Simultaneous Measurement of the Ratio R � B�t! Wb�=B�t! Wq� and the Top-Quark Pair Production Cross Section with the D0 Detector at ��� s p � 1:96 TeV 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 V. 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. Grünendahl,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,x 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,‡ PRL 100, 192003 (2008) P H Y S I C A L R E V I E W L E T T E R S week ending 16 MAY 2008 0031-9007=08=100(19)=192003(7) 192003-1 © 2008 The American Physical Society V. M. Podstavkov,51 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,x 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,k 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 30University College Dublin, Dublin, Ireland PRL 100, 192003 (2008) P H Y S I C A L R E V I E W L E T T E R S week ending 16 MAY 2008 192003-2 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 10 January 2008; published 14 May 2008) We present the first simultaneous measurement of the ratio of branching fractions, R � B�t! Wb�=B�t! Wq�, with q being a d, s, or b quark, and the top-quark pair production cross section �t�t in the lepton plus jets channel using 0:9 fb�1 of p �p collision data at ��� s p � 1:96 TeV collected with the D0 detector. We extract R and �t�t by analyzing samples of events with 0, 1, and � 2 identified b jets. We PRL 100, 192003 (2008) P H Y S I C A L R E V I E W L E T T E R S week ending 16 MAY 2008 192003-3 measure R � 0:97�0:09 �0:08�stat� syst� and �t�t � 8:18�0:90 �0:84�stat� syst� � 0:50�lumi� pb, in agreement with the standard model prediction. DOI: 10.1103/PhysRevLett.100.192003 PACS numbers: 13.85.Lg, 12.15.Hh, 13.85.Qk, 14.65.Ha Within the standard model (SM) the top quark decays to a W boson and a down-type quark q (q � d, s, b) with a rate proportional to the squared Cabibbo-Kobayashi- Maskawa (CKM) matrix element, jVtqj2 [1]. Under the assumption of three fermion families and a unitary 3� 3 CKM matrix, the jVtqj elements are severely constrained, with jVtbj � 0:999100�0:000034 �0:000004 [2]. However, in several extensions of the SM the 3� 3 CKM submatrix would not appear unitary and jVtqj elements can significantly deviate from their SM values. This would affect the rate for single top-quark production via the electroweak inter- action [3] and the ratio R of the top-quark branching fractions, which can be expressed in terms of the CKM matrix elements as R � B�t! Wb� B�t! Wq� � j Vtb j 2 j Vtb j2 � j Vts j2 � j Vtd j2 : A precise measurement of R is therefore a necessary ingredient for performing direct measurements of the jVtqj elements via the combination with future measure- ments of the single top-quark production in s and t chan- nels [4], free of assumptions about the number of quark families or the unitarity of the CKM matrix. We report the first simultaneous measurement of R and the top-quark pair (t�t) production cross section �t�t. R was measured by the CDF and D0 collaborations [5,6]. The simultaneous measurement of R and �t�t, in contrast to previous measurements [7,8], allows one to extract �t�t without assuming B�t! Wb� � 1, and to achieve a higher precision on both quantities by exploiting their different sensitivity to systematic uncertainties. The current measurement is based on data collected with the D0 detector [9] between August 2002 and December 2005 at the Fermilab Tevatron p �p collider at ��� s p � 1:96 TeV, corresponding to an integrated luminosity of about 0:9 fb�1. We use the top-quark pair decay channel tt! W�qW�q, with the subsequent decay of one W boson into two quarks, and the other one into an electron or muon and a neutrino, referred to as the lepton plus jets (‘� jets) channel. We select a data sample enriched in t�t events by requiring � 3 jets with transverse momentum pT > 20 GeV and pseudorapidity j�j< 2:5 [10], one iso- lated electron (muon) with pT > 20 GeV and j�j< 1:1 (j�j< 2:0), and missing transverse energy E6 T > 20 GeV (e� jets) or E6 T > 25 GeV (�� jets). The