Search for a Neutral Higgs Boson Decaying to a W Boson Pair in p p Collisions at p s

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Search for a Neutral Higgs Boson Decaying to a W Boson Pair in p p Collisions at p s

1:96 TeV

A. Abulencia,²³D. Acosta,¹⁷J. Adelman,¹³T. Affolder,¹⁰T. Akimoto,⁵⁵M. G. Albrow,¹⁶D. Ambrose,¹⁶S. Amerio,⁴³ D. Amidei,³⁴A. Anastassov,⁵²K. Anikeev,¹⁶A. Annovi,¹⁸J. Antos,¹M. Aoki,⁵⁵G. Apollinari,¹⁶J.-F. Arguin,³³ T. Arisawa,⁵⁷A. Artikov,¹⁴W. Ashmanskas,¹⁶A. Attal,⁸F. Azfar,⁴²P. Azzi-Bacchetta,⁴³P. Azzurri,⁴⁶N. Bacchetta,⁴³ H. Bachacou,²⁸W. Badgett,¹⁶A. Barbaro-Galtieri,²⁸V. E. Barnes,⁴⁸B. A. Barnett,²⁴S. Baroiant,⁷V. Bartsch,³⁰G. Bauer,³²

F. Bedeschi,⁴⁶S. Behari,²⁴S. Belforte,⁵⁴G. Bellettini,⁴⁶J. Bellinger,⁵⁹A. Belloni,³²E. Ben Haim,⁴⁴D. Benjamin,¹⁵ A. Beretvas,¹⁶J. Beringer,²⁸T. Berry,²⁹A. Bhatti,⁵⁰M. Binkley,¹⁶D. Bisello,⁴³R. E. Blair,²C. Blocker,⁶B. Blumenfeld,²⁴ A. Bocci,¹⁵A. Bodek,⁴⁹V. Boisvert,⁴⁹G. Bolla,⁴⁸A. Bolshov,³²D. Bortoletto,⁴⁸J. Boudreau,⁴⁷A. Boveia,¹⁰B. Brau,¹⁰

C. Bromberg,³⁵E. Brubaker,¹³J. Budagov,¹⁴H. S. Budd,⁴⁹S. Budd,²³K. Burkett,¹⁶G. Busetto,⁴³P. Bussey,²⁰ K. L. Byrum,²S. Cabrera,¹⁵M. Campanelli,¹⁹M. Campbell,³⁴F. Canelli,⁸A. Canepa,⁴⁸D. Carlsmith,⁵⁹R. Carosi,⁴⁶ S. Carron,¹⁵M. Casarsa,⁵⁴A. Castro,⁵P. Catastini,⁴⁶D. Cauz,⁵⁴M. Cavalli-Sforza,³A. Cerri,²⁸L. Cerrito,⁴²S. H. Chang,²⁷ J. Chapman,³⁴Y. C. Chen,¹M. Chertok,⁷G. Chiarelli,⁴⁶G. Chlachidze,¹⁴F. Chlebana,¹⁶I. Cho,²⁷K. Cho,²⁷D. Chokheli,¹⁴

J. P. Chou,²¹P. H. Chu,²³S. H. Chuang,⁵⁹K. Chung,¹²W. H. Chung,⁵⁹Y. S. Chung,⁴⁹M. Ciljak,⁴⁶C. I. Ciobanu,²³ M. A. Ciocci,⁴⁶A. Clark,¹⁹D. Clark,⁶M. Coca,¹⁵G. Compostella,⁴³M. E. Convery,⁵⁰J. Conway,⁷B. Cooper,³⁰ K. Copic,³⁴M. Cordelli,¹⁸G. Cortiana,⁴³F. Cresciolo,⁴⁶A. Cruz,¹⁷C. Cuenca Almenar,⁷J. Cuevas,¹¹R. Culbertson,¹⁶

D. Cyr,⁵⁹S. DaRonco,⁴³S. D’Auria,²⁰M. D’Onofrio,³D. Dagenhart,⁶P. de Barbaro,⁴⁹S. De Cecco,⁵¹A. Deisher,²⁸ G. De Lentdecker,⁴⁹M. Dell’Orso,⁴⁶F. Delli Paoli,⁴³S. Demers,⁴⁹L. Demortier,⁵⁰J. Deng,¹⁵M. Deninno,⁵D. De Pedis,⁵¹

P. F. Derwent,¹⁶C. Dionisi,⁵¹J. R. Dittmann,⁴P. DiTuro,⁵²C. Do¨rr,²⁵S. Donati,⁴⁶M. Donega,¹⁹P. Dong,⁸J. Donini,⁴³ T. Dorigo,⁴³S. Dube,⁵²K. Ebina,⁵⁷J. Efron,³⁹J. Ehlers,¹⁹R. Erbacher,⁷D. Errede,²³S. Errede,²³R. Eusebi,¹⁶ H. C. Fang,²⁸S. Farrington,²⁹I. Fedorko,⁴⁶W. T. Fedorko,¹³R. G. Feild,⁶⁰M. Feindt,²⁵J. P. Fernandez,³¹R. Field,¹⁷ G. Flanagan,⁴⁸L. R. Flores-Castillo,⁴⁷A. Foland,²¹S. Forrester,⁷G. W. Foster,¹⁶M. Franklin,²¹J. C. Freeman,²⁸I. Furic,¹³

