Combination of t(t)over-bar cross section measurements and constraints on the mass of the top quark and its decays into charged Higgs bosons

Combination of

t

t

_{cross section measurements and constraints on the mass of the top quark and its}

decays into charged Higgs bosons

V. M. Abazov,37B. Abbott,75M. Abolins,65B. S. Acharya,30M. Adams,51T. Adams,49E. Aguilo,6M. Ahsan,59 G. D. Alexeev,37G. Alkhazov,41A. Alton,64,*G. Alverson,63G. A. Alves,2L. S. Ancu,36T. Andeen,53M. S. Anzelc,53

M. Aoki,50Y. Arnoud,14M. Arov,60M. Arthaud,18A. Askew,49,†B. A˚ sman,42O. Atramentov,49,†C. Avila,8 J. BackusMayes,82F. Badaud,13L. Bagby,50B. Baldin,50D. V. Bandurin,59S. Banerjee,30E. Barberis,63A.-F. Barfuss,15

P. Bargassa,80P. Baringer,58J. Barreto,2J. F. Bartlett,50U. Bassler,18D. Bauer,44S. Beale,6A. Bean,58M. Begalli,3 M. Begel,73C. Belanger-Champagne,42L. Bellantoni,50A. Bellavance,50J. A. Benitez,65S. B. Beri,28G. Bernardi,17 R. Bernhard,23I. Bertram,43M. Besanc¸on,18R. Beuselinck,44V. A. Bezzubov,40P. C. Bhat,50V. Bhatnagar,28G. Blazey,52 S. Blessing,49K. Bloom,67A. Boehnlein,50D. Boline,62T. A. Bolton,59E. E. Boos,39G. Borissov,43T. Bose,62A. Brandt,78

R. Brock,65G. Brooijmans,70A. Bross,50D. Brown,19X. B. Bu,7D. Buchholz,53M. Buehler,81V. Buescher,22 V. Bunichev,39S. Burdin,43,‡T. H. Burnett,82C. P. Buszello,44P. Calfayan,26B. Calpas,15S. Calvet,16J. Cammin,71

M. A. Carrasco-Lizarraga,34E. Carrera,49W. Carvalho,3B. C. K. Casey,50H. Castilla-Valdez,34S. Chakrabarti,72 D. Chakraborty,52K. M. Chan,55A. Chandra,48E. Cheu,46S. Chevalier-The´ry,18D. K. Cho,62S. Choi,33B. Choudhary,29

T. Christoudias,44S. Cihangir,50D. Claes,67J. Clutter,58M. Cooke,50W. E. Cooper,50M. Corcoran,80F. Couderc,18 M.-C. Cousinou,15S. Cre´pe´-Renaudin,14V. Cuplov,59D. Cutts,77M. C´ wiok,31A. Das,46G. Davies,44K. De,78 S. J. de Jong,36E. De La Cruz-Burelo,34K. DeVaughan,67F. De´liot,18M. Demarteau,50R. Demina,71D. Denisov,50 S. P. Denisov,40S. Desai,50H. T. Diehl,50M. Diesburg,50A. Dominguez,67T. Dorland,82A. Dubey,29L. V. Dudko,39 L. Duflot,16D. Duggan,49A. Duperrin,15S. Dutt,28A. Dyshkant,52M. Eads,67D. Edmunds,65J. Ellison,48V. D. Elvira,50

Y. Enari,77S. Eno,61P. Ermolov,39,‡‡M. Escalier,15H. Evans,54A. Evdokimov,73V. N. Evdokimov,40G. Facini,63 A. V. Ferapontov,59T. Ferbel,61,71F. Fiedler,25F. Filthaut,36W. Fisher,50H. E. Fisk,50M. Fortner,52H. Fox,43S. Fu,50

S. Fuess,50T. Gadfort,70C. F. Galea,36A. Garcia-Bellido,71V. Gavrilov,38P. Gay,13W. Geist,19W. Geng,15,65 C. E. Gerber,51Y. Gershtein,49,†D. Gillberg,6G. Ginther,50,71B. Go´mez,8A. Goussiou,82P. D. Grannis,72S. Greder,19

H. Greenlee,50Z. D. Greenwood,60E. M. Gregores,4G. Grenier,20Ph. Gris,13J.-F. Grivaz,16A. Grohsjean,26 S. Gru¨nendahl,50M. W. Gru¨newald,31F. Guo,72J. Guo,72G. Gutierrez,50P. Gutierrez,75A. Haas,70N. J. Hadley,61 P. Haefner,26S. Hagopian,49J. Haley,68I. Hall,65R. E. Hall,47L. Han,7K. Harder,45A. Harel,71J. M. Hauptman,57 J. Hays,44T. Hebbeker,21D. Hedin,52J. G. Hegeman,35A. P. Heinson,48U. Heintz,62C. Hensel,24I. Heredia-De La Cruz,34

K. Herner,64G. Hesketh,63M. D. Hildreth,55R. Hirosky,81T. Hoang,49J. D. Hobbs,72B. Hoeneisen,12M. Hohlfeld,22 S. Hossain,75P. Houben,35Y. Hu,72Z. Hubacek,10N. Huske,17V. Hynek,10I. Iashvili,69R. Illingworth,50A. S. Ito,50 S. Jabeen,62M. Jaffre´,16S. Jain,75K. Jakobs,23D. Jamin,15C. Jarvis,61R. Jesik,44K. Johns,46C. Johnson,70M. Johnson,50

