Search for the Standard Model Higgs Boson in the Missing Energy and Acoplanar
b-Jet Topology
at
p
ffiffiffi
s
¼ 1:96 TeV
V. M. Abazov,36B. Abbott,75M. Abolins,65B. S. Acharya,29M. Adams,51T. Adams,49E. Aguilo,6M. Ahsan,59 G. D. Alexeev,36G. Alkhazov,40A. Alton,64,*G. Alverson,63G. A. Alves,2M. Anastasoaie,35L. S. Ancu,35T. Andeen,53
B. Andrieu,17M. S. Anzelc,53M. Aoki,50Y. Arnoud,14M. Arov,60M. Arthaud,18A. Askew,49B. A˚ sman,41 A. C. S. Assis Jesus,3O. Atramentov,49C. Avila,8F. Badaud,13L. Bagby,50B. Baldin,50D. V. Bandurin,59P. Banerjee,29
S. Banerjee,29E. Barberis,63A.-F. Barfuss,15P. Bargassa,80P. Baringer,58J. Barreto,2J. F. Bartlett,50U. Bassler,18 D. Bauer,43S. Beale,6A. Bean,58M. Begalli,3M. Begel,73C. Belanger-Champagne,41L. Bellantoni,50A. Bellavance,50
J. A. Benitez,65S. B. Beri,27G. Bernardi,17R. Bernhard,23I. Bertram,42M. Besanc¸on,18R. Beuselinck,43 V. A. Bezzubov,39P. C. Bhat,50V. Bhatnagar,27C. Biscarat,20G. Blazey,52F. Blekman,43S. Blessing,49K. Bloom,67
A. Boehnlein,50D. Boline,62T. A. Bolton,59E. E. Boos,38G. Borissov,42T. Bose,77A. Brandt,78R. Brock,65 G. Brooijmans,70A. Bross,50D. Brown,81X. B. Bu,7N. J. Buchanan,49D. Buchholz,53M. Buehler,81V. Buescher,22 V. Bunichev,38S. Burdin,42,†T. H. Burnett,82C. P. Buszello,43J. M. Butler,62P. Calfayan,25S. Calvet,16J. Cammin,71 E. Carrera,49W. Carvalho,3B. C. K. Casey,50H. Castilla-Valdez,33S. Chakrabarti,18D. Chakraborty,52K. M. Chan,55 A. Chandra,48E. Cheu,45F. Chevallier,14D. K. Cho,62S. Choi,32B. Choudhary,28L. Christofek,77T. Christoudias,43 S. Cihangir,50D. Claes,67J. Clutter,58M. Cooke,50W. E. Cooper,50M. Corcoran,80F. Couderc,18M.-C. Cousinou,15 S. Cre´pe´-Renaudin,14V. Cuplov,59D. Cutts,77M. C´ wiok,30H. da Motta,2A. Das,45G. Davies,43K. De,78S. J. de Jong,35
E. De La Cruz-Burelo,33C. De Oliveira Martins,3K. DeVaughan,67J. D. Degenhardt,64F. De´liot,18M. Demarteau,50 R. Demina,71D. Denisov,50S. P. Denisov,39S. Desai,50H. T. Diehl,50M. Diesburg,50A. Dominguez,67H. Dong,72
T. Dorland,82A. Dubey,28L. V. Dudko,38L. Duflot,16S. R. Dugad,29D. Duggan,49A. Duperrin,15J. Dyer,65 A. Dyshkant,52M. Eads,67D. Edmunds,65J. Ellison,48V. D. Elvira,50Y. Enari,77S. Eno,61P. Ermolov,38,††H. Evans,54 A. Evdokimov,73V. N. Evdokimov,39A. V. Ferapontov,59T. Ferbel,71F. Fiedler,24F. Filthaut,35W. Fisher,50H. E. Fisk,50 M. Fortner,52H. Fox,42S. Fu,50S. Fuess,50T. Gadfort,70C. F. Galea,35C. Garcia,71A. Garcia-Bellido,71V. Gavrilov,37 P. Gay,13W. Geist,19W. Geng,15,65C. E. Gerber,51Y. Gershtein,49D. Gillberg,6G. Ginther,71N. Gollub,41B. Go´mez,8
A. Goussiou,82P. D. Grannis,72H. Greenlee,50Z. D. Greenwood,60E. M. Gregores,4G. Grenier,20Ph. Gris,13 J.-F. Grivaz,16A. Grohsjean,25S. Gru¨nendahl,50M. W. Gru¨newald,30F. Guo,72J. Guo,72G. Gutierrez,50P. Gutierrez,75 A. Haas,70N. J. Hadley,61P. Haefner,25S. Hagopian,49J. Haley,68I. Hall,65R. E. Hall,47L. Han,7K. Harder,44A. Harel,71
