Article
Reference
Search for Resonant tt Production in pp Collisions at s√=1.96 TeV
CDF Collaboration
CLARK, Allan Geoffrey (Collab.), et al.
Abstract
We report on a search for narrow-width particles decaying to a top and antitop quark pair. The data set used in the analysis corresponds to an integrated luminosity of 680 pb−1 collected with the Collider Detector at Fermilab in run II. We present 95% confidence level upper limits on the cross section times branching ratio. Assuming a specific top-color-assisted technicolor production model, the leptophobic Z′ with width ΓZ′=0.012MZ′, we exclude the mass range MZ′
CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . Search for Resonant tt Production in pp Collisions at s√=1.96 TeV. Physical Review Letters , 2008, vol. 100, no. 23, p. 231801
DOI : 10.1103/PhysRevLett.100.231801
Available at:
http://archive-ouverte.unige.ch/unige:38501
Disclaimer: layout of this document may differ from the published version.
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Search for Resonant t t Production in p p Collisions at p s
1:96 TeV
T. Aaltonen,23A. Abulencia,24J. Adelman,13T. Akimoto,54M. G. Albrow,17B. A´ lvarez Gonza´lez,11S. Amerio,42 D. Amidei,34A. Anastassov,51A. Annovi,19J. Antos,14G. Apollinari,17A. Apresyan,47T. Arisawa,56A. Artikov,15 W. Ashmanskas,17A. Attal,3A. Aurisano,52F. Azfar,41P. Azzi-Bacchetta,42P. Azzurri,45N. Bacchetta,42W. Badgett,17
A. Barbaro-Galtieri,28V. E. Barnes,47B. A. Barnett,25S. Baroiant,7V. Bartsch,30G. Bauer,32P.-H. Beauchemin,33 F. Bedeschi,45P. Bednar,14S. Behari,25G. Bellettini,45J. Bellinger,58A. Belloni,32D. Benjamin,16A. Beretvas,17 J. Beringer,28T. Berry,29A. Bhatti,49M. Binkley,17D. Bisello,42I. Bizjak,30R. E. Blair,2C. Blocker,6B. Blumenfeld,25 A. Bocci,16A. Bodek,48V. Boisvert,48G. Bolla,47A. Bolshov,32D. Bortoletto,47J. Boudreau,46A. Boveia,10B. Brau,10
L. Brigliadori,5C. Bromberg,35E. Brubaker,13J. Budagov,15H. S. Budd,48S. Budd,24K. Burkett,17G. Busetto,42 P. Bussey,21A. Buzatu,33K. L. Byrum,2S. Cabrera,16,sM. Campanelli,35M. Campbell,34F. Canelli,17A. Canepa,44 D. Carlsmith,58R. Carosi,45S. Carrillo,18,mS. Carron,33B. Casal,11M. Casarsa,17A. Castro,5P. Catastini,45D. Cauz,53 M. Cavalli-Sforza,3A. Cerri,28L. Cerrito,30,qS. H. Chang,27Y. C. Chen,1M. Chertok,7G. Chiarelli,45G. Chlachidze,17 F. Chlebana,17K. Cho,27D. Chokheli,15J. P. Chou,22G. Choudalakis,32S. H. Chuang,51K. Chung,12W. H. Chung,58 Y. S. Chung,48C. I. Ciobanu,24M. A. Ciocci,45A. Clark,20D. Clark,6G. Compostella,42M. E. Convery,17J. Conway,7 B. Cooper,30K. Copic,34M. Cordelli,19G. Cortiana,42F. Crescioli,45C. Cuenca Almenar,7,sJ. Cuevas,11,pR. Culbertson,17 J. C. Cully,34D. Dagenhart,17M. Datta,17T. Davies,21P. de Barbaro,48S. De Cecco,50A. Deisher,28G. De Lentdecker,48,e