D. Johnston,67A. Jonckheere,50P. Jonsson,44A. Juste,50E. Kajfasz,15D. Karmanov,39P. A. Kasper,50I. Katsanos,67 V. Kaushik,78R. Kehoe,79S. Kermiche,15N. Khalatyan,50A. Khanov,76A. Kharchilava,69Y. N. Kharzheev,37 D. Khatidze,70T. J. Kim,32M. H. Kirby,53M. Kirsch,21B. Klima,50J. M. Kohli,28J.-P. Konrath,23A. V. Kozelov,40

J. Kraus,65T. Kuhl,25A. Kumar,69A. Kupco,11T. Kurcˇa,20V. A. Kuzmin,39J. Kvita,9F. Lacroix,13D. Lam,55 S. Lammers,54G. Landsberg,77P. Lebrun,20W. M. Lee,50A. Leflat,39J. Lellouch,17J. Li,78,‡‡L. Li,48Q. Z. Li,50 S. M. Lietti,5J. K. Lim,32D. Lincoln,50J. Linnemann,65V. V. Lipaev,40R. Lipton,50Y. Liu,7Z. Liu,6A. Lobodenko,41 M. Lokajicek,11P. Love,43H. J. Lubatti,82R. Luna-Garcia,34,xA. L. Lyon,50A. K. A. Maciel,2D. Mackin,80P. Ma¨ttig,27

A. Magerkurth,64P. K. Mal,82H. B. Malbouisson,3S. Malik,67V. L. Malyshev,37Y. Maravin,59B. Martin,14 R. McCarthy,72C. L. McGivern,58M. M. Meijer,36A. Melnitchouk,66L. Mendoza,8D. Menezes,52P. G. Mercadante,5 M. Merkin,39K. W. Merritt,50A. Meyer,21J. Meyer,24J. Mitrevski,70R. K. Mommsen,45N. K. Mondal,30R. W. Moore,6

T. Moulik,58G. S. Muanza,15M. Mulhearn,70O. Mundal,22L. Mundim,3E. Nagy,15M. Naimuddin,50M. Narain,77 H. A. Neal,64J. P. Negret,8P. Neustroev,41H. Nilsen,23H. Nogima,3S. F. Novaes,5T. Nunnemann,26G. Obrant,41 C. Ochando,16D. Onoprienko,59J. Orduna,34N. Oshima,50N. Osman,44J. Osta,55R. Otec,10G. J. Otero y Garzo´n,1 M. Owen,45M. Padilla,48P. Padley,80M. Pangilinan,77N. Parashar,56S.-J. Park,24S. K. Park,32J. Parsons,70R. Partridge,77

N. Parua,54A. Patwa,73G. Pawloski,80B. Penning,23M. Perfilov,39K. Peters,45Y. Peters,45P. Pe´troff,16R. Piegaia,1 J. Piper,65M.-A. Pleier,22P. L. M. Podesta-Lerma,34,kV. M. Podstavkov,50Y. Pogorelov,55M.-E. Pol,2P. Polozov,38 A. V. Popov,40C. Potter,6W. L. Prado da Silva,3S. Protopopescu,73J. Qian,64A. Quadt,24B. Quinn,66A. Rakitine,43 M. S. Rangel,16K. Ranjan,29P. N. Ratoff,43P. Renkel,79P. Rich,45M. Rijssenbeek,72I. Ripp-Baudot,19F. Rizatdinova,76

S. Robinson,44R. F. Rodrigues,3M. Rominsky,75C. Royon,18P. Rubinov,50R. Ruchti,55G. Safronov,38G. Sajot,14 A. Sa´nchez-Herna´ndez,34M. P. Sanders,17B. Sanghi,50G. Savage,50L. Sawyer,60T. Scanlon,44D. Schaile,26

R. D. Schamberger,72Y. Scheglov,41H. Schellman,53T. Schliephake,27S. Schlobohm,82C. Schwanenberger,45 R. Schwienhorst,65J. Sekaric,49H. Severini,75E. Shabalina,24M. Shamim,59V. Shary,18A. A. Shchukin,40 R. K. Shivpuri,29V. Siccardi,19V. Simak,10V. Sirotenko,50P. Skubic,75P. Slattery,71D. Smirnov,55G. R. Snow,67

J. Snow,74S. Snyder,73S. So¨ldner-Rembold,45L. Sonnenschein,21A. Sopczak,43M. Sosebee,78K. Soustruznik,9 B. Spurlock,78J. Stark,14V. Stolin,38D. A. Stoyanova,40J. Strandberg,64S. Strandberg,42M. A. Strang,69E. Strauss,72 M. Strauss,75R. Stro¨hmer,26D. Strom,53L. Stutte,50S. Sumowidagdo,49P. Svoisky,36M. Takahashi,45A. Tanasijczuk,1

W. Taylor,6B. Tiller,26F. Tissandier,13M. Titov,18V. V. Tokmenin,37I. Torchiani,23D. Tsybychev,72B. Tuchming,18 C. Tully,68P. M. Tuts,70R. Unalan,65L. Uvarov,41S. Uvarov,41S. Uzunyan,52B. Vachon,6P. J. van den Berg,35 R. Van Kooten,54W. M. van Leeuwen,35N. Varelas,51E. W. Varnes,46I. A. Vasilyev,40P. Verdier,20L. S. Vertogradov,37 M. Verzocchi,50D. Vilanova,18P. Vint,44P. Vokac,10M. Voutilainen,67,{R. Wagner,68H. D. Wahl,49M. H. L. S. Wang,71 J. Warchol,55G. Watts,82M. Wayne,55G. Weber,25M. Weber,50,**L. Welty-Rieger,54A. Wenger,23,††M. Wetstein,61