J. M. Hauptman,57J. Hays,43T. Hebbeker,21D. Hedin,52J. G. Hegeman,34A. P. Heinson,48U. Heintz,62C. Hensel,22,x K. Herner,72G. Hesketh,63M. D. Hildreth,55R. Hirosky,81J. D. Hobbs,72B. Hoeneisen,12H. Hoeth,26M. Hohlfeld,22 S. Hossain,75P. Houben,34Y. Hu,72Z. Hubacek,10V. Hynek,9I. Iashvili,69R. Illingworth,50A. S. Ito,50S. Jabeen,62
M. Jaffre´,16S. Jain,75K. Jakobs,23C. Jarvis,61R. Jesik,43K. Johns,45C. Johnson,70M. Johnson,50D. Johnston,67 A. Jonckheere,50P. Jonsson,43A. Juste,50E. Kajfasz,15J. M. Kalk,60D. Karmanov,38P. A. Kasper,50I. Katsanos,70 D. Kau,49V. Kaushik,78R. Kehoe,79S. Kermiche,15N. Khalatyan,50A. Khanov,76A. Kharchilava,69Y. M. Kharzheev,36
D. Khatidze,70T. J. Kim,31M. H. Kirby,53M. Kirsch,21B. Klima,50J. M. Kohli,27J.-P. Konrath,23A. V. Kozelov,39 J. Kraus,65T. Kuhl,24A. Kumar,69A. Kupco,11T. Kurcˇa,20V. A. Kuzmin,38J. Kvita,9F. Lacroix,13D. Lam,55 S. Lammers,70G. Landsberg,77P. Lebrun,20W. M. Lee,50A. Leflat,38J. Lellouch,17J. Li,78,††L. Li,48Q. Z. Li,50 S. M. Lietti,5J. K. Lim,31J. G. R. Lima,52D. Lincoln,50J. Linnemann,65V. V. Lipaev,39R. Lipton,50Y. Liu,7Z. Liu,6
A. Lobodenko,40M. Lokajicek,11P. Love,42H. J. Lubatti,82R. Luna,3A. L. Lyon,50A. K. A. Maciel,2D. Mackin,80 R. J. Madaras,46P. Ma¨ttig,26C. Magass,21A. Magerkurth,64P. K. Mal,82H. B. Malbouisson,3S. Malik,67V. L. Malyshev,36
Y. Maravin,59B. Martin,14R. McCarthy,72A. Melnitchouk,66L. Mendoza,8P. G. Mercadante,5M. Merkin,38 K. W. Merritt,50A. Meyer,21J. Meyer,22,xJ. Mitrevski,70R. K. Mommsen,44N. K. Mondal,29R. W. Moore,6T. Moulik,58 G. S. Muanza,20M. Mulhearn,70O. Mundal,22L. Mundim,3E. Nagy,15M. Naimuddin,50M. Narain,77N. A. Naumann,35
H. A. Neal,64J. P. Negret,8P. Neustroev,40H. Nilsen,23H. Nogima,3S. F. Novaes,5T. Nunnemann,25V. O’Dell,50 D. C. O’Neil,6G. Obrant,40C. Ochando,16D. Onoprienko,59N. Oshima,50N. Osman,43J. Osta,55R. Otec,10 G. J. Otero y Garzo´n,50M. Owen,44P. Padley,80M. Pangilinan,77N. Parashar,56S.-J. Park,22,xS. K. Park,31J. Parsons,70
R. Partridge,77N. Parua,54A. Patwa,73G. Pawloski,80B. Penning,23M. Perfilov,38K. Peters,44Y. Peters,26P. Pe´troff,16 M. Petteni,43R. Piegaia,1J. Piper,65M.-A. Pleier,22P. L. M. Podesta-Lerma,33,‡V. M. Podstavkov,50Y. Pogorelov,55 M.-E. Pol,2P. Polozov,37B. G. Pope,65A. V. Popov,39C. Potter,6W. L. Prado da Silva,3H. B. Prosper,49S. Protopopescu,73
J. Rieger,54M. Rijssenbeek,72I. Ripp-Baudot,19F. Rizatdinova,76S. Robinson,43R. F. Rodrigues,3M. Rominsky,75 C. Royon,18P. Rubinov,50R. Ruchti,55G. Safronov,37G. Sajot,14A. Sa´nchez-Herna´ndez,33M. P. Sanders,17B. Sanghi,50
G. Savage,50L. Sawyer,60T. Scanlon,43D. Schaile,25R. D. Schamberger,72Y. Scheglov,40H. Schellman,53 T. Schliephake,26S. Schlobohm,82C. Schwanenberger,44A. Schwartzman,68R. Schwienhorst,65J. Sekaric,49 H. Severini,75E. Shabalina,51M. Shamim,59V. Shary,18A. A. Shchukin,39R. K. Shivpuri,28V. Siccardi,19V. Simak,10
V. Sirotenko,50P. Skubic,75P. Slattery,71D. Smirnov,55G. R. Snow,67J. Snow,74S. Snyder,73S. So¨ldner-Rembold,44 L. Sonnenschein,17A. Sopczak,42M. Sosebee,78K. Soustruznik,9B. Spurlock,78J. Stark,14J. Steele,60V. Stolin,37 D. A. Stoyanova,39J. Strandberg,64S. Strandberg,41M. A. Strang,69E. Strauss,72M. Strauss,75R. Stro¨hmer,25D. Strom,53