G. De Lorenzo,3M. Dell’Orso,45L. Demortier,49J. Deng,16M. Deninno,5D. De Pedis,50P. F. Derwent,17 G. P. Di Giovanni,43C. Dionisi,50B. Di Ruzza,53J. R. Dittmann,4M. D’Onofrio,3S. Donati,45P. Dong,8J. Donini,42 T. Dorigo,42S. Dube,51J. Efron,38R. Erbacher,7D. Errede,24S. Errede,24R. Eusebi,17H. C. Fang,28S. Farrington,29 W. T. Fedorko,13R. G. Feild,59M. Feindt,26J. P. Fernandez,31C. Ferrazza,45R. Field,18G. Flanagan,47R. Forrest,7 S. Forrester,7M. Franklin,22J. C. Freeman,28I. Furic,18M. Gallinaro,49J. Galyardt,12F. Garberson,10J. E. Garcia,45 A. F. Garfinkel,47H. Gerberich,24D. Gerdes,34S. Giagu,50P. Giannetti,45K. Gibson,46J. L. Gimmell,48C. Ginsburg,17 N. Giokaris,15,bM. Giordani,53P. Giromini,19M. Giunta,45V. Glagolev,15D. Glenzinski,17M. Gold,36N. Goldschmidt,18 J. Goldstein,41,dA. Golossanov,17G. Gomez,11G. Gomez-Ceballos,32M. Goncharov,52O. Gonza´lez,31I. Gorelov,36
A. T. Goshaw,16K. Goulianos,49A. Gresele,42S. Grinstein,22C. Grosso-Pilcher,13R. C. Group,17U. Grundler,24 J. Guimaraes da Costa,22Z. Gunay-Unalan,35C. Haber,28K. Hahn,32S. R. Hahn,17E. Halkiadakis,51A. Hamilton,20
B.-Y. Han,48J. Y. Han,48R. Handler,58F. Happacher,19K. Hara,54D. Hare,51M. Hare,55S. Harper,41R. F. Harr,57 R. M. Harris,17M. Hartz,46K. Hatakeyama,49J. Hauser,8C. Hays,41M. Heck,26A. Heijboer,44B. Heinemann,28 J. Heinrich,44C. Henderson,32M. Herndon,58J. Heuser,26S. Hewamanage,4D. Hidas,16C. S. Hill,10,dD. Hirschbuehl,26
A. Hocker,17S. Hou,1M. Houlden,29S.-C. Hsu,9B. T. Huffman,41R. E. Hughes,38U. Husemann,59J. Huston,35 J. Incandela,10G. Introzzi,45M. Iori,50A. Ivanov,7B. Iyutin,32E. James,17B. Jayatilaka,16D. Jeans,50E. J. Jeon,27
S. Jindariani,18W. Johnson,7M. Jones,47K. K. Joo,27S. Y. Jun,12J. E. Jung,27T. R. Junk,24T. Kamon,52D. Kar,18 P. E. Karchin,57Y. Kato,40R. Kephart,17U. Kerzel,26V. Khotilovich,52B. Kilminster,38D. H. Kim,27H. S. Kim,27 J. E. Kim,27M. J. Kim,17S. B. Kim,27S. H. Kim,54Y. K. Kim,13N. Kimura,54L. Kirsch,6S. Klimenko,18M. Klute,32
B. Knuteson,32B. R. Ko,16S. A. Koay,10K. Kondo,56D. J. Kong,27J. Konigsberg,18A. Korytov,18A. V. Kotwal,16 J. Kraus,24M. Kreps,26J. Kroll,44N. Krumnack,4M. Kruse,16V. Krutelyov,10T. Kubo,54S. E. Kuhlmann,2T. Kuhr,26
N. P. Kulkarni,57Y. Kusakabe,56S. Kwang,13A. T. Laasanen,47S. Lai,33S. Lami,45S. Lammel,17M. Lancaster,30 R. L. Lander,7K. Lannon,38A. Lath,51G. Latino,45I. Lazzizzera,42T. LeCompte,2J. Lee,48J. Lee,27Y. J. Lee,27 S. W. Lee,52,rR. Lefe`vre,20N. Leonardo,32S. Leone,45S. Levy,13J. D. Lewis,17C. Lin,59C. S. Lin,17M. Lindgren,17 E. Lipeles,9A. Lister,7D. O. Litvintsev,17T. Liu,17N. S. Lockyer,44A. Loginov,59M. Loreti,42L. Lovas,14R.-S. Lu,1 D. Lucchesi,42J. Lueck,26C. Luci,50P. Lujan,28P. Lukens,17G. Lungu,18L. Lyons,41J. Lys,28R. Lysak,14E. Lytken,47
P. Mack,26D. MacQueen,33R. Madrak,17K. Maeshima,17K. Makhoul,32T. Maki,23P. Maksimovic,25S. Malde,41 S. Malik,30G. Manca,29A. Manousakis,15,bF. Margaroli,47C. Marino,26C. P. Marino,24A. Martin,59M. Martin,25 V. Martin,21,kM. Martı´nez,3R. Martı´nez-Balları´n,31T. Maruyama,54P. Mastrandrea,50T. Masubuchi,54M. E. Mattson,57 P. Mazzanti,5K. S. McFarland,48P. McIntyre,52R. McNulty,29,jA. Mehta,29P. Mehtala,23S. Menzemer,11,lA. Menzione,45 P. Merkel,47C. Mesropian,49A. Messina,35T. Miao,17N. Miladinovic,6J. Miles,32R. Miller,35C. Mills,22M. Milnik,26
A. Mitra,1G. Mitselmakher,18H. Miyake,54S. Moed,20N. Moggi,5C. S. Moon,27R. Moore,17M. Morello,45 P. Movilla Fernandez,28J. Mu¨lmensta¨dt,28A. Mukherjee,17Th. Muller,26R. Mumford,25P. Murat,17M. Mussini,5 J. Nachtman,17Y. Nagai,54A. Nagano,54J. Naganoma,56K. Nakamura,54I. Nakano,39A. Napier,55V. Necula,16C. Neu,44