A. White,78D. Wicke,25M. R. J. Williams,43G. W. Wilson,58S. J. Wimpenny,48M. Wobisch,60D. R. Wood,63 T. R. Wyatt,45Y. Xie,77C. Xu,64S. Yacoob,53R. Yamada,50W.-C. Yang,45T. Yasuda,50Y. A. Yatsunenko,37Z. Ye,50

H. Yin,7K. Yip,73H. D. Yoo,77S. W. Youn,53J. Yu,78C. Zeitnitz,27S. Zelitch,81T. Zhao,82B. Zhou,64J. Zhu,72 M. Zielinski,71D. Zieminska,54L. Zivkovic,70V. Zutshi,52and E. G. Zverev39

(The D0 Collaboration)

1

Universidad de Buenos Aires, Buenos Aires, Argentina 2_{LAFEX, Centro Brasileiro de Pesquisas Fı´sicas, Rio de Janeiro, Brazil}

3

Universidade do Estado do Rio de Janeiro, Rio de Janeiro, Brazil 4_{Universidade Federal do ABC, Santo Andre´, Brazil}

5_{Instituto de Fı´sica Teo´rica, Universidade Estadual Paulista, Sa˜o Paulo, Brazil}

6_{University of Alberta, Edmonton, Alberta, Canada; Simon Fraser University, Burnaby, British Columbia, Canada;}

York University, Toronto, Ontario, Canada; and McGill University, Montreal, Quebec, Canada 7

University of Science and Technology of China, Hefei, People’s Republic of China 8_{Universidad de los Andes, Bogota´, Colombia}

9

Center for Particle Physics, Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic 10_{Czech Technical University in Prague, Prague, Czech Republic}

11

Center for Particle Physics, Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic 12_{Universidad San Francisco de Quito, Quito, Ecuador}

13_{LPC, Universite´ Blaise Pascal, CNRS/IN2P3, Clermont, France} 14

LPSC, , USAUniversite´ Joseph Fourier Grenoble 1, CNRS/IN2P3, Institut National Polytechnique de Grenoble, Grenoble, France 15_{CPPM, Aix-Marseille Universite´, CNRS/IN2P3, Marseille, France}

16

LAL, , USAUniversite´ Paris-Sud, IN2P3/CNRS, Orsay, France 17_{LPNHE, IN2P3/CNRS, , USAUniversite´s Paris VI and VII, Paris, France}

18

CEA, Irfu, SPP, Saclay, France

19_{IPHC, Universite´ de Strasbourg, CNRS/IN2P3, Strasbourg, France}

20_{IPNL, Universite´ Lyon 1, CNRS/IN2P3, Villeurbanne, France and Universite´ de Lyon, Lyon, France} 21_{III. Physikalisches Institut A, RWTH Aachen University, Aachen, Germany}

22_{Physikalisches Institut, Universita¨t Bonn, Bonn, Germany} 23

Physikalisches Institut, Universita¨t Freiburg, Freiburg, Germany 24_{II. Physikalisches Institut, Georg-August-Universita¨t G Go¨ttingen, Germany}

25

Institut fu¨r Physik, Universita¨t Mainz, Mainz, Germany 26_{Ludwig-Maximilians-Universita¨t Mu¨nchen, Mu¨nchen, Germany} 27

Fachbereich Physik, University of Wuppertal, Wuppertal, Germany 28_{Panjab University, Chandigarh, India}

29_{Delhi University, Delhi, India} 30

Tata Institute of Fundamental Research, Mumbai, India 31_{University College Dublin, Dublin, Ireland} 32

Korea Detector Laboratory, Korea University, Seoul, Korea 33_{SungKyunKwan University, Suwon, Korea}

34

CINVESTAV, Mexico City, Mexico

35_{FOM-Institute NIKHEF and University of Amsterdam/NIKHEF, Amsterdam, The Netherlands} 36_{Radboud University Nijmegen/NIKHEF, Nijmegen, The Netherlands}

37_{Joint Institute for Nuclear Research, Dubna, Russia} 38_{Institute for Theoretical and Experimental Physics, Moscow, Russia}

39_{Moscow State University, Moscow, Russia} 40_{Institute for High Energy Physics, Protvino, Russia} 41_{Petersburg Nuclear Physics Institute, St. Petersburg, Russia}

42_{Stockholm University, Stockholm, Sweden, and Uppsala University, Uppsala, Sweden} 43

Lancaster University, Lancaster, United Kingdom 44_{Imperial College, London, United Kingdom} 45

University of Manchester, Manchester, United Kingdom 46_{University of Arizona, Tucson, Arizona 85721, USA} 47

California State University, Fresno, California 93740, USA 48_{University of California, Riverside, California 92521, USA} 49_{Florida State University, Tallahassee, Florida 32306, USA} 50

Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA 51_{University of Illinois at Chicago, Chicago, Illinois 60607, USA}

52

Northern Illinois University, DeKalb, Illinois 60115, USA 53_{Northwestern University, Evanston, Illinois 60208, USA}

54

Indiana University, Bloomington, Indiana 47405, USA 55_{University of Notre Dame, Notre Dame, Indiana 46556, USA}

56_{Purdue University Calumet, Hammond, Indiana 46323, USA} 57_{Iowa State University, Ames, Iowa 50011, USA} 58_{University of Kansas, Lawrence, Kansas 66045, USA} 59

Kansas State University, Manhattan, Kansas 66506, USA 60_{Louisiana Tech University, Ruston, Louisiana 71272, USA} 61