L. Stutte,50S. Sumowidagdo,49P. Svoisky,55A. Sznajder,3P. Tamburello,45A. Tanasijczuk,1W. Taylor,6B. Tiller,25 F. Tissandier,13M. Titov,18V. V. Tokmenin,36I. Torchiani,23D. Tsybychev,72B. Tuchming,18C. Tully,68P. M. Tuts,70
R. Unalan,65L. Uvarov,40S. Uvarov,40S. Uzunyan,52B. Vachon,6P. J. van den Berg,34R. Van Kooten,54 W. M. van Leeuwen,34N. Varelas,51E. W. Varnes,45I. A. Vasilyev,39P. Verdier,20L. S. Vertogradov,36M. Verzocchi,50
D. Vilanova,18F. Villeneuve-Seguier,43P. Vint,43P. Vokac,10M. Voutilainen,67,kR. Wagner,68H. D. Wahl,49 M. H. L. S. Wang,50J. Warchol,55G. Watts,82M. Wayne,55G. Weber,24M. Weber,50,{L. Welty-Rieger,54A. Wenger,23,** N. Wermes,22M. Wetstein,61A. White,78D. Wicke,26M. Williams,42G. W. Wilson,58S. J. Wimpenny,48M. Wobisch,60 D. R. Wood,63T. R. Wyatt,44Y. Xie,77S. Yacoob,53R. Yamada,50W.-C. Yang,44T. Yasuda,50Y. A. Yatsunenko,36H. Yin,7 K. Yip,73H. D. Yoo,77S. W. Youn,53J. Yu,78C. Zeitnitz,26S. Zelitch,81T. Zhao,82B. Zhou,64J. Zhu,72M. Zielinski,71
D. Zieminska,54A. Zieminski,54,††L. Zivkovic,70V. Zutshi,52and E. G. Zverev38 (D0 Collaboration)
1Universidad de Buenos Aires, Buenos Aires, Argentina 2LAFEX, Centro Brasileiro de Pesquisas Fı´sicas, Rio de Janeiro, Brazil
3
Universidade do Estado do Rio de Janeiro, Rio de Janeiro, Brazil
4Universidade Federal do ABC, Santo Andre´, Brazil
5Instituto de Fı´sica Teo´rica, Universidade Estadual Paulista, Sa˜o 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, Bogota´, 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 12
Universidad San Francisco de Quito, Quito, Ecuador
13LPC, Universite´ Blaise Pascal, CNRS/IN2P3, Clermont, France
14LPSC, Universite´ Joseph Fourier Grenoble 1, CNRS/IN2P3, Institut National Polytechnique de Grenoble, Grenoble, France 15CPPM, Aix-Marseille Universite´, CNRS/IN2P3, Marseille, France
16LAL, Universite´ Paris-Sud, IN2P3/CNRS, Orsay, France 17LPNHE, IN2P3/CNRS, Universite´s Paris VI and VII, Paris, France
18CEA, Irfu, SPP, Saclay, France
19IPHC, Universite´ Louis Pasteur, CNRS/IN2P3, Strasbourg, France
20IPNL, Universite´ Lyon 1, CNRS/IN2P3, Villeurbanne, France and Universite´ de Lyon, Lyon, France 21III. Physikalisches Institut A, RWTH Aachen University, Aachen, Germany
22Physikalisches Institut, Universita¨t Bonn, Bonn, Germany 23Physikalisches Institut, Universita¨t Freiburg, Freiburg, Germany
24Institut fu¨r Physik, Universita¨t Mainz, Mainz, Germany 25Ludwig-Maximilians-Universita¨t Mu¨nchen, Mu¨nchen, Germany 26Fachbereich Physik, University of Wuppertal, Wuppertal, Germany
27Panjab University, Chandigarh, India 28
Delhi University, Delhi, India
29Tata Institute of Fundamental Research, Mumbai, India 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
42Lancaster University, Lancaster, United Kingdom 43Imperial College, London, United Kingdom 44University of Manchester, Manchester, United Kingdom