leading jet pT is required to exceed 40 GeV. Events containing a second isolated lepton with pT > 15 GeV are rejected. The lepton isolation criteria are based on calorimeter and tracking information. Details of lepton, jets, and E6 T identification are described elsewhere [10]. We identify b jets using a neural-network tagging algo- rithm [11]. It combines variables that characterize the presence and properties of secondary vertices and tracks with high impact parameter inside the jet. In the simula- tion, we assign a probability for each jet to be b tagged based on its flavor, pT , and �. These probabilities are determined from data control samples, and can be com- bined to yield a probability for each t�t event to have 0, 1, or � 2 b-tagged jets [7]. We split the ‘� jets sample into subsamples according to lepton flavor (e or�), jet multiplicity (3 or� 4 jets) and number of identified b jets (0, 1 or � 2), thus obtaining 12 disjoint data sets. We simultaneously fit R and �t�t to the observed number of 1 b tag and� 2 b tag events, and, in 0 b tag events with� 4 jets, to the shape of a discriminant D that exploits kinematic differences between t�t signal and background. We do not use a discriminant in events with 3 jets and 0 b tags, since the signal-to-background ratio is about 5 times smaller. The dominant background is the production ofW bosons in association with heavy and light flavor jets (W � jets). Smaller contributions arise from Z� jets, diboson, and single top-quark production. Multijet events enter the se- lected sample if a jet is misidentified as an electron, or a muon in a jet from a heavy quark or an in-flight pion or kaon decay appears isolated. We model W � jets and Z� jets processes with the ALPGEN [12] leading-order generator for the matrix ele- ment calculation and PYTHIA [13] for parton showering and hadronization. Diboson samples are generated with PYTHIA. Single top-quark production is modeled with the SINGLETOP [14] event generator. The t�t signal is simulated with PYTHIA for a top-quark mass of mtop � 175 GeV and includes three decay modes t�t! W�bW� �b, t�t! W�bW� �ql (or t�t! W�qlW� �b) and t�t! W�qlW� �ql, where ql denotes a light down-type (d or s) quark. These three decay modes are referred to as bb, bql and qlql. We pass the generated events through a GEANT-based [15] simulation of the D0 detector. Additional corrections [10] are applied to the reconstructed objects to improve the agreement between data and simulation. The determination of the background composition starts with the evaluation of the multijet background for each jet multiplicity and lepton flavor before b-jet tagging by counting events in the corresponding control data samples and applying the matrix method [7]. We estimate the number of events with a lepton originating from a W or Z boson decay by subtracting the multijet background from the observed event yield before b tagging. We further subtract diboson, single top quark and Z� jets contribu- PRL 100, 192003 (2008) P H Y S I C A L R E V I E W L E T T E R S week ending 16 MAY 2008 192003-4 http://dx.doi.org/10.1103/PhysRevLett.100.192003 tions, normalized to the next-to-leading-order cross sec- tions [16]. The remaining data events are assumed to come from t�t and W � jets. In every step of the fitting procedure used to extract �t�t and R, we iteratively redetermine the expected number of t�t events and reevaluate the W � jets background. Since the probability to tag a t�t event depends on the jet flavor, it depends on R. Assuming three t�t decay modes bb, bql and qlql, the probability for a t�t event to pass our selection criteria and to have n b-tagged jets is: Pntotal�t�t� � R2A�bb�Pnt �bb� � 2R�1� R�A�bql�P n t �bql� � �1� R�2A�qlql�P n t �qlql�; (1) where A (Pnt ) describes the acceptance (tagging probabil- ity) for each mode. Figure 1(a) shows Pnt as a function of R for t�t events with � 4 jets. Table I presents the sample composition for the measured �t�t and R � 1. The topological discriminant D [10] exploits