M. S. Neubauer,24J. Nielsen,28,gL. Nodulman,2M. Norman,9O. Norniella,24E. Nurse,30S. H. Oh,16Y. D. Oh,27 I. Oksuzian,18T. Okusawa,40R. Oldeman,29R. Orava,23K. Osterberg,23S. Pagan Griso,42C. Pagliarone,45E. Palencia,17
V. Papadimitriou,17A. Papaikonomou,26A. A. Paramonov,13B. Parks,38S. Pashapour,33J. Patrick,17G. Pauletta,53 M. Paulini,12C. Paus,32D. E. Pellett,7A. Penzo,53T. J. Phillips,16G. Piacentino,45J. Piedra,43L. Pinera,18K. Pitts,24
C. Plager,8L. Pondrom,58X. Portell,3O. Poukhov,15N. Pounder,41F. Prakoshyn,15A. Pronko,17J. Proudfoot,2 F. Ptohos,17,iG. Punzi,45J. Pursley,58J. Rademacker,41,dA. Rahaman,46V. Ramakrishnan,58N. Ranjan,47I. Redondo,31 B. Reisert,17V. Rekovic,36P. Renton,41M. Rescigno,50S. Richter,26F. Rimondi,5L. Ristori,45A. Robson,21T. Rodrigo,11 E. Rogers,24S. Rolli,55R. Roser,17M. Rossi,53R. Rossin,10P. Roy,33A. Ruiz,11J. Russ,12V. Rusu,17H. Saarikko,23 A. Safonov,52W. K. Sakumoto,48G. Salamanna,50O. Salto´,3L. Santi,53S. Sarkar,50L. Sartori,45K. Sato,17P. Savard,33
A. Savoy-Navarro,43T. Scheidle,26P. Schlabach,17E. E. Schmidt,17M. P. Schmidt,59M. Schmitt,37T. Schwarz,7 L. Scodellaro,11A. L. Scott,10A. Scribano,45F. Scuri,45A. Sedov,47S. Seidel,36Y. Seiya,40A. Semenov,15 L. Sexton-Kennedy,17A. Sfyrla,20S. Z. Shalhout,57M. D. Shapiro,28T. Shears,29P. F. Shepard,46D. Sherman,22 M. Shimojima,54,oM. Shochet,13Y. Shon,58I. Shreyber,20A. Sidoti,45P. Sinervo,33A. Sisakyan,15A. J. Slaughter,17 J. Slaunwhite,38K. Sliwa,55J. R. Smith,7F. D. Snider,17R. Snihur,33M. Soderberg,34A. Soha,7S. Somalwar,51V. Sorin,35 J. Spalding,17F. Spinella,45T. Spreitzer,33P. Squillacioti,45M. Stanitzki,59R. St. Denis,21B. Stelzer,8O. Stelzer-Chilton,41
D. Stentz,37J. Strologas,36D. Stuart,10J. S. Suh,27A. Sukhanov,18H. Sun,55I. Suslov,15T. Suzuki,54A. Taffard,24,f R. Takashima,39Y. Takeuchi,54R. Tanaka,39M. Tecchio,34P. K. Teng,1K. Terashi,49J. Thom,17,hA. S. Thompson,21
G. A. Thompson,24E. Thomson,44P. Tipton,59V. Tiwari,12S. Tkaczyk,17D. Toback,52S. Tokar,14K. Tollefson,35 T. Tomura,54D. Tonelli,17S. Torre,19D. Torretta,17S. Tourneur,43W. Trischuk,33Y. Tu,44N. Turini,45F. Ukegawa,54 S. Uozumi,54S. Vallecorsa,20N. van Remortel,23A. Varganov,34E. Vataga,36F. Va´zquez,18,mG. Velev,17C. Vellidis,45,b
V. Veszpremi,47M. Vidal,31R. Vidal,17I. Vila,11R. Vilar,11T. Vine,30M. Vogel,36I. Volobouev,28,rG. Volpi,45 F. Wu¨rthwein,9P. Wagner,44R. G. Wagner,2R. L. Wagner,17J. Wagner,26W. Wagner,26R. Wallny,8S. M. Wang,1 A. Warburton,33D. Waters,30M. Weinberger,52W. C. Wester III,17B. Whitehouse,55D. Whiteson,44,fA. B. Wicklund,2
E. Wicklund,17G. Williams,33H. H. Williams,44P. Wilson,17B. L. Winer,38P. Wittich,17,hS. Wolbers,17C. Wolfe,13 T. Wright,34X. Wu,20S. M. Wynne,29A. Yagil,9K. Yamamoto,40J. Yamaoka,51T. Yamashita,39C. Yang,59U. K. Yang,13,n
Y. C. Yang,27W. M. Yao,28G. P. Yeh,17J. Yoh,17K. Yorita,13T. Yoshida,40G. B. Yu,48I. Yu,27S. S. Yu,17J. C. Yun,17 L. Zanello,50A. Zanetti,53I. Zaw,22X. Zhang,24Y. Zheng,8,cand S. Zucchelli5
(CDF Collaboration)a
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
14Comenius University, 842 48 Bratislava, Slovakia; Institute of Experimental Physics, 040 01 Kosice, Slovakia
15Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
16Duke University, Durham, North Carolina 27708, USA
17Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
18University of Florida, Gainesville, Florida 32611, USA
19Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
20University of Geneva, CH-1211 Geneva 4, Switzerland
21Glasgow University, Glasgow G12 8QQ, United Kingdom
22Harvard University, Cambridge, Massachusetts 02138, USA
23Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland
24University of Illinois, Urbana, Illinois 61801, USA
231801-2
25The Johns Hopkins University, Baltimore, Maryland 21218, USA
26Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany
27Center for High Energy Physics: Kyungpook National University, Taegu 702-701, Korea;
Seoul National University, Seoul 151-742, Korea; SungKyunKwan University, Suwon 440-746, Korea;
Korea Institute of Science and Technology Information, Daejeon, 305-806, Korea;
Chonnam National University, Gwangju, 500-757, 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
36University of New Mexico, Albuquerque, New Mexico 87131, USA
37Northwestern University, Evanston, Illinois 60208, USA
38The Ohio State University, Columbus, Ohio 43210, USA
39Okayama University, Okayama 700-8530, Japan
40Osaka City University, Osaka 588, Japan
41University of Oxford, Oxford OX1 3RH, United Kingdom
42University of Padova, Istituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy
43LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France
44University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
45Istituto Nazionale di Fisica Nucleare Pisa, Universities of Pisa, Siena and Scuola Normale Superiore, I-56127 Pisa, Italy
46University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
47Purdue University, West Lafayette, Indiana 47907, USA
48University of Rochester, Rochester, New York 14627, USA
49The Rockefeller University, New York, New York 10021, USA
50Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, University of Rome ‘‘La Sapienza,’’ I-00185 Roma, Italy
51Rutgers University, Piscataway, New Jersey 08855, USA
52Texas A&M University, College Station, Texas 77843, USA
53Istituto Nazionale di Fisica Nucleare, University of Trieste/Udine, Italy
54University of Tsukuba, Tsukuba, Ibaraki 305, Japan
55Tufts University, Medford, Massachusetts 02155, USA
56Waseda University, Tokyo 169, Japan
57Wayne State University, Detroit, Michigan 48201, USA
58University of Wisconsin, Madison, Wisconsin 53706, USA
59Yale University, New Haven, Connecticut 06520, USA (Received 6 September 2007; published 11 June 2008)
We report on a search for narrow-width particles decaying to a top and antitop quark pair. The data set used in the analysis corresponds to an integrated luminosity of 680 pb1 collected with the Collider Detector at Fermilab in run II. We present 95% confidence level upper limits on the cross section times branching ratio. Assuming a specific top-color-assisted technicolor production model, the leptophobicZ0 with widthZ00:012MZ0, we exclude the mass rangeMZ0<725 GeV=c2at the 95% confidence level.