45University of Arizona, Tucson, Arizona 85721, USA
46Lawrence Berkeley National Laboratory and University of California, Berkeley, California 94720, USA 47California State University, Fresno, California 93740, USA
48University of California, Riverside, California 92521, USA 49Florida State University, Tallahassee, Florida 32306, USA 50Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
51University of Illinois at Chicago, Chicago, Illinois 60607, USA 52Northern Illinois University, DeKalb, Illinois 60115, USA
53Northwestern University, Evanston, Illinois 60208, USA 54Indiana University, Bloomington, Indiana 47405, USA 55University of Notre Dame, Notre Dame, Indiana 46556, USA
56Purdue University Calumet, Hammond, Indiana 46323, USA 57
Iowa State University, Ames, Iowa 50011, USA
58University of Kansas, Lawrence, Kansas 66045, USA 59Kansas State University, Manhattan, Kansas 66506, USA 60Louisiana Tech University, Ruston, Louisiana 71272, USA 61University of Maryland, College Park, Maryland 20742, USA
62Boston University, Boston, Massachusetts 02215, USA 63Northeastern University, Boston, Massachusetts 02115, USA
64University of Michigan, Ann Arbor, Michigan 48109, USA 65Michigan State University, East Lansing, Michigan 48824, USA
66University of Mississippi, University, Mississippi 38677, USA 67University of Nebraska, Lincoln, Nebraska 68588, USA 68Princeton University, Princeton, New Jersey 08544, USA 69State University of New York, Buffalo, New York 14260, USA
70Columbia University, New York, New York 10027, USA 71University of Rochester, Rochester, New York 14627, USA 72State University of New York, Stony Brook, New York 11794, USA
73Brookhaven National Laboratory, Upton, New York 11973, USA 74Langston University, Langston, Oklahoma 73050, USA 75University of Oklahoma, Norman, Oklahoma 73019, USA 76Oklahoma State University, Stillwater, Oklahoma 74078, USA
77Brown University, Providence, Rhode Island 02912, USA 78University of Texas, Arlington, Texas 76019, USA 79Southern Methodist University, Dallas, Texas 75275, USA
80
Rice University, Houston, Texas 77005, USA
81University of Virginia, Charlottesville, Virginia 22901, USA 82University of Washington, Seattle, Washington 98195, USA
(Received 15 August 2008; published 17 December 2008)
We report a search for the standard model Higgs boson in the missing energy and acoplanarb-jet topology, using an integrated luminosity of 0:93 fb1 recorded by the D0 detector at the Fermilab Tevatronp p Collider. The analysis includes signal contributions from p p ! ZH ! b b, as well as fromWH production in which the charged lepton from the W boson decay is undetected. Neural networks are used to separate signal from background. In the absence of a signal, we set limits on ðp p ! VHÞ BðH ! b bÞ at the 95% C.L. of 2.6–2.3 pb, for Higgs boson masses in the range 105–135 GeV, whereV ¼ W, Z. The corresponding expected limits range from 2.8 to 2.0 pb.