M. Gallinaro,⁵⁰J. Galyardt,¹²J. E. Garcia,⁴⁶M. Garcia Sciveres,²⁸A. F. Garfinkel,⁴⁸C. Gay,⁶⁰H. Gerberich,²³ D. Gerdes,³⁴S. Giagu,⁵¹P. Giannetti,⁴⁶A. Gibson,²⁸K. Gibson,¹²C. Ginsburg,¹⁶N. Giokaris,¹⁴K. Giolo,⁴⁸M. Giordani,⁵⁴ P. Giromini,¹⁸M. Giunta,⁴⁶G. Giurgiu,¹²V. Glagolev,¹⁴D. Glenzinski,¹⁶M. Gold,³⁷N. Goldschmidt,³⁴J. Goldstein,⁴²

G. Gomez,¹¹G. Gomez-Ceballos,¹¹M. Goncharov,⁵³O. Gonza´lez,³¹I. Gorelov,³⁷A. T. Goshaw,¹⁵Y. Gotra,⁴⁷ K. Goulianos,⁵⁰A. Gresele,⁴³M. Griffiths,²⁹S. Grinstein,²¹C. Grosso-Pilcher,¹³R. C. Group,¹⁷U. Grundler,²³ J. Guimaraes da Costa,²¹Z. Gunay-Unalan,³⁵C. Haber,²⁸S. R. Hahn,¹⁶K. Hahn,⁴⁵E. Halkiadakis,⁵²A. Hamilton,³³ B.-Y. Han,⁴⁹J. Y. Han,⁴⁹R. Handler,⁵⁹F. Happacher,¹⁸K. Hara,⁵⁵M. Hare,⁵⁶S. Harper,⁴²R. F. Harr,⁵⁸R. M. Harris,¹⁶

K. Hatakeyama,⁵⁰J. Hauser,⁸C. Hays,¹⁵A. Heijboer,⁴⁵B. Heinemann,²⁹J. Heinrich,⁴⁵M. Herndon,⁵⁹D. Hidas,¹⁵ C. S. Hill,¹⁰D. Hirschbuehl,²⁵A. Hocker,¹⁶A. Holloway,²¹S. Hou,¹M. Houlden,²⁹S.-C. Hsu,⁹B. T. Huffman,⁴² R. E. Hughes,³⁹J. Huston,³⁵J. Incandela,¹⁰G. Introzzi,⁴⁶M. Iori,⁵¹Y. Ishizawa,⁵⁵A. Ivanov,⁷B. Iyutin,³²E. James,¹⁶

D. Jang,⁵²B. Jayatilaka,³⁴D. Jeans,⁵¹H. Jensen,¹⁶E. J. Jeon,²⁷S. Jindariani,¹⁷M. Jones,⁴⁸K. K. Joo,²⁷S. Y. Jun,¹² T. R. Junk,²³T. Kamon,⁵³J. Kang,³⁴P. E. Karchin,⁵⁸Y. Kato,⁴¹Y. Kemp,²⁵R. Kephart,¹⁶U. Kerzel,²⁵V. Khotilovich,⁵³ B. Kilminster,³⁹D. H. Kim,²⁷H. S. Kim,²⁷J. E. Kim,²⁷M. J. Kim,¹²S. B. Kim,²⁷S. H. Kim,⁵⁵Y. K. Kim,¹³L. Kirsch,⁶

S. Klimenko,¹⁷M. Klute,³²B. Knuteson,³²B. R. Ko,¹⁵H. Kobayashi,⁵⁵K. Kondo,⁵⁷D. J. Kong,²⁷J. Konigsberg,¹⁷ A. Korytov,¹⁷A. V. Kotwal,¹⁵A. Kovalev,⁴⁵A. Kraan,⁴⁵J. Kraus,²³I. Kravchenko,³²M. Kreps,²⁵J. Kroll,⁴⁵ N. Krumnack,⁴M. Kruse,¹⁵V. Krutelyov,⁵³S. E. Kuhlmann,²Y. Kusakabe,⁵⁷S. Kwang,¹³A. T. Laasanen,⁴⁸S. Lai,³³

