematic un- certainty was assigned. The estimated c �c production con- tribution was varied by the indicated uncertainty. In the determination of NDs1 , the functional forms of the signal and background models were varied in a number of ways to determine the sensitivity of the candidate yield. In addi- tion, the scaling of the widths was varied by �10% to check the sensitivity to uncertainty in mass resolution. By comparing the pTð�Þ distribution for the signal using the default ISGW2 decay model [16] to the HQET semi- leptonic decay model [9], a weighting factor was found and applied to the fully simulated signal MC events, and the efficiency determined again. The difference observed was assigned as a contribution to the systematic uncertainty of �K0 S and R gen D� . When estimating �K0 S , the uncertainty due to modeling of the b hadron pT spectrum was derived by using an alternate weighting technique. The cuts on the pT and decay length of the K0 S were varied and a systematic uncertainty on the efficiency due to this source was also assigned. Discrepancies in track reconstruction efficiencies between data and MC in low-pT tracks were accounted for by assigning a systematic uncertainty to each of the pion tracks in the K0 S reconstruction [17,18]. The uncertainty in R gen D� is due to a combination of MC statistics and uncertainties in PDG branching ratio values and production fractions, fð �b ! b hadronÞ. The uncorre- lated systematic uncertainty is given in Table I. The estimated systematic uncertainties were added in quadrature to obtain a total estimated systematic uncer- tainty on the branching ratio product of 16.8%. The branch- ing ratio product was determined to be: fð �b ! B0 sÞBrðB0 s ! D� s1� þ��XÞBrðD� s1 ! D��K0 SÞ ¼ ½2:66� 0:52ðstatÞ � 0:45ðsystÞ� � 10�4: To assess the systematic uncertainty on the mass mea- surement, the same variations of theDs1ð2536Þmass signal model, as well as background functional form, were ap- plied as described above. The mass values used for the mass constraints on the decay products were varied within their PDG uncertainties and were also set to the D0 central fit values. Ensemble tests indicated that the statistical error is correct. From the observed variations, a total systematic mass uncertainty of 0:5 MeV=c2 was taken, for a mass measurement of: mðDs1Þ ¼ 2535:7� 0:6ðstatÞ � 0:5ðsystÞ MeV=c2: This measured mass value is in good agreement with the PDG average value of 2535:34� 0:31 MeV=c2 [5]. To allow comparison of this measurement to theoreti- cal predictions, the semileptonic branching ratio alone as TABLE I. Estimated systematic uncertainties. Source Systematic uncertainty Brð �b ! D��XÞ 6.9% ND�� 2.9% NDs1 5.5% �K0 S 11.0% R gen D� 8.6% Total 16.8% PRL 102, 051801 (2009) P HY S I CA L R EV I EW LE T T E R S week ending 6 FEBRUARY 2009 051801-6 shown in Table II is extracted by taking the hadronization fraction into B0 s as fð �b ! B0 sÞ ¼ 0:103� 0:014 [5] and also assuming that BrðDs1ð2536Þ ! D�K0 SÞ ¼ 0:25 [9]. This is the first experimental measurement of this semi- leptonic branching ratio and is compared to a number of theoretical predictions [1,19,20] of the exclusive rate in Table II. The systematic uncertainty on this quantity is as described earlier, and the error labeled ‘‘(prod. frac.)’’ is due to the current uncertainty on fð �b ! B0 sÞ. The first two theoretical predictions include relativistic and 1=mQ cor- rections, while the third does not. The result is found to be consistent within uncertainties with the first two theo- retical predictions, and demonstrates the need for such corrections. In summary, using 1:3 fb�1 of integrated luminosity collected with the D0 detector, a