The Higgs mechanism, postulated to explain electro-weak symmetry breaking, predicts the existence of the Higgs boson, which has yet to be found. The CERN LEP eþe Collider experiments placed a lower limit on its
mass of 114.4 GeV at 95% C.L. [1]. Global fits to pre-cision electroweak data suggest a mass ofMH< 185 GeV at 95% C.L. when combined with the direct searches [2]. In this range, the Fermilab Tevatron p p Collider has significant discovery potential. Searches in the missing energy and acoplanar b-jet channel have been published by CDF [3] and D0 [4]. This channel is sensitive toZH associated production when the Z decays to neutrinos and WH production when the charged lepton from the W decay is undetected. It is complementary to searches with visible leptons in the final state [5–8] and con-tributes significantly to the discovery potential of a low mass Higgs boson. At the Tevatron it provides the best sensitivity along with the search for WH ! ‘‘b b. The result in this Letter supersedes our previous work. As well as benefitting from more data, this analysis is enhanced through the use of artificial neural networks (NN) for heavy flavor tagging (b tagging) and in event selection.
The D0 detector is described in Ref. [9]. Dedicated triggers selected events with acoplanar jets and large im-balance in transverse momentum, (E6T), as defined by energy deposited in the D0 calorimeters. After imposing data quality requirements the data correspond to an inte-grated luminosity of 0:93 fb1 [10]. Time-dependent ad-justments have been made to the trigger requirements to compensate for the increasing peak instantaneous luminos-ity of the Tevatron. The selection criteria therefore varied somewhat, but typical requirements using jets recon-structed at the highest level trigger [9] were H6 T > 30 GeV, where H6 T is the imbalance in transverse momen-tum from jets, and an azimuthal angle between the two leading (highestpT) jets ofðjet1; jet2Þ < 170.
Event selection requires at least two jets with pT > 20 GeV, jj < 1:1 (central calorimeter) or 1:4 < jj < 2:5 (end calorimeters) and ðjet1; jet2Þ < 165, where
is the pseudorapidity measured from the center of the detector. Reconstructed jets are corrected based on the expected calorimeter response, energy lost due to shower-ing out of the jet cone, and energy deposited in the jet cone not associated with the jet [11]. We require the distance along the beam axis of the primary vertex from the center of the detector (zPV) to be less than 35 cm, and at least three tracks attached to the primary vertex to ensureb tagging capability. In order to reduce the contribution from tt background, we also require E6T > 50 GeV and HT < 240 GeV, where HT is the scalar sum of the transverse
momenta of selected jets. A significant proportion of W=Z þ jets events in which the bosons decay into charged leptons are rejected by vetoing isolated leptons (electrons or muons).
Signal samples ofZH ! b b and WH ! ‘‘b b (‘ ¼ e, , ) were generated for 105 MH 135 GeV in
10 GeV increments usingPYTHIA[12]. There are two types of backgrounds: physical processes modeled by Monte Carlo (MC) generators and instrumental back-ground predicted from data. ALPGEN [13] was used to simulatettproduction with up to four jets. Samples of W þ jets (W decays to all three lepton pairs for light jets jj, b b and cc jets) and Z þ jets (including Z ! and Z ! þprocesses forjj, b b and cc jets) were also generated
separately using ALPGEN. Diboson processes (WW, WZ and ZZ) were generated with PYTHIA. The samples gen-erated with ALPGEN were processed through PYTHIA for showering and hadronization. Next-to-leading order (NLO) cross sections were used for normalizing all pro-cesses (NNLO fortt). All samples were processed through the GEANT3-based D0 detector simulation [14] and the reconstruction software. The trigger requirements were modeled using a parametrized trigger simulation deter-mined from data.