S. Lami,⁴⁶S. Lammel,¹⁶M. Lancaster,³⁰R. L. Lander,⁷K. Lannon,³⁹A. Lath,⁵²G. Latino,⁴⁶I. Lazzizzera,⁴³ T. LeCompte,²J. Lee,⁴⁹J. Lee,²⁷Y. J. Lee,²⁷S. W. Lee,⁵³R. Lefe`vre,³N. Leonardo,³²S. Leone,⁴⁶S. Levy,¹³J. D. Lewis,¹⁶ C. Lin,⁶⁰C. S. Lin,¹⁶M. Lindgren,¹⁶E. Lipeles,⁹T. M. Liss,²³A. Lister,¹⁹D. O. Litvintsev,¹⁶T. Liu,¹⁶N. S. Lockyer,⁴⁵ A. Loginov,³⁶M. Loreti,⁴³P. Loverre,⁵¹R.-S. Lu,¹D. Lucchesi,⁴³P. Lujan,²⁸P. Lukens,¹⁶G. Lungu,¹⁷L. Lyons,⁴²J. Lys,²⁸ R. Lysak,¹E. Lytken,⁴⁸P. Mack,²⁵D. MacQueen,³³R. Madrak,¹⁶K. Maeshima,¹⁶T. Maki,²²P. Maksimovic,²⁴S. Malde,⁴²

G. Manca,²⁹F. Margaroli,⁵R. Marginean,¹⁶C. Marino,²³A. Martin,⁶⁰V. Martin,³⁸M. Martı´nez,³T. Maruyama,⁵⁵ P. Mastrandrea,⁵¹H. Matsunaga,⁵⁵M. E. Mattson,⁵⁸R. Mazini,³³P. Mazzanti,⁵K. S. McFarland,⁴⁹P. McIntyre,⁵³ R. McNulty,²⁹A. Mehta,²⁹S. Menzemer,¹¹A. Menzione,⁴⁶P. Merkel,⁴⁸C. Mesropian,⁵⁰A. Messina,⁵¹M. von der Mey,⁸

T. Miao,¹⁶N. Miladinovic,⁶J. Miles,³²R. Miller,³⁵J. S. Miller,³⁴C. Mills,¹⁰M. Milnik,²⁵R. Miquel,²⁸A. Mitra,¹ G. Mitselmakher,¹⁷A. Miyamoto,²⁶N. Moggi,⁵B. Mohr,⁸R. Moore,¹⁶M. Morello,⁴⁶P. Movilla Fernandez,²⁸ J. Mu¨lmensta¨dt,²⁸A. Mukherjee,¹⁶Th. Muller,²⁵R. Mumford,²⁴P. Murat,¹⁶J. Nachtman,¹⁶J. Naganoma,⁵⁷S. Nahn,³²

I. Nakano,⁴⁰A. Napier,⁵⁶D. Naumov,³⁷V. Necula,¹⁷C. Neu,⁴⁵M. S. Neubauer,⁹J. Nielsen,²⁸T. Nigmanov,⁴⁷

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L. Nodulman,²O. Norniella,³E. Nurse,³⁰T. Ogawa,⁵⁷S. H. Oh,¹⁵Y. D. Oh,²⁷T. Okusawa,⁴¹R. Oldeman,²⁹R. Orava,²² K. Osterberg,²²C. Pagliarone,⁴⁶E. Palencia,¹¹R. Paoletti,⁴⁶V. Papadimitriou,¹⁶A. A. Paramonov,¹³B. Parks,³⁹

S. Pashapour,³³J. Patrick,¹⁶G. Pauletta,⁵⁴M. Paulini,¹²C. Paus,³²D. E. Pellett,⁷A. Penzo,⁵⁴T. J. Phillips,¹⁵ G. Piacentino,⁴⁶J. Piedra,⁴⁴L. Pinera,¹⁷K. Pitts,²³C. Plager,⁸L. Pondrom,⁵⁹X. Portell,³O. Poukhov,¹⁴N. Pounder,⁴²

F. Prakoshyn,¹⁴A. Pronko,¹⁶J. Proudfoot,²F. Ptohos,¹⁸G. Punzi,⁴⁶J. Pursley,²⁴J. Rademacker,⁴²A. Rahaman,⁴⁷ A. Rakitin,³²S. Rappoccio,²¹F. Ratnikov,⁵²B. Reisert,¹⁶V. Rekovic,³⁷N. van Remortel,²²P. Renton,⁴²M. Rescigno,⁵¹ S. Richter,²⁵F. Rimondi,⁵L. Ristori,⁴⁶W. J. Robertson,¹⁵A. Robson,²⁰T. Rodrigo,¹¹E. Rogers,²³S. Rolli,⁵⁶R. Roser,¹⁶

M. Rossi,⁵⁴R. Rossin,¹⁷C. Rott,⁴⁸A. Ruiz,¹¹J. Russ,¹²V. Rusu,¹³H. Saarikko,²²S. Sabik,³³A. Safonov,⁵³ W. K. Sakumoto,⁴⁹G. Salamanna,⁵¹O. Salto´,³D. Saltzberg,⁸C. Sanchez,³L. Santi,⁵⁴S. Sarkar,⁵¹L. Sartori,⁴⁶K. Sato,⁵⁵