first measurement of the semileptonic B0 s decay into the narrow D� s1ð2536Þ state has been made and compared with theory. In addition, the mass of the D� s1ð2536Þ was measured and found to be in good agreement with the PDG value. We thank the staffs at Fermilab and collaborating insti- tutions, and acknowledge support from the DOE and NSF (USA); CEA and CNRS/IN2P3 (France); FASI, Rosatom and RFBR (Russia); CAPES, CNPq, FAPERJ, FAPESP, and FUNDUNESP (Brazil); DAE and DST (India); Colciencias (Colombia); CONACyT (Mexico); KRF and KOSEF (Korea); CONICET and UBACyT (Argentina); FOM (The Netherlands); Science and Technology Facilities Council (United Kingdom); MSMT and GACR (Czech Republic); CRC Program, CFI, NSERC and WestGrid Project (Canada); BMBF and DFG (Germany); SFI (Ireland); The Swedish Research Council (Sweden); CAS and CNSF (China); Alexander von Humboldt Foundation; and the Marie Curie Program. *Deceased. [1] D. Scora and N. Isgur, Phys. Rev. D 52, 2783 (1995), using the results of Ref. [20] for input. [2] B. Aubert et al. (BABAR Collaboration), Phys. Rev. Lett. 90, 242001 (2003); D. Besson et al. (CLEO Collaboration), Phys. Rev. D 68, 032002 (2003); P. Krokovny et al. (Belle Collaboration), Phys. Rev. Lett. 91, 262002 (2003). [3] Y. Kubota et al. (CLEO Collaboration) Phys. Rev. Lett. 72, 1972 (1994); A.V. Evdokimov et al. (SELEX Collaboration), ibid. 93, 242001 (2004); B. Aubert et al. (BABAR Collaboration), ibid. 97, 222001 (2006); J. Brodzicka et al. (Belle Collaboration), ibid. 100, 092001 (2008). [4] A. H. Heister et al. (ALEPH Collaboration), Phys. Lett. B 526, 34 (2002); A. H. Heister et al., (OPAL Collaboration), Z. Phys. C 76, 425 (1997); J. Alexander et al. (CLEO Collaboration), Phys. Lett. B 303, 377 (1993); H. Albrecht et al. (ARGUS Collaboration), ibid. 297, 425 (1992); B. Aubert et al. (BABAR Collaboration), Phys. Rev. D 74, 032007 (2006). [5] W.-M. Yao et al., J. Phys. G 33, 1 (2006). [6] V. Abazov et al. (D0 Collaboration), Nucl. Instrum. Methods Phys. Res., Sect. A 565, 463 (2006). [7] J. Rieger, Report No. FERMILAB-THESIS-2007-46, 2007. [8] T. Sjöstrand et al., Comput. Phys. Commun. 135, 238 (2001). [9] D. J. Lange, Nucl. Instrum. Methods Phys. Res., Sect. A 462, 152 (2001). For the signal, the ISGW2 semileptonic decay model [18] is used, and the VVS_PWAVE option for the spin-1 Ds1 decay. [10] R. Brun et al., CERN Report No. DD/EE/84-1, 1984. [11] V. Abazov et al. (D0 Collaboration), Phys. Rev. Lett. 94, 182001 (2005). [12] B. Aubert et al. (BABAR Collaboration), arXiv:hep-ex/ 0607084. [13] V. Abazov et al. (D0 Collaboration), Phys. Rev. Lett. 94, 071802 (2005). [14] V. Abazov et al. (D0 Collaboration), Phys. Rev. D 74, 112002 (2006). [15] D. Acosta et al. (CDF Collaboration), Phys. Rev. Lett. 91, 241804 (2003); A. F. Falk and M. E. Peskin, Phys. Rev. D 49, 3320 (1994). [16] N. Isgur, D. Scora, B. Grinstein, and M.B. Wise, Phys. Rev. D 39, 799 (1989). [17] V. Abazov et al. (D0 Collaboration), Phys. Rev. Lett. 94, 232001 (2005). [18] V. Abazov et al. (D0 Collaboration), Phys. Rev. Lett. 95, 171803 (2005). [19] D. Ebert, R. N. Faustov, and V.O. Galkin, Phys. Rev. D 61, 014016 (1999). [20] H. B. Mayorga, A. Moreno Briceno, and J. H. Munoz, J. Phys. G 29, 2059 (2003). TABLE II. Experimental measurement compared with various theoretical predictions. Source BrðB0 s ! D� s1ð2536Þ�þ��XÞ This result ½1:03� 0:20ðstatÞ � 0:17ðsystÞ � 0:14ðprod:frac:Þ�% Theoretical Predictions BrðB0 s ! D� s1ð2536Þ�þ��Þ ISGW2 [1] ð0:53� 0:27Þ% Relativistic Quark Model & 1=mQ corrections [19] ð1:06� 0:16Þ% Nonrel. HQET and ISGW [20] 0.195% PRL 102, 051801 (2009) P HY S I CA L R EV I EW LE T T E R S week ending 6 FEBRUARY 2009 051801-7