University of Maryland, College Park, Maryland 20742, USA 62_{Boston University, Boston, Massachusetts 02215, USA} 63

Northeastern University, Boston, Massachusetts 02115, USA 64_{University of Michigan, Ann Arbor, Michigan 48109, USA} 65_{Michigan State University, East Lansing, Michigan 48824, USA}

66

University of Mississippi, University, Mississippi 38677, USA 67_{University of Nebraska, Lincoln, Nebraska 68588, USA} 68

Princeton University, Princeton, New Jersey 08544, USA 69_{State University of New York, Buffalo, New York 14260, USA}

70

Columbia University, New York, New York 10027, USA 71_{University of Rochester, Rochester, New York 14627, USA} 72_{State University of New York, Stony Brook, New York 11794, USA}

73_{Brookhaven National Laboratory, Upton, New York 11973, USA} 74_{Langston University, Langston, Oklahoma 73050, USA} 75

University of Oklahoma, Norman, Oklahoma 73019, USA 76_{Oklahoma State University, Stillwater, Oklahoma 74078, USA}

77

Brown University, Providence, Rhode Island 02912, USA 78_{University of Texas, Arlington, Texas 76019, USA} 79_{Southern Methodist University, Dallas, Texas 75275, USA}

80_{Rice University, Houston, Texas 77005, USA} 81_{University of Virginia, Charlottesville, Virginia 22901, USA}

82

University of Washington, Seattle, Washington 98195, USA

(Received 31 March 2009; published 19 October 2009)

We combine measurements of the top quark pair production cross section in pp collisions in the

‘_þjets,‘‘, and‘ final states (where ‘is an electron or muon) at a center of mass energy ofpffiffiffi_s

¼

1:96 TeVin1 fb 1_{of data collected with the D0 detector. For a top quark mass of170 GeV=c}2_{, we obtain}

tt¼8:18þ00::9887 pbin agreement with the theoretical prediction. Based on predictions from higher order

quantum chromodynamics, we extract a mass for the top quark from the combined ttcross section,

consistent with the world average of the top quark mass. In addition, the ratios ofttcross sections in

different final states are used to set upper limits on the branching fractionsB_ðt_!Hþ_b!þ_bÞand

x_{Visitor from Centro de Investigacion en Computacion-IPN,}

Mexico City, Mexico. ‡‡Deceased.

††_{Visitor from Universita¨t Zu¨rich, Zu¨rich, Switzerland.}

**Visitor from Universita¨t Bern, Bern, Switzerland.

*Visitor from Augustana College, Sioux Falls, SD, USA. kVisitor from ECFM, Universidad Autonoma de Sinaloa,

Culiaca´n, Mexico.

†_{Visitor from Rutgers University, Piscataway, NJ, USA.}

{_{Visitor from Helsinki Institute of Physics, Helsinki, Finland.}

‡_{Visitor from The University of Liverpool, Liverpool, United}

Kingdom.

B_ðt_!Hþ_{b!csbÞas a function of the charged Higgs boson mass.}

DOI:10.1103/PhysRevD.80.071102 PACS numbers: 14.65.Ha, 13.85.Lg, 13.85.Qk, 13.85.Rm

Precise measurements of the production and decay prop-erties of the heaviest known fermion, the top quark, pro-vide important tests of the standard model (SM) and offer a window for searches for new physics. In this paper we measure the top-antitop quark pair (tt) production cross section and compare it with the SM prediction, extract the top quark pole mass from this measurement, and search for new physics in top quark decays analyzing ratios of thett cross sections measured in different decay channels.

The inclusivettproduction cross section (_tt) is mea-sured in differentttdecay channels assuming SM branch-ing fractions. The comparison of the results to predictions in next-to-leading order perturbative quantum chromody-namics (QCD), including higher order soft-gluon resum-mations [1–4], yields a direct test of the SM. Ratios of_tt measured in different final states are particularly sensitive to non-SM particles that may appear in top quark decays, especially if the boson in the decay is not a SMW boson. An example is the decay into a charged Higgs boson (t_! Hþ_{b), which, as predicted in some models [5], can}

com-pete with the SM decay t_!Wþ_{b. Additionally, many}

experimental uncertainties cancel in the ratios. Furthermore, since _tt depends on the mass of the top

quark (m_t), it can be used to extractm_t. Such a measure-ment is less accurate than direct mass measuremeasure-ments, but provides complementary information with different ex-perimental and theoretical uncertainties.

Within the SM, each quark of thettpair is expected to decay nearly 100% of the times into aW boson and a b quark [6].W bosons can decay hadronically intoqq0_pairs

or leptonically intoe_e,, andwith the in turn decaying into an electron, a muon, or hadrons, and asso-ciated neutrinos. If one of the W bosons decays hadroni-cally while the other one produces a direct electron or muon or a secondary electron or muon fromdecay, the final state is referred to as the‘þjets (or ‘j) channel. If bothW bosons decay leptonically, this leads to a dilepton final state containing a pair of electrons, a pair of muons, or an electron and a muon (the‘‘channel), or a hadronically decaying tau accompanied either by an electron or a muon (the‘channel).

Measurements of the individualttcross sections in ‘‘ and‘channels using about1 fb 1_{ofppdata from the D0} detector at the Fermilab Tevatron collider at pffiffiffi_s

¼

1:96 TeVare available in Ref. [7]. In the‘_þjets channel, we use the same selection and background estimation as in Ref. [8], but a slightly larger data set and a unified treat-ment of systematic uncertainties with the‘‘and‘ chan-nels. We provide a brief summary of the event selection and analysis procedures below.