P. Savard,³³A. Savoy-Navarro,⁴⁴T. Scheidle,²⁵P. Schlabach,¹⁶E. E. Schmidt,¹⁶M. P. Schmidt,⁶⁰M. Schmitt,³⁸ T. Schwarz,³⁴L. Scodellaro,¹¹A. L. Scott,¹⁰A. Scribano,⁴⁶F. Scuri,⁴⁶A. Sedov,⁴⁸S. Seidel,³⁷Y. Seiya,⁴¹A. Semenov,¹⁴

L. Sexton-Kennedy,¹⁶I. Sfiligoi,¹⁸M. D. Shapiro,²⁸T. Shears,²⁹P. F. Shepard,⁴⁷D. Sherman,²¹M. Shimojima,⁵⁵ M. Shochet,¹³Y. Shon,⁵⁹I. Shreyber,³⁶A. Sidoti,⁴⁴P. Sinervo,³³A. Sisakyan,¹⁴J. Sjolin,⁴²A. Skiba,²⁵A. J. Slaughter,¹⁶ K. Sliwa,⁵⁶J. R. Smith,⁷F. D. Snider,¹⁶R. Snihur,³³M. Soderberg,³⁴A. Soha,⁷S. Somalwar,⁵²V. Sorin,³⁵J. Spalding,¹⁶ M. Spezziga,¹⁶F. Spinella,⁴⁶T. Spreitzer,³³P. Squillacioti,⁴⁶M. Stanitzki,⁶⁰A. Staveris-Polykalas,⁴⁶R. St. Denis,²⁰

B. Stelzer,⁸O. Stelzer-Chilton,⁴²D. Stentz,³⁸J. Strologas,³⁷D. Stuart,¹⁰J. S. Suh,²⁷A. Sukhanov,¹⁷K. Sumorok,³² H. Sun,⁵⁶T. Suzuki,⁵⁵A. Taffard,²³R. Takashima,⁴⁰Y. Takeuchi,⁵⁵K. Takikawa,⁵⁵M. Tanaka,²R. Tanaka,⁴⁰ N. Tanimoto,⁴⁰M. Tecchio,³⁴P. K. Teng,¹K. Terashi,⁵⁰S. Tether,³²J. Thom,¹⁶A. S. Thompson,²⁰E. Thomson,⁴⁵

P. Tipton,⁴⁹V. Tiwari,¹²S. Tkaczyk,¹⁶D. Toback,⁵³S. Tokar,¹⁴K. Tollefson,³⁵T. Tomura,⁵⁵D. Tonelli,⁴⁶ M. To¨nnesmann,³⁵S. Torre,¹⁸D. Torretta,¹⁶S. Tourneur,⁴⁴W. Trischuk,³³R. Tsuchiya,⁵⁷S. Tsuno,⁴⁰N. Turini,⁴⁶ F. Ukegawa,⁵⁵T. Unverhau,²⁰S. Uozumi,⁵⁵D. Usynin,⁴⁵A. Vaiciulis,⁴⁹S. Vallecorsa,¹⁹A. Varganov,³⁴E. Vataga,³⁷

G. Velev,¹⁶G. Veramendi,²³V. Veszpremi,⁴⁸R. Vidal,¹⁶I. Vila,¹¹R. Vilar,¹¹T. Vine,³⁰I. Vollrath,³³I. Volobouev,²⁸ G. Volpi,⁴⁶F. Wu¨rthwein,⁹P. Wagner,⁵³R. G. Wagner,²R. L. Wagner,¹⁶W. Wagner,²⁵R. Wallny,⁸T. Walter,²⁵Z. Wan,⁵²

S. M. Wang,¹A. Warburton,³³S. Waschke,²⁰D. Waters,³⁰W. C. Wester III,¹⁶B. Whitehouse,⁵⁶D. Whiteson,⁴⁵ A. B. Wicklund,²E. Wicklund,¹⁶G. Williams,³³H. H. Williams,⁴⁵P. Wilson,¹⁶B. L. Winer,³⁹P. Wittich,¹⁶S. Wolbers,¹⁶ C. Wolfe,¹³T. Wright,³⁴X. Wu,¹⁹S. M. Wynne,²⁹A. Yagil,¹⁶K. Yamamoto,⁴¹J. Yamaoka,⁵²T. Yamashita,⁴⁰C. Yang,⁶⁰ U. K. Yang,¹³Y. C. Yang,²⁷W. M. Yao,²⁸G. P. Yeh,¹⁶J. Yoh,¹⁶K. Yorita,¹³T. Yoshida,⁴¹G. B. Yu,⁴⁹I. Yu,²⁷S. S. Yu,¹⁶