As b tagging is applied later, jets are required to be ‘‘taggable’’, i.e., satisfy certain minimal tracking and ver-texing criteria; a jet must have at least two tracks, one with pT> 1 and the other with >0:5 GeV, each with 2 hits in
the silicon vertex detector, and Rðtrack; jetÞ < 0:5, where R ¼pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðÞ2þ ðÞ2, with being the azimu-thal angle. The fraction of taggable jets was investigated as a function ofpT, and zPVusing aW þ jets data sample. The taggability of simulated jets is corrected by the ratio of taggabilities measured in data and MC samples, which is found to depend only on . Correction factors of 0:97 0:01 and 0:95 0:03 (statistical errors) are used for the central and end calorimeters, respectively.
For events originating from hard processes with genuine missing transverse energy, theH6 T,E6TandT6T(whereT6T is
) T H , T E A( -0.3 -0.2 -0.1 0 0.1 0.2 Events/ 0.01 100 200 300 400 500 600 700 800 900 Data Physics Bkg Instrumental Bkg -1
DØ, 0.93 fb
FIG. 1 (color online). AðE6T; H6 TÞ for data, MC physics back-ground and instrumental backback-ground in the signal region, before implementing b tagging. The final selection corresponds to 0:1 < AðE6T; H6 TÞ < 0:2.
the negative of the vector sum of thepTof all tracks) point in the same direction and are correlated. However, dijet events in which one of the jets has been mismeasured typically haveE6T pointing along the direction of one of the jets. Instrumental effects produce events that tend to haveE6T andT6T misaligned. To reduce instrumental back-ground, we require: (i) minfiðE6T; jetiÞg > 0:15, where minfiðE6T; jetiÞg is the minimum of the difference in
azimuthal angle between the direction of E6T and any of the jets; (ii) E6TðGeVÞ > 40 minfiðE6T; jetiÞg þ 80; (iii)ðE6T; T6TÞ < =2, where ðE6T; T6TÞ is the differ-ence in azimuthal angle between the directions ofE6T and T6T; (iv) 0:1 < AðE6T; H6 TÞ < 0:2, where AðE6T; H6 TÞ
ðE6T H6 TÞ=ðE6Tþ H6 TÞ is the asymmetry between E6T
andH6 T.
The residual contribution of the instrumental back-ground is determined from distributions in AðE6T; H6 TÞ and ðE6T; T6TÞ. The instrumental background peaks at AðE6T; H6 TÞ < 0 because it is dominated by poor quality
jets that are taken into account when calculatingE6Tbut not H6 T. Signal and sideband regions are defined as having ðE6T; T6TÞ < =2 and ðE6T; T6TÞ > =2, respectively.
The shape of the backgrounds from simulated processes, for both regions, are taken directly from the MC calcula-tions. We fit a sixth-order polynomial to the AðE6T; H6 TÞ distribution in the sideband region to determine the shape (before b-jet tagging) for the instrumental background (after subtracting the MC background contribution) and a triple Gaussian for the signal region. We then do a com-bined physics and instrumental backgrounds fit to data in the signal region, as shown in Fig.1. For this combined fit, the simulation and instrumental background shapes are fixed to those from previous fits, and only the absolute scale of the two types of background is allowed to float. The normalization of the background for simulated (MC) processes is found to be1:06 0:02 (statistical error), in good agreement with the expected cross sections. The
invariant mass distribution of the two leading jets after final background normalization is shown in Fig.2.
The standard D0 neural network b tagging algorithm employs lifetime based information involving track impact parameters and secondary vertices [15]. We optimize the choice ofb tagging operating points for best signal signifi-cance and require one tightb-tag (b-tag efficiency 50% for a mistag rate of 0:4%) and one loose b-tag (b-tag efficiency 70% for a mistag rate of 4:5%). Table I shows the number of expected events from MC and in-strumental backgrounds along with the number of events observed in data, before and after b tagging. After b tagging, 134 18 events are expected and 140 are observed.
Invariant Mass (GeV)
0 50 100 150 200 250 300 350 400 Events/ 10 GeV 0 200 400 600 800 1000 1200 1400 Data Instrumental Bkg Diboson W+jets Z+jets t t -1
DØ, 0.93 fb
FIG. 2 (color online). Invariant mass distribution of the two leading jets beforeb tagging requirements.