In each final state we select data samples enriched intt events by requiring one or two isolated high transverse

momentum (p_T) leptons for the ‘_þjets or ‘‘ channel, respectively. At least two highp_T jets are required for‘‘ and ‘ events, and at least three for ‘_þjets events. Further, in all but theechannel, large transverse missing energy (E_{6 T}) is required to account for the large transverse momenta of neutrinos fromWboson orlepton decays. In theefinal state, a requirement on the sum of thep_Tof the highest p_T (leading) lepton and the two leading jets is imposed instead. In the channel, theE_{6 T} requirement is supplemented with a requirement on the significance of theE_{6 T} measurement, estimated from thep_T of muons and jets, and their expected resolutions. Additional criteria are applied on the invariant mass of the two opposite charge leptons of the same flavor in the eeand channels to reduce the dominant background from Z=_!‘þ_‘

events. In the‘_þjets and‘channels we require a mini-mum azimuthal angle separation between the E_{6 T} vector and the leptonp_T,_ð‘; E_{6 TÞ}, to reduce background from multijet events, where jets are misidentified as electron, muon, or. Details of lepton, jet, andE₆ T identification are

provided in Refs. [9,10]. The final selection in these chan-nels demands at least one identified b jet via a neural-network based algorithm [11]. In the‘þjets channel we separate events with one or 2b-tagged jets due to their different signal over background ratio and systematic uncertainties.

To simplify the combination and extraction of cross section ratios, all channels are constructed to be mutually exclusive. In particular, events with two identified leptons are excluded from the‘_þjets selection, and all‘ candi-dates are removed from the rest of the channels.

The compositions of the samples in the‘_þjets,‘‘, and ‘ channels are shown in Table I. W_þjets production dominates the background for the ‘_þjets events, while multijet production is the most important background in the ‘ channel. Background in the ‘‘ channels comes mainly from Z_þjets production. In the‘‘channel, con-tributions fromW_þjets production are part of the multijet background. The smaller contribution from diboson pro-duction is included in the category labeled ‘‘other back-ground.’’ This category also includes the contribution from single top quark production in the ‘_þjets and‘ chan-nels. The signal, W_þjets and Z_þjets backgrounds are simulated using ALPGEN [12] for the matrix element cal-culation andPYTHIA[13] for parton showering and hadro-nization. Diboson and single top backgrounds are simulated with thePYTHIAandSINGLETOP[14] generators, respectively. We estimate the multijet background from the control data samples. The difference in the ratio ofttand Wþjets events in theeþjets andþjets final states is the result of the larger efficiency and misidentified lepton rate in the e_þjets channel compensating for the lower

lepton acceptance (j_j<1:1) compared to the _þjets channel (j_j<2:0). In addition, the wider rapidity distri-bution of the W_þjets events compared to tt events in-creases the W_þjets background contribution in the _þjets channel.

To calculate the combined cross section, we define a joint likelihood function as the product of Poisson proba-bilities for the 14 disjoint subsamples, as listed in TableI. Fourteen additional Poisson terms constrain the multijet background in the‘_þjets and‘channels. In particular, for the e and channels, the multijet background is determined by counting events with an electron or muon and associatedof the same electric charge, introducing a corresponding Poisson term per channel. In the ‘þjets channel, we estimate the multijet background separately for each of the eight subchannels by using corresponding control data samples [15]. Four additional terms arise from applying this same method in evaluating the multijet back-ground beforebtagging.

Each systematic uncertainty is included in the likelihood function through one free ‘‘nuisance’’ parameter [15]. Each of these parameters is represented by a Gaussian probability density function with zero mean and a standard deviation of one; all are allowed to float in the maximiza-tion of the likelihood funcmaximiza-tion, thereby changing the cen-tral value of the measuredtt. Correlations are taken into account by using the same nuisance parameter for a com-mon source of systematic uncertainty in different channels scaled by the corresponding standard deviation (SD) of each individual channel. Thus, the likelihood function to be maximized is represented by the product

L _¼Y

14

i¼1

P_ðn_i; m_iÞ Y

14

j¼1

P_ðn_j; m_jÞ Y K

k¼1 SD_ik

G_ðk; 0;1Þ; (1)

where Pðn; m_Þ is the Poisson probability to observe n events given the expectation of m events. The predicted number of events in each channel is the sum of the pre-dicted background and expectedttevents, which depends ontt. In the product,iruns over the subsamples andjruns over the multijet background subsamples. The Gaussian distributions SD_ikG_ðk; 0;1_Þ describe the systematic uncertainties,Kis the total number of independent sources of systematic uncertainty, _k are the individual nuisance parameters, andSDikis 1 standard deviation for the source

of uncertainty kin subsamplei.

Systematic uncertainties on the measured_ttare

eval-uated from sources that include electron and muon identi-fication; and jet identification and energy calibration; b-jet identification; modeling of triggers, signal, and back-ground; and integrated luminosity. All these uncertainties are treated as fully correlated among channels and between signal and background. Systematic uncertainties arising from limited statistics of data or Monte Carlo samples used in estimating signal or backgrounds are considered to be uncorrelated. A detailed discussion on systematic uncertainties can be found in Refs. [7,8]. TableII shows a breakdown of uncertainties on the combined cross sec-tion. We evaluate the effect from each source by setting all uncertainties to zero except the one in question and redoing the likelihood maximization with respect to only the cor-responding nuisance parameter. Since the method allows each uncertainty to change the central value, the total uncertainty on _ttdiffers slightly from the quadratic sum

of the statistical and individual systematic uncertainties. The total systematic uncertainty on_ttexceeds the

statis-tical contribution. The luminosity uncertainty of 6.1% which enters into the estimation of the majority of the backgrounds and the luminosity measurement of the se-lected samples is the dominant source of systematic uncertainty.