J. C. Yun,¹⁶L. Zanello,⁵¹A. Zanetti,⁵⁴I. Zaw,²¹F. Zetti,⁴⁶X. Zhang,²³J. Zhou,⁵²and S. Zucchelli⁵ (CDF Collaboration)

1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China

2Argonne National Laboratory, Argonne, Illinois 60439, USA

3Institut de Fisica d’Altes Energies, Universitat Autonoma de Barcelona, E-08193 Bellaterra (Barcelona), Spain

4Baylor University, Waco, Texas 76798, USA

5Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy

6Brandeis University, Waltham, Massachusetts 02254, USA

7University of California, Davis, Davis, California 95616, USA

8University of California, Los Angeles, Los Angeles, California 90024, USA

9University of California, San Diego, La Jolla, California 92093, USA

10University of California, Santa Barbara, Santa Barbara, California 93106, USA

11Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain

12Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA

13Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA

14Joint Institute for Nuclear Research, RU-141980 Dubna, Russia

15Duke University, Durham, North Carolina 27708, USA

16Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA

17University of Florida, Gainesville, Florida 32611, USA

18Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy

19University of Geneva, CH-1211 Geneva 4, Switzerland

20Glasgow University, Glasgow G12 8QQ, United Kingdom

21Harvard University, Cambridge, Massachusetts 02138, USA

22Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014 Helsinki, Finland

23University of Illinois, Urbana, Illinois 61801, USA

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24The Johns Hopkins University, Baltimore, Maryland 21218, USA

25Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany

26High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305, Japan

27Center for High Energy Physics: Kyungpook National University, Taegu 702-701, Korea;

Seoul National University, Seoul 151-742, Korea;

and SungKyunKwan University, Suwon 440-746, Korea

28Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

29University of Liverpool, Liverpool L69 7ZE, United Kingdom

30University College London, London WC1E 6BT, United Kingdom

31Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain

32Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

33Institute of Particle Physics: McGill University, Montre´al, Canada H3A 2T8;

and University of Toronto, Toronto, Canada M5S 1A7

34University of Michigan, Ann Arbor, Michigan 48109, USA

35Michigan State University, East Lansing, Michigan 48824, USA

36Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia

37University of New Mexico, Albuquerque, New Mexico 87131, USA

38Northwestern University, Evanston, Illinois 60208, USA

39The Ohio State University, Columbus, Ohio 43210, USA

40Okayama University, Okayama 700-8530, Japan

41Osaka City University, Osaka 588, Japan

42University of Oxford, Oxford OX1 3RH, United Kingdom

43Istituto Nazionale di Fisica Nucleare, University of Padova, Sezione di Padova-Trento, I-35131 Padova, Italy

44LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris F-75252, France

45University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA

46Istituto Nazionale di Fisica Nucleare Pisa, Universities of Pisa, Siena and Scuola Normale Superiore, I-56127 Pisa, Italy

47University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA

48Purdue University, West Lafayette, Indiana 47907, USA

49University of Rochester, Rochester, New York 14627, USA

50The Rockefeller University, New York, New York 10021, USA

51Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, University of Rome ‘‘La Sapienza,’’ I-00185 Roma, Italy

52Rutgers University, Piscataway, New Jersey 08855, USA

53Texas A&M University, College Station, Texas 77843, USA

54Istituto Nazionale di Fisica Nucleare, University of Trieste/Udine, Italy

55University of Tsukuba, Tsukuba, Ibaraki 305, Japan

56Tufts University, Medford, Massachusetts 02155, USA

57Waseda University, Tokyo 169, Japan

58Wayne State University, Detroit, Michigan 48201, USA

59University of Wisconsin, Madison, Wisconsin 53706, USA

60Yale University, New Haven, Connecticut 06520, USA (Received 31 May 2006; published 25 August 2006)

We present the results of a search for standard model Higgs boson production with decay toWW, identified through the leptonic final statesee; e and. This search uses 360 pb¹of data collected from pp collisions at

ps

1:96 TeV by the upgraded Collider Detector at Fermilab (CDF II). We observe no signal excess and set 95% confidence level upper limits on the production cross section times branching ratio for the Higgs boson toWWor any new scalar particle with similar decay products. These upper limits range from 5.5 to 3.2 pb for Higgs boson masses between 120 and 200 GeV=c².

DOI:10.1103/PhysRevLett.97.081802 PACS numbers: 14.80.Bn, 13.85.Ni, 13.85.Qk, 13.85.Rm

The Higgs mechanism is a leading candidate for electroweak symmetry breaking and consequently for mass generation of the W and Z bosons without violation of local gauge invariance. A manifestation of this mechanism is the existence of a neutral scalar particle, the Higgs boson [1], which has not been observed to date.