TABLE I. Number of events after selections. Sample Nob-tag Doubleb-tag ZHðMH¼ 115 GeVÞ 2:46 0:34 0:88 0:12 WHðMH¼ 115 GeVÞ 1:75 0:25 0:61 0:08 Wjj 5180 670 7:6 1:4 Wb b 397 52 35:4 7:1 Wcc 1170 150 9:3 1:9 Zð! þÞjj 107 14 0:25 0:05 Zð! Þjj 2130 280 0:63 0:12 Zð! þÞb b 6:39 0:83 0:63 0:13 Zð! Þb b 229 30 24:9 5:0 Zð! þÞcc 12:8 1:7 0:18 0:04 Zð! Þcc 467 61 4:9 1:0 tt 172 34 29:1 6:1 Diboson 228 25 3:84 0:50 Total MC Bkg 10100 750 117 17 Instrumental Bkg 2560 330 17:2 3:4 Total Bkg 12700 800 134 18 Observed Events 12500 140 NN output 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Events/ 0.08 0 5 10 15 20 25 30 35 40 NN output 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Events/ 0.08 0 5 10 15 20 25 30 35 40 Data Instrumental Bkg Diboson W+jets Z+jets t t Signal x 50 -1
DØ, 0.93 fb
FIG. 3 (color online). NN output distributions for MH¼ 115 GeV after b tagging. The MC expectation for the Higgs signal is scaled up by a factor of 50.
Further signal-to-background discrimination is achieved by combining several kinematic variables using a NN. Independent MC samples are used for NN training, NN testing and limit setting. The instrumental background contribution is not taken into account during training, as its inclusion does not improve the expected sensitivity. The signal sample used for training is a combination of theZH andWH contributions, weighted such that the total con-tribution from each sample is that expected afterb tagging. The NN input variables are the invariant mass of the two leading jets in the event,R between the two jets, pT of the leading jet,pT of the next-to-leading jet,E6T,H6 T and HT. The input variables are selected for their ability to
separate signal and background and to provide good mod-eling of data. The NN outputs for signal, background and data are shown in Fig.3.
Systematic uncertainties affect the expected number of signal and background events (‘‘overall uncertainties’’) as well as the shape of the distribution in the NN output (‘‘differential uncertainties’’). We estimate overall system-atic uncertainties associated with luminosity (6.1%), trig-ger efficiencies (5%), jet identification (5%), b tagging (7%), background MC cross section (6%–18%) and instru-mental background (20%). All systematic uncertainties are common and correlated between signal and backgrounds, except for the uncertainties on the cross sections and the instrumental background. Differential uncertainties are es-timated from the difference in the shape of the NN output by varying the jet energy scale (JES) by its uncertainties in a correlated way for all signal and background MC samples at each mass point. The difference in the distribution of the NN output from the uncertainty in the shape of the MC di-b-jet mass spectrum associated with the parameters of the generator is also taken into account at eachMH point. The JES uncertainty was estimated to be 10% and that for the mass spectrum 8%. Additionally, the impact on the NN output of the discrepancy in the low mass region in Fig.2was investigated and found to be negligible.
In the absence of any significant enhancement we set a limit on the Higgs production cross section using a modi-fied frequentist approach with a Poisson log-likelihood ratio (LLR) statistic [16,17]. The NN distribution is used to construct the LLR test statistic. The impact of systematic
uncertainties is incorporated through ‘‘marginalization’’ of the Poisson probability distributions for signal and back-ground, assuming Gaussian distributions. We adjust each component of systematic uncertainty by introducing nui-sance multipliers for each and maximizing the likelihood for the agreement between prediction and data with respect to the nuisance parameters, constrained by the prior Gaussian uncertainties for each (profiling). All correlations in the systematics are maintained between signal and background. The resulting limits are presented in TableII. In summary, we have performed a search for the stan-dard model Higgs boson produced in association with either a Z or W boson (denoted as VH), in the final state topology requiring missing transverse momentum and two b-tagged jets in 0:93 fb1 of data. In the absence of a
significant excess in data above the background expecta-tion, we set limits on ðp p ! VHÞ BðH ! b bÞ at the 95% confidence level of 2.6–2.3 pb for Higgs boson masses in the range 105–135 GeV. The corresponding expected limits range from 2.8–2.0 pb. The expected and observed limits, along with the SM prediction, are shown in Fig.4as a function of Higgs mass. This is the most stringent limit to date using the missing energy and acoplanarb-jet topology at a hadron collider. The sensitivity should increase sig-nificantly in the near future with the continuing accumu-lation of luminosity from the Tevatron and improvement in analysis techniques.