TABLE I. Expected numbers of background and signal events fortt¼8:18 pb, observed numbers of data events and measuredtt

at top mass of170 GeV=c2_{. Quoted uncertainties include both statistical and systematic uncertainties, added in quadrature.}

Channel Luminosity (pb 1_{) Wþjets Zþjets Multijet Other bkg tt Total Observed}

tt(pb)

e_þjets (3 jets,1btag) 1038 53:4þ6:0

6:0 6:0þ11::22 31:5þ33::55 11:4þ11::45 81:7þ66::47184:0þ99::02 183 8:06þ11::8971

_þjets (3 jets,1btag) 996 59:2þ5:5

5:6 6:5þ11::33 9:7þ22::88 9:5þ11::22 59:0þ55::76143:9þ88::11 133 6:43þ22::2201

eþjets (3 jets,2btags) 1038 5:0þ0:8

0:8 0:6þ00::22 2:7þ00::33 2:4þ00::44 30:7þ33::99 41:5þ44::76 40 7:78þ22::4101

_þjets (3 jets,2btags) 996 5:8þ0:9

0:9 0:7þ00::22 1:0þ00::33 2:1þ00::33 23:8þ33::42 33:5þ43::19 31 7:29þ22::7325

e_þjets (4jets,1btag) 1038 8:5þ2:7

2:7 2:2þ00::55 7:9þ11::00 3:0þ00::55 81:6þ89::71103:3þ77::36 113 9:38þ11::8252

_þjets (4jets,1btag) 996 13:6þ2:6

2:7 2:5þ00::76 0:0þ00::00 2:4þ00::44 65:9þ67::92 84:3þ56::93 99 10:44þ21::1176

e_þjets (4jets,2btags) 1038 1:0þ0:3

0:3 0:2þ00::11 1:1þ00::11 0:9þ00::22 41:7þ66::00 44:9þ66::00 30 5:12þ11::5928

_þjets (4jets,2btags) 996 1:5þ0:4

0:4 0:3þ00::11 0:0þ00::00 0:7þ00::11 35:6þ55::01 38:2þ55::12 34 7:60þ21::1170

ee 1074 2:3þ0:5

0:5 0:6þ00::44 0:5þ00::11 11:6þ11::22 15:0þ11::55 17 9:61þ32::4784

e(1 jet) 1070 5:5þ0:7

0:8 0:9þ00::32 3:1þ00::77 8:9þ11::44 18:4þ11::99 21 10:61þ54::3323

e(2jets) 1070 5:4þ0:9

1:0 2:6þ00::65 1:4þ00::44 36:4þ33::66 45:8þ44::55 39 6:66þ11::8152

1009 5:6þ1:1

1:2 0:2þ00::22 0:6þ00::11 9:1þ11::00 15:4þ11::98 12 5:08þ33::8206

e(1btag) 1038 0:6þ0:0

0:1 0:6þ00::11 3:0þ11::77 0:2þ00::11 10:7þ11::33 15:0þ22::22 16 8:94þ43::0332

(1btag) 996 0:8þ0:1

0:2 1:2þ00::33 8:0þ22::88 0:2þ00::00 12:6þ11::44 22:7þ33::22 20 6:40þ33::8843

TableIII summarizes the individual _tt measurements

for the individual channels, as well as some of their com-binations. Within uncertainties, all measurements are con-sistent with each other. The combined cross section for ‘_þjets, ‘‘, and ‘ final states for a top quark mass of 170 GeV=c2 _{is evaluated to be}

_tt¼8:18þ00::9887 pb; (2)

in agreement with theoretical predictions [1–4]. The un-certainty is comparable to the one on the cross section combination from different methods in the‘_þjets chan-nel performed by D0 [8]. The observed number of events in the different channels is compared to the sum of the background and combinedttsignal in Fig.1(a).

We compute ratios R of measured cross sections, R‘‘=‘j ¼‘‘_tt=

‘j tt andR

‘=‘‘-‘j

¼‘_tt= ‘j&‘‘

tt , by

generat-ing pseudodata sets in the numerator and denominator in order to take into account the correlation between system-atic uncertainties. channel

tt represent the measured cross

sections in the corresponding channel. The pseudodata sets are created by varying the number of signal and background events around the expected number according to Poisson probabilities. All independent sources of sys-tematic uncertainties are varied within a Gaussian distri-bution. Although the individual channels considered are exclusive, each channel can receive signal contributions

from differentttdecay modes. We calculate the contribu-tion from dilepton events to the‘_þjets final state as well as the contribution from dilepton and‘_þjets events to the ‘ final states using the corresponding observed cross sections in the individual channels when generating pseu-dodata sets. For each pseupseu-dodata set, we perform the max-imization of Eq. (1) separately in the numerator and denominator, and divide the results. The central value is obtained from the mode of the distribution ofR, and the

uncertainties are derived from the interval containing 68% of the pseudoexperiments. From these pseudoexperiments we obtain R‘‘=‘j ¼0:86þ00::1917 and R

‘=‘‘-‘j

¼0:97þ00::3229, which is consistent with the SM expectation ofR_¼1.