Its mass is a free parameter in the standard model (SM),

but its couplings to other particles of known mass are fully specified at tree level. Direct searches at the CERN ee collider (LEP) yielded a lower limit for the Higgs boson mass ofm_H>114:4 GeV=c² at 95% confidence level (C.L.) [2]. Precision electroweak measure- ments indirectly predict a Higgs boson mass of 91⁴⁵₃₂ GeV=c² [3].

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At the Tevatron, the dominant production mechanism for the SM Higgs boson is gluon-gluon fusion through heavy quark loops. Branching fractions for the various decay channels of the Higgs boson depend on its mass.

For masses below about135 GeV=c² the dominant decay is H!bb, while heavier Higgs bosons decay predomi- nantly toWW[4], whereWindicates aWboson that can be off mass shell. For thebbdecay mode, the requirement of associated production of the Higgs boson with vector bosons (pp !WH=ZH) can greatly improve the signal purity [5]. For theWW decay mode, the leptonic decays ofW bosons give a clean enough signature that the inclusive single Higgs production process gives the best search sensitivity. The next-to-leading order (NLO) production cross section [4] times branching ratio for a SM Higgs boson,pp !H BRH!WW, ranges from 0.036 to 0.25 pb for Higgs masses of110–200 GeV=c².

This Letter presents the results of a direct search for a Higgs boson in the channelgg!H!WW !‘‘ (‘e; ; ), identified by the ‘‘dilepton’’ final states ee, e, or . We also include the efficiency for leptonically decaying taus toeor . This is the first search in this channel by the CDF Collaboration. A similar search in this channel was recently performed by the D0 Collaboration [6]. The data sample used for this analysis were collected with the CDF II detector at the Fermilab Tevatron between 2002 and 2004, and corresponds to an integrated luminosity of approximately360 pb¹ [7]. For this integrated luminosity, the cross-section limits we are able to place on Higgs production are a factor of approximately 10 –50 larger than the SM expectation, based on the NLO calculation. However, the production cross section can be enhanced in extensions to the SM due to new particles, e.g., a fourth generation fermion family [8], contributing at higher order to the gluon-gluon fusion Higgs production process.

CDF II is a detector with approximate azimuthal and forward-backward symmetry and it is fully described else- where [9]. It consists of a charged-particle tracking system in a 1.4 T magnetic field and segmented electromagnetic and hadronic calorimeters surrounded by muon detectors.

The electromagnetic and hadronic sampling calorimeters surrounding the solenoid are used to measure the energy of interacting particles in the pseudorapidity rangejj<3:6 [10]. The calorimeters are divided into projective geometry towers. This analysis uses both central (jj<1:1) and end- plug detectors (1:2<jj<2:0) to identify electron candidates. A set of drift chambers located outside the central hadron calorimeters and another set behind a 60 cm iron shield help detect muons in the region jj<0:6. Addi- tional drift chambers and scintillation counters detect muons in the region0:6 jj 1:0.

Events used for this analysis are collected using the following triggers [11,12]: an inclusive central electron (jj<1:1) trigger requiring an electron with E_T >

18 GeV, an inclusive central muon (jj<1:0) trigger requiring a muon with p_T >18 GeV=c, or a trigger for events with a forward electron (1:2 jj 2:0) with E_T>20 GeV and missing transverse energy, 6E_T>

15 GeV[13].

After the event reconstruction, event selection criteria which retain highH!WWsignal efficiency while min- imizing the effect of background contamination are applied. Some selection requirements are mass dependent, as the event kinematics and topology change as functions ofm_H.

The selection requires two oppositely charged lepton candidates originating from the same vertex, with p_T>

20 GeV=cfor the trigger lepton and p_T>10 GeV=cfor the second one. The leptons are also required to be isolated in both the calorimeter and the tracking chamber [14], and the dilepton invariant mass m_‘‘ is required to be greater than16 GeV=c², in order to remove events from thecc=bb resonances.

After removal of events identified as cosmic rays or electrons from photon conversions [11], we count the jets [15] with E_T>15 GeV andjj<2:5. Signal events do not typically have high-E_T jets in the final state, but can occasionally have lower-E_T jets from initial state gluon radiation. On the other hand, tt pairs decay primarily to WWbband thus tend to have at least two jets in the final state. This background is reduced by selecting only events satisfying one of the following criteria: no jets withE_T>

15 GeV, or only one jet with15< E_T<55 GeV, or two jets each with15< E_T <40 GeV. Events with more than two jets withE_T>15 GeVare also rejected.

After the selection criteria described above, the dominant surviving background is Drell-Yan production of

‘‘ pairs, which is suppressed by requiring that 6E_T>

m_H=4. The events with missing energy due to a mismea- surement of the jet energy, orZ!events with missing energy arising from a leptonic tau decay, are removed by requiring the azimuthal angle between the 6E_T and the closest jet or lepton to be at least 20, if 6E_T<50 GeV.