the kine- matic differences between t�t and W � jets events to achieve a better constraint on the number of t�t events in the subsample with � 4 jets and 0 b tags. We select variables well described by the background model that provide a good separation between t�t and W � jets back- ground. Only the four highest-pT jets are considered for these variables to reduce the sensitivity to soft radiation. The optimal set of variables is chosen to minimize the expected statistical uncertainty on the fitted fraction of t�t events. Because of differences in acceptance and sample composition, the discriminants are constructed from differ- ent sets of variables in the e� jets and �� jets channels. In the e� jets channel we use five variables: the leading jet pT , the maximum �R [10] between two jets, A, CM, and DM [17]. In the �� jets channel we use six variables: A, DM, the scalar sum of the pT of jets and the muon, the scalar sum of the pT of the third and fourth jet, the transverse mass of all jets, and the ratio of the mass of the three leading jets to the mass of the event, defined as the invariant mass of all jets, the lepton and E6 T . The discriminant function is built using simulated W � jets and t�t events. We evaluate it for each physics process considered and build corresponding template distributions consisting of ten bins. For t�t we obtain a distribution for each of the three decay modes. The shapes of the discrimi- nant distributions for Z� jets, diboson and single top backgrounds are found to be similar to that of the W � jets events and we use the latter to model them. The discriminant shape for the multijet background is obtained from a sample of data events where the lepton fails the isolation criteria. We define a likelihood function as the product of Poisson probabilities over all 30 subsamples and bins of the discriminant. In each subsample the expected number of events is estimated as a function of R and �t�t. We include 12 additional Poisson terms to constrain the multi- jet background in each subsample. The systematic uncer- tainties are incorporated in the fit using nuisance parameters [7], each represented by a Gaussian term in the likelihood. In this approach, each source of systematic uncertainty is allowed to affect the central value of R and �t�t during the fit, yielding a combined statistical and TABLE I. Sample composition for the measured �t�t and R � 1. Total uncertainties are given. Njets Sample 0 b tags 1 b tag � 2 b tags 3 W � jets 1394:4� 65:1 102:5� 9:4 8:3� 1:2 Multijet 287:4� 35:9 28:1� 3:5 3:3� 0:4 Other 254:0� 35:2 29:4� 3:5 5:2� 0:7 t�t 109:7� 6:6 143:3� 5:1 54:3� 4:3 Total 2045:5� 82:5 303:3� 11:8 71:2� 4:5 Observed 2050 294 76 4 W � jets 188:2� 38:0 17:3� 3:8 1:8� 0:4 Multijet 66:9� 9:9 6:6� 1:0 0:8� 0:1 Other 62:2� 11:8 8:0� 1:4 1:7� 0:3 t�t 83:8� 9:4 126:4� 11:4 64:2� 4:5 Total 401:1� 42:1 158:3� 12:1 69:5� 4:5 Observed 389 179 58 R p ro b ab ili ty R 0.2 R 0 0.2 0.4 0.6 0.8 1 0 0.4 0.6 0.8 1 0 tag 1 tag 2 tags≥ DØ Run II 0 1 2≥ N u m b er o f ev en ts Number of tagged jets 0 200 400 600 DØ Data tt other W+jets Multijet -1L=0.9 fb Likelihood discriminant N u m b er o f ev en ts 0 20 40 60 80 100 120 0 0.2 0.4 0.6 0.8 1 0 20 40 60 80 100 120 DØ Data tt other W+jets Multijet -1L=0.9 fb)c()b()a( FIG. 1. (a) Probability of t�t events to have 0, 1, and� 2 b tags as a function of R for events with� 4 jets; (b) predicted and observed number of events in the 0, 1 and� 2 b tag samples for the measured R and �t�t for events with� 4 jets and (c) predicted and observed discriminant distribution in the 0 b tag sample with � 4 jets. PRL 100, 192003 (2008) P H Y S I C A L R E V I E W L E T T E R S week ending 16 MAY 2008 192003-5 systematic uncertainty. The result of the fit is R � 0:97�0:09 �0:08�stat� syst� and �t�t � 8:18�0:90 �0:84�stat� syst� � 0:50�lumi� pb; (2) for a top-quark mass of 175 GeV. Figures 1(b) and 1(c) compare the distribution of the data to the sum of predicted background and measured signal. We observe no signifi- cant