Extensions of the SM, based on supersymmetry or grand unification [5], require the existence of additional Higgs multiplets beyond the Higgs doublet of the SM. Some of these models, such as the two Higgs-doublet model or the minimal supersymmetric standard model, foresee the ex-istence of physical degrees of freedom which can be asso-ciated with a charged scalar particle, the charged Higgs boson. If this charged Higgs boson is lighter than the top quark, it will appear in the top quark decays. We use the ratios to extract upper limits on the branching ratio B

B_ðt_!Hþ_{bÞ. In particular, a charged Higgs boson}

decay-ing into a tau and a neutrino (B_ðHþ_{!Þ ¼1) results in}

more events in the‘channel, while fewer events appear in the ‘‘ and‘_þjets final states compared to the SM pre-diction. In case of a leptophobic (B_ðHþ_{!csÞ ¼1)}

model, the number of dilepton events decreases faster than the number of ‘_þjets events for increasingB_ðt_! Hþ_{bÞ. We therefore useR}‘‘=‘j

to set limits on the

lepto-phobic model, whileR‘=‘‘ -‘jis used to search for decays

in which the charged Higgs bosons are assumed to decay exclusively to taus.

To extract the limits, we generate pseudodata sets as-suming different branching fractions Bðt!Hþ_{bÞ. The}

signal for a charged Higgs boson is simulated using the PYTHIA Monte Carlo event generator [13], and includes decays of tt_!Wþ_{bH band its charge conjugate (WH)} andtt_!Hþ_{bH b(HH). For a given branching fraction} B, we calculate the expected number ofttevents per final state,

Ntt¼ ½ð1 BÞ2WW þ2Bð1 BÞ WH

þB2

HHŠttL; (3)

where are the selection efficiencies for the different decays (WW refers tott_!Wþ_{bW b) and Lis the} inte-grated luminosity. We addN_ttto the expected background and treat the sum as a new number of expected events in each channel. We then perform the likelihood maximiza-tion to extract ttfrom these pseudodata as if they con-tained only SM ttproduction. This provides distributions for the ratios of cross sections for each generatedB, which are compared to the observed ratio. We set limits onBby

TABLE III. Summary of measured tt in different channels

formt¼170 GeV=c2.

Channel tt(pb)

‘_þjets 8:46þ1:09

0:97

‘‘[7] 7:46þ1:60

1:37

‘_þjets and‘‘ 8:18þ0:99

0:87

‘[7] 7:77þ2:90

2:47

‘_þjets,‘‘, and‘ 8:18þ0:98

0:87

TABLE II. Summary of uncertainties on the combinedtt.

Source tt(pb)

Statistical _þ0:47 0:46

Lepton identification _þ0:15 0:14

Tau identification _þ0:02 0:02

Jet identification _þ0:11 0:11

Jet energy scale _þ0:19 0:16

Tau energy scale _þ0:02 0:02

Trigger modeling _þ0:11 0:07

b-jet identification _þ0:34 0:32

Signal modeling _þ0:17 0:15

Background estimation _þ0:14 0:14

Multijet background _þ0:12 0:12

Luminosity _þ0:56 0:48

Other _þ0:15 0:14

Total systematic uncertainty _þ0:78 0:69

using the frequentist approach of Feldman and Cousins [16].

The observed and expected (i.e., forR_¼1) limits for the tauonic and the leptophobic charged Higgs boson models are shown in Figs. 1(b)and 1(c), respectively. In the tauonic model the upper 95% C.L. limits onB range from 15% to 40% for80 GeV=c2 _M

Hþ155 GeV=c2,

improving the limits given in [17]. For the leptophobic charged Higgs boson model, which is investigated here for the first time, the upper limit on the B range is between 48% and 57% for the same mass range. Although indirect bounds as those from the measured rate of b_!s[18] appear stronger than the results from the direct search presented here, they can be invalidated by the presence of new physics contributions.

The interpretation of the direct measurement of the top quark mass [6] has become a subject of intense discussion in terms of its renormalization scheme [19]. The extraction of this parameter from the measured cross section provides complementary information, with different sensitivity to theoretical and experimental uncertainties, relative to di-rect methods that rely on kinematic details of the top quark reconstruction. Simulated samples ofttevents generated at different values of the top quark mass are used to estimate the signal acceptance. The resulting measurements of_tt are fitted as a function ofm_t[2]:

_ttðmtÞ ¼

1 m4

t½

a_þb_ðm_t m0Þ þcðmt m0Þ2

þd_ðm_t m_0Þ3_{Š (4)}

where_ttandmtare in pb andGeV=c2, respectively, and m0 ¼170 GeV=c2[20]. The dependence on the top quark mass is due to the mass dependence of the selection efficiencies.

We compare this parametrization to a prediction in pure next-to-leading-order (NLO) QCD [1], to a calculation including NLO QCD and all higher-order soft-gluon re-summations in next-to-leading logarithms (NLL) [2], to an approximation to the next-to-next-to-leading-order

(NNLO) QCD cross section that includes all next-to-next-to-leading logarithms relevant in NNLO QCD [3], and to a calculation that employs full kinematics in the double differential cross section beyond NLL using the soft anomalous dimension matrix to calculate the soft-gluon contributions at NNLO [4]. Figure2shows the experimen-tal and the theoretical [1–3]ttcross sections as a function of the top quark mass.

Following the method of Refs. [7,8], we extract the most probable top quark mass values and the 68% C.L. band. Since the theoretical predictions are performed in the pole mass scheme, this defines the extracted parameter here. The results are given in Table IV. All values are in good agreement with the current world average of 171:2

2:1 GeV=c2 _[6].