To further reduce the largeZ= background, the dilepton invariant mass is required to be m_‘‘<m_H=2 5 GeV=c². Finally, the scalar sum of the p_T of the two leptons and the6E_Tis required to be below the Higgs boson mass.

The kinematic cuts described above exploit the correla- tions in the W pairs produced by the decay of a Higgs boson and suppress SM WW production. These correla- tions are due to angular momentum conservation in the decay of a spin-zero Higgs boson. Since W bosons decay into left-handed leptons and right-handed antileptons, and since theWbosons in the decayH!WWhave opposite helicities, the final state lepton pairs and also the neutrino pairs tend to be azimuthally aligned in Higgs decay. This implies that the signal events tend to have smallerm_‘‘and azimuthal angle between leptons () and larger 6E_T, as compared with production of SMWWpairs. These differ-

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ences are further exploited in the final stages of the analysis, when thedistribution of the data is compared with the background and signal predictions.

The acceptance for identifying H!WW !‘‘

events with the above selection criteria is calculated as a function of the Higgs boson mass using the PYTHIA[16]

Monte Carlo program, after aGEANT-based [17] simulation of the CDF detector response. The total acceptance is a product of the geometric and kinematic acceptance, the lepton identification efficiencies, the trigger efficiencies, and the topological cut efficiencies. It does not include the branching fraction ofW leptonic decays. The total acceptance ranges from 3.0% to 6.5%, depending on the Higgs mass, and is summarized in TableI. Approximately 25% of the expected signal areeeevents, 25%, and 50%e.

The systematic uncertainty on the acceptance is 6%

resulting from uncertainties in the modeling of the initial state radiation byPYTHIA (3%), and uncertainties on the gluon parton distribution functions (4%) [18], jet energy scale (1%), track isolation (<2%), electron and muon trigger efficiencies (<1%), and electron and muon identification efficiencies (2%). In addition, a 6% uncertainty on the integrated luminosity is applied to the expected number of events for all processes [19].

After all selection requirements, the background events come predominantly from WW pair production (about 70% of the total form_H 160 GeV=c²) [20],Z=,W jets, and W. Smaller backgrounds include WZ, ZZ, andttproduction. A summary of these contributions as a function of Higgs mass is given in TableII. The diboson (WW,WZ,ZZ),Z=, andttbackgrounds are determined using thePYTHIA Monte Carlo program, followed by the

CDF II detector simulation. We normalize the total number of events for these processes to recent theoretical cross sections [21,22]. To estimate the W background we use a matrix element generator [23] and usePYTHIAfor the initial state QCD radiation and hadronization.

The background from Wjets, where a jet or track is misidentified as a lepton (electron or muon), is determined from the data and called the ‘‘fake background.’’ We first determine the probability that a jet with a large fraction of its energy deposited in the electromagnetic calorimeter is

TABLE II. The expected number of signal and SM background events are presented. The number of events observed in the data, with them_Hdependent selection criteria, is also shown. The errors include all systematic effects.

mH(GeV=c²) 120 140 160 180 200

WW 5:490:66 7:980:96 9:791:18 9:891:19 9:191:11

Z= 1:630:42 1:010:26 0:760:20 0:830:21 0:960:25

Wjets= 4:570:90 3:490:81 2:480:69 1:700:46 1:200:37

WZZZ 0:250:03 0:370:05 0:400:05 0:490:07 1:160:15

tt 0:120:01 0:210:02 0:350:04 0:460:05 0:580:06

Total background 12:061:19 13:081:28 13:781:38 13:371:30 13:091:21

H!WW 0:0900:008 0:320:03 0:580:05 0:410:03 0:200:02

Data 7 14 16 19 17

TABLE I. The branching ratioBRH!WW and the total acceptance of the signal after all the selection criteria. The total acceptance is calculated with respect to the number ofpp! H!WW!‘‘ events.

m_H (GeV=c²) 120 140 160 180 200

BRH!WW(%) 13 48 90 94 74

Total acceptance (%) 3.15 4.56 6.47 6.41 5.54

°) Dilepton Azimuthal Separation ( 0 20 40 60 80 100 120 140 160 180

°Events / 22.5

0 1 2 3 4 5 6 7 8

°) Dilepton Azimuthal Separation ( 0 20 40 60 80 100 120 140 160 180

°Events / 22.5

0 1 2 3 4 5 6 7

8 Data

→ll DY/Z

γ W+jet/

WW t WZ+ZZ+t

2)

=140 GeV/c )(mH

WW*

→ 10x(H

°) Dilepton Azimuthal Separation (

0 20 40 60 80 100 120 140 160 180

°Events / 22.5

0 1 2 3 4 5 6 7 8

°) Dilepton Azimuthal Separation (

0 20 40 60 80 100 120 140 160 180

°Events / 22.5

0 1 2 3 4 5 6 7

8 Data

→ll DY/Z

γ W+jet/

WW t WZ+ZZ+t

2)

=160 GeV/c ) (mH

WW*

→ 10x(H

FIG. 1. Dilepton azimuthal distributions for SM backgrounds, HWWsignal, and data, for two Higgs masses:140 GeV=c²(top panel) and 160 GeV=c² (bottom panel). Note that the Higgs signal is scaled by a factor of 10 and is not included in the cumulative background distributions.