dependence of R on mtop within �10 GeV around the assumed value while �t�t changes by 0:09 pb per 1 GeV within the same range. We find a correlation between R and �t�t of �58%. Table II summarizes the statistical and lead- ing systematic uncertainties on R and �t�t excluding the 6.1% uncertainty on the integrated luminosity [18]. The total uncertainty on R is about 9%, compared to 17% achieved in the previous measurement [6]. The largest uncertainty comes from the limited statistics. Since the b-tagging efficiency drives the distribution of the events among the b-tag subsamples and is strongly anticorrelated with R, the systematic uncertainty is dominated by the b-tagging efficiency estimation, responsible for 90% of the total systematic uncertainty. The total uncertainty on �t�t, excluding luminosity, is 10:5%, representing a 30% improvement over the pre- vious measurement [7] assuming R � 1. Part of the im- provement results from a fourfold reduction in the systematic uncertainties due to b-tagging, which is mostly absorbed by the R measurement. We extract a limit on R and jVtbj following the Feldman- Cousins procedure [19]. We generate pseudoexperiments with all systematic uncertainties included for various input values of R (Rtrue). We obtain R> 0:88 at 68% C.L. and R> 0:79 at 95% C.L., illustrated in Fig. 2. From R we determine the ratio of jVtbj2 to the off-diagonal matrix elements to be jVtbj2 jVtsj2�jVtdj2 > 3:8 at 95% C.L. Assuming a unitary CKM matrix with three fermion generations we derive jVtbj> 0:89 at 95% C.L. In summary, we have performed a simultaneous mea- surement of R and �t�t yielding the most precise measure- ments to date, both in good agreement with the SM [1,20]. This measurement of R will be a key ingredient in a future model-independent direct determination of the jVtqj CKM matrix elements. 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. *Visitor from Augustana College, Sioux Falls, SD, USA. †Visitor from The University of Liverpool, Liverpool, United Kingdom. ‡Visitor from ICN-UNAM, Mexico City, Mexico. xVisitor from II. Physikalisches Institut, Georg-August- University, Göttingen, Germany. kVisitor from Helsinki Institute of Physics, Helsinki, Finland. {Deceased. [1] N. Cabbibo, Phys. Rev. Lett. 10, 531 (1963); M. Kobayashi and T. Maskawa, Prog. Theor. Phys. 49, 652 (1973). [2] W.-M. Yao et al., J. Phys. G 33, 1 (2006). [3] V. Abazov et al. (D0 Collaboration), Phys. Rev. Lett. 98, 181802 (2007). [4] J. Alwall et al., Eur. Phys. J. C 49, 791 (2007). [5] T. Affolder et al. (CDF Collaboration), Phys. Rev. Lett. 86, 3233 (2001); D. Acosta et al. (CDF Collaboration), Phys. Rev. Lett. 95, 102002 (2005). [6] V. Abazov et al. (D0 Collaboration), Phys. Lett. B 639, 616 (2006). FIG. 2 (color online). The 68% (inner band), 95% (middle band), and 99% (outer band) C.L. bands for Rtrue as a function of R. The dotted black line indicates the measured value R � 0:97. TABLE II. Summary of uncertainties on �t�t and R. Source ��t�t (pb) �R Statistical �0:67� 0:64 �0:067� 0:065 Lepton identification �0:32� 0:27 n/a Jet energy scale �0:32� 0:23 n/a W � jets background �0:21� 0:23 n/a Multijet background �0:17� 0:17 �0:016� 0:016 Signal modeling �0:12� 0:25 n/a b-tagging efficiency �0:10� 0:09 �0:059� 0:047 Other �0:24� 0:13 �0:015� 0:014 Total uncertainty �0:90� 0:84 �0:092� 0:083 PRL 100, 192003 (2008) P H Y S I C A L R E V I E W L E T T E R S week ending 16 MAY 2008 192003-6 [7] V. Abazov et al. (D0 Collaboration), Phys. Rev. D 74, 112004 (2006). [8] D. Acosta et al. (CDF Collaboration), Phys. Rev. D 71, 072005 (2005); A. Abulencia et al. (CDF Collaboration), Phys. Rev. Lett. 97, 082004 (2006). [9] V. Abazov et al. (D0 Collaboration), Nucl. Instrum. Methods Phys. Res., Sect. A 565, 463 (2006). [10] V. Abazov et al. (D0 Collaboration), Phys. Rev. D 76, 092007 (2007). [11] T. Scanlon, Ph.D. thesis, University of London, 2006. [12] M. L. Mangano et al., J. High Energy Phys. 07 (2003) 001. [13] T. 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