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); 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 (United Kingdom); MSMT and GACR (Czech Republic); CRC Program, CFI, NSERC and WestGrid Project (Canada); BMBF and DFG (Germany); SFI (Ireland); The Swedish Research TABLE II. Expected (Exp.) and observed (Obs.) limits in pb
and as a ratio to the SM Higgs cross section (in parentheses), assumingH ! b b.
Higgs Mass (GeV) 105 115 125 135 ZH Exp. 1.6 (15) 1.5 (19) 1.4 (29) 1.2 (47) ZH Obs. 1.5 (14) 1.5 (20) 1.4 (30) 1.3 (51) WH Exp. 4.8 (25) 4.3 (33) 3.8 (47) 3.6 (84) WH Obs. 4.4 (23) 5.0 (39) 4.4 (55) 4.2 (99) VH Exp. 2.8 (9.1) 2.5 (12) 2.3 (18) 2.0 (30) VH Obs. 2.6 (8.7) 2.7 (13) 2.5 (20) 2.3 (34)
Higgs Mass (GeV)
110 120 130 140 ) (pb) b b → B(H × VH) → p (p σ -1 10 1 10 -1
DØ, 0.93 fb
Expected Limit Observed Limit SM PredictionFIG. 4 (color online). 95% C.L. upper limit on ðp p ! VHÞ BðH ! b bÞ (and corresponding expected limit) for VH production versus Higgs boson mass.
Council (Sweden); CAS and CNSF (China); and the Alexander von Humboldt Foundation (Germany).
*Visitor from Augustana College, Sioux Falls, SD, USA.
†Visitor from The University of Liverpool, Liverpool,
United Kingdom.
‡Visitor from ECFM, Universidad Autonoma de Sinaloa,
Culiaca´n, Mexico.
x
Visitor from II. Physikalisches Institut, Georg-August-University, Go¨ttingen, Germany.
k
Visitor from Helsinki Institute of Physics, Helsinki, Finland.
{Visitor from Universita¨t Bern, Bern, Switzerland.
**Visitor from Universita¨t Zu¨rich, Zu¨rich, Switzerland.
††Deceased.
[1] G. Abbiendi et al. (ALEPH, DELPHI, L3, and OPAL Collaborations), Phys. Lett. B 565, 61 (2003).
[2] LEP Electroweak Working Group, http://lepewwg.web. cern.ch/LEPEWWG/.
[3] T. Aaltonen et al. (CDF Collaboration), Phys. Rev. Lett. 100, 211801 (2008).
[4] V. M. Abazov et al. (D0 Collaboration), Phys. Rev. Lett. 97, 161803 (2006).
[5] V. M. Abazov et al. (D0 Collaboration), Phys. Lett. B 663, 26 (2008).
[6] T. Aaltonen et al. (CDF Collaboration), Phys. Rev. Lett. 100, 041801 (2008).
[7] V. M. Abazov et al. (D0 Collaboration), Phys. Lett. B 655 209 (2007).
[8] T. Aaltonen et al. (CDF Collaboration), arXiv:0807.4493 [Phys. Rev. Lett. (to be published)].
[9] V. M. Abazov et al. (D0 Collaboration), Nucl. Instrum. Methods Phys. Res., Sect. A 565, 463 (2006).
[10] T. Andeen et al., Fermilab Report No. FERMILAB-TM-2365, 2007.
[11] V. M. Abazov et al. (D0 Collaboration), Phys. Rev. Lett. 101, 062001 (2008).
[12] T. Sjo¨strand et al., arXiv:hep-ph/0308153, 2003. We used
PYTHIAversion 6.323.
[13] M. L. Mangano et al., J. High Energy Phys. 07 (2003) 001. [14] R. Brun and F. Carminati, CERN Program Library Long
Writeup Report No. W5013, 1993.
[15] T. Scanlon, Fermilab Report No. FERMILAB-THESIS-2006-43, 2006.
[16] T. Junk, Nucl. Instrum. Methods Phys. Res., Sect. A 434, 435 (1999).
[17] W. Fisher, Fermilab Report No. FERMILAB-TM-2386-E, 2007.