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misidentified as an electron, and the probability that a minimum ionizing track is misidentified as a muon.

These probabilities are termed fake rates. The fake rate for each lepton type is calculated using an average of four inclusive jet samples (triggered with at least one jet with E_T>20, 50, 70, or 100 GeV). We subtract the contribution from sources of real leptons (W and Z decays) and pa- rametrize the fake rates as a function of jet transverse energy (for electrons) or track transverse momentum (for muons). The background is determined by weighting the jets from a data sample ofW !‘ jets events by the fake rates.

For data events passing the previously described selection criteria, we search for an excess of events in the azimuthal angle distribution between the leptons,. A binned likelihood is used to compare the azimuthal angle distribution in the data with a combination of expected distributions from the SM background processes. Figure1 shows thedistributions for SM backgrounds, for Higgs masses of 140 and 160 GeV=c², and for the data. We observe no evidence for a signal over the SM expectations.

We calculate upper limits on the production cross section times branching ratio, _H BRH!WW, using a Bayesian procedure. We consider three components in the data: H!WW, SM WW, and other SM processes (WZ,ZZ, Drell-Yan,Wjets=) labeled as ‘‘other.’’ The expected number of events in eachbin is

f_WWn_WW f_othern_other

f_HWWL_HBRH!WW;

wheref_WW,f_other, andf_HWW represent the expected fraction of the specified categories of events falling in each bin,n_WW andn_otherare the expected numbers ofWW and non-WWbackground events, and,L, and_Hcorrespond to efficiency, integrated luminosity, andHproduction cross section. A posterior density is obtained by multiplying the Poisson likelihood function with Gaussian prior densities for the integrated luminosity, background normalizations, and the signal efficiency:

L^NY^bins

i1

ⁿ_iⁱeⁱ

n_i! Gn_WW; _WWGn_other; _other G; GL; _L;

wheren_iis the number of events observed in the data, and Gn; _n are Gaussian constraints for parameter n with

uncertainty _n. The prior density for BRH! WW is assumed uniform. The posterior density is then integrated over all parameters except for BRH! WW, for which a 95% C.L. upper limit is obtained by calculating the 95th percentile of the resulting distribution.

The expected and observed upper limits on the cross section times branching ratio, BRH!WW, for different Higgs masses are shown in Table III. The expected limits are calculated using 1000 simulated experiments, assuming no signal, for each Higgs mass. The median value of the limits obtained from these experiments is chosen as thea prioriupper limit.

In conclusion, observing no signal in the direct search forH!WW, with the subsequent decay of theWbosons to leptons, we have set mass dependent limits at 95% C.L.

on pp !H BRH!WW. This search is poten- tially sensitive to other new physics models such as the example in Fig.2.

We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contributions.

This work was supported by the U.S. Department of Energy and National Science Foundation; the Italian Istituto Nazionale di Fisica Nucleare; the Ministry of Education, Culture, Sports, Science and Technology of Japan; the Natural Sciences and Engineering Research Council of Canada; the National Science Council of the Republic of China; the Swiss National Science Founda- tion; the A. P. Sloan Foundation; the Bundesministerium fu¨r Bildung und Forschung, Germany; the Korean Science and Engineering Foundation and the Korean Research Foundation; the Particle Physics and Astronomy Research Council and the Royal Society, U.K.; the Russian Foundation for Basic Research; the Comisio´n Interministerial de Ciencia y Tecnologı´a, Spain; in part by the European Community’s Human Potential TABLE III. The expected and observed 95% C.L. limits on

pp!H BRH!WW.

mH(GeV=c²) 120 140 160 180 200

Expected limits (pb) 7.1 4.8 3.5 3.4 4.0 Observed limits (pb) 4.5 4.6 3.2 4.3 5.2

110 120 130 140 150 160 170 180 190 200 10-1

1 10 102

2) Higgs Mass (GeV/c 110 120 130 140 150 160 170 180 190 200

) (pb)* WW→ x BR(HHσ95% C.L. ^-110

1 10 102

Standard Model 4th Generation Model

Expected Limit 95% C.L. Limit

Excluded LEP

FIG. 2 (color online). Summary of the run II CDF 95% C.L.

upper limits onpp!H BRH!WW. Shown for com- parison are the standard model prediction, the fourth generation model prediction [8], and the region excluded by the LEP experiments. The prediction for the fourth generation model assumes that fourth family fermions have a mass m₄ 200 GeV=c².

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Programme under Contract No. HPRN-CT-2002-00292;

and the Academy of Finland.

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