In summary, we have combined the tt cross section measurements in‘_þjets,‘‘, and‘channels to measure _tt¼8:18þ00::9887 pbfor a top quark mass of 170 GeV=c2. For the first time, we have also calculated ratios of cross

l+jets 1 tag l+jets 2 tag dilepton τ+lepton

event N 0 200 400 600 =8.18 pb tt σ Data + background t t background -1 DØ, L=1.0 fb

(GeV/c ) 2 +

H

M

80 100 120 140 160

) + b H τ B(t 0 0.2 0.4 0.6 0.8 1 )=1 ν τ τ + B(H

Expected 95% CL limit

Observed 95% CL limit

-1

DØ, L=1.0 fb

ττν)=1

+

B(H

Expected 95% CL limit

Observed 95% CL limit

(GeV/c ) 2 +

H

M

80 100 120 140 160

) + b H τ B(t 0 0.2 0.4 0.6 0.8 1 )=1 s c τ + B(H

Expected 95% CL limit

Observed 95% CL limit

-1

DØ, L=1.0 fb

)=1 s c τ + B(H

Expected 95% CL limit

Observed 95% CL limit

(a) (b) (c)

FIG. 1 (color online). (a) Expected and observed numbers of events versus channel used in measuring the combinedtt. The dashed

band around the prediction indicates the total uncertainty. Upper limits onB_ðt_!Hþ_{bÞfor (b) tauonic and (c) leptophobicH}þ_decays.

The yellow band shows the1standard deviation band around the expected limit.

2

Top Mass (GeV/c )

(pb)_tt σ 2 4 6 8 10 12 14 tt σ Measured

Nadolsky et al., PRD 78, 013004 (2008) Cacciari et al., JHEP 09, 127 (2008) Moch and Uwer, PRD 78, 034003 (2008)

150 160 170 180 190

(pb)_tt

σ

-1

DØ, L = 1 fb

FIG. 2 (color online). Experimental and theoretical [1–3]tt

as a function of m_t. The colored dashed lines represent the

theoretical uncertainties due to the choice of the PDF and the renormalization and factorization scales. The point shows the

measured combinedtt, the black dot-dashed line shows the fit

with Eq. (4), and the gray band shows the corresponding total

experimental uncertainty.

sections and interpreted them in terms of limits on non-standard model top quark decays into a charged Higgs boson. All results are in good agreement with the SM expectations. Finally, using different theoretical predic-tions given in the pole mass scheme, we have extracted the top quark mass from the combinedttand have found

the result to be consistent with the world average top quark mass [6] from direct measurements.

We thank the staffs at Fermilab and the collaborating institutions, and acknowledge support from the DOE and NSF (USA); CEA and CNRS/IN2P3 (France); FASI, Rosatom, and RFBR (Russia); 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); STFC and the Royal Society (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); and the Alexander von Humboldt Foundation (Germany).

[1] P. M. Nadolskyet al., Phys. Rev. D78, 013004 (2008); W.

Beenakker, H. Kuijf, W. L. vanNeerven, and J. Smith,

Phys. Rev. D40, 54 (1989).

[2] M. Cacciariet al., J. High Energy Phys. 09 (2008) 127; M.

Cacciari (private communications).

[3] S. Moch and P. Uwer, Phys. Rev. D 78, 034003 (2008);

(private communications).

[4] N. Kidonakis and R. Vogt, Phys. Rev. D 78, 074005

(2008); N. Kidonakis (private communications).

[5] J. Guasch, R. A. Jimenez, and J. Sola, Phys. Lett. B360,

47 (1995).

[6] C. Amsleret al.(Particle Data Group), Phys. Lett. B667, 1

(2008).

[7] V. M. Abazovet al.(D0 Collaboration), Phys. Lett. B679,

177 (2009).

[8] V. M. Abazovet al. (D0 Collaboration), Phys. Rev. Lett.

100, 192004 (2008).

[9] V. M. Abazovet al.(D0 Collaboration), Phys. Rev. D76,

092007 (2007).

[10] V. M. Abazovet al.(D0 Collaboration), Phys. Rev. D76,

052006 (2007).

[11] T. Scanlon, Ph.D. thesis, Imperial College, London [Institution Report No. FERMILAB-THESIS-2006-43, 2006].

[12] M. L. Manganoet al., J. High Energy Phys. 07 (2003) 001.

[13] T. Sjo¨strand et al., Comput. Phys. Commun. 135, 238

(2001).

[14] E. E. Booset al., Phys. At. Nucl.69, 1317 (2006).

[15] V. M. Abazovet al.(D0 Collaboration), Phys. Rev. D74,

112004 (2006).

[16] G. J. Feldman and R. D. Cousins, Phys. Rev. D57, 3873

(1998).

[17] A. Abulenciaet al.(CDF Collaboration), Phys. Rev. Lett.

96, 042003 (2006).

[18] M. Misiaket al., Phys. Rev. Lett.98, 022002 (2007).

[19] A. H. Hoang and I. W. Stewart, Nucl. Phys. B, Proc. Suppl.

185, 220 (2008).

[20] We obtaina_¼6:823 50109_{,b¼1:104 8010}8_,c¼

8:805 52105_{, andd¼ 1:76710}3_{for Eq. (4).}

TABLE IV. Top quark mass with 68% C.L. region for different

theoretical predictions oftt. Combined experimental and

theo-retical uncertainties are shown.

Theoretical prediction mt (GeV=c2)

NLO [1] 165:5þ6:1

5:9

NLO_þNLL[2] 167:5þ5:8 5:6

Approximate NNLO [3] 169:1þ5:9

5:2

Approximate NNLO [4] 168:2þ5:9

5:4