DOI:10.1103/PhysRevLett.100.231801 PACS numbers: 13.85.Rm, 12.60.Cn, 12.60.Nz, 14.70.Pw
The discovery of the top quark in 1995 at Fermilab [1]
concluded a long search that occupied particle physicists for about two decades. Nevertheless, a decade after the top quark discovery there is still much to investigate about this massive but fundamental particle. There are several ways in which new physics can make an appearance through the top quark. The large mass, in particular, suggests that the top quark may play a special role in the dynamics of electroweak symmetry breaking and act as a powerful probe in this physics sector.
Among the technicolor scenarios, Ref. [2] predicts the existence of a heavy meson T produced in gluon-gluon
interactions (gg!T !tt). Top-color models, later ex- tended to top-color assisted technicolor [3,4], account for the spontaneous breaking of electroweak symmetry by introducing new strong dynamics, which would explain the large top quark mass. In this scheme the breaking of the SU31U11SU32U12 gauge structure at a scale of 1 TeV results in colored ‘‘top gluons’’ and in a singlet vectorZ0, which couples to the third quark genera- tion. A narrow leptophobic Z0 decaying to tt has been predicted in [5] and searched for in Tevatron run I data [6]. Otherttproduction mechanisms can be found in uni- versal extra dimensions theories. The Randall-Sundrum
scenario with the standard model in the bulk predicts the existence of Kaluza-Klein excitations of the gluon, which decay primarily into top quarks [7].
In this Letter, we present a search for narrow resonances decaying tottby looking for an anomalous peak in thett invariant mass distribution. The search algorithm was de- veloped for a narrowttresonance, like aZ0, before looking at the data. At the Tevatron, the Z0 resonance can be produced via qq annihilation. The standard model (SM) predicts that the top quark decays more than 99% of the time into aWbpair and then theW can decay either lep- tonically (BR32:4%) or hadronically (BR67:6%) [8].
The Collider Detector at Fermilab II (CDFII) [9] is an azimuthally and forward-backward symmetric apparatus designed to studypp collisions at
ps
1:96 TeVat the Tevatron. The detector has a charged particle tracking system [10] in a 1.4 T magnetic field directed parallel to the proton beam direction. This system consists of a silicon microstrip detector, which covers the radial range from 1.35 cm to 28 cm, and an open-cell drift chamber in the radial range from 40 cm to 137 cm. Outside the tracking system are the electromagnetic and hadronic calorimeters, with projective segmentation in- [11]. A set of drift chambers and scintillation counters detects muons in the central region (jj<1). The beam luminosity is deter- mined by measuring the inelastic pp collision rate with gas Cherenkov detectors [12].
We search for events in which one W boson decays hadronically and one leptonically. The partonic final state for this decay channel is‘‘bbq q0 (‘e; ); therefore, the final state events have one high momentum charged lepton, large missing transverse energy (6ET) [11] due to the undetected neutrino, and four jets from the quarks’ parton shower and hadronization processes (‘‘leptonjets’’
channel).
The data sample for this analysis has been collected using high-pT triggers for electrons and muons [9]. The off-line selection requires an electron (or muon) candidate with ETpT>20 GeV contained in the central detector region. A jet is defined as an energy cluster in the calo- rimeter and is reconstructed using a fixed cone algorithm with a cone of radius 0.4 in-space. This cone size is chosen to minimize jet merging due to overlapping cones in these multijet events. After correcting the raw jet ener- gies and 6ET to account for multiple pp interactions and inhomogeneities in the detector according to [13], we select events containing at least 4 jets with jj<2 and ET>15 GeV, and with 6ET>20 GeV. Jets originating from the hadronization of b quarks are identified (b tagged), with about 40% efficiency, by reconstructing their displaced vertices [14]. The tagging information is not used for event selection, but only to reduce jet combina- torics during event reconstruction.
Standard model processes that result in the same final state as the resonance are backgrounds to this search. The
dominant components areW boson production in associa- tion with jets (Wjets), tt production, multijet events where a jet is misidentified as a charged lepton and missing energy is generated by jet energy mismeasurement, and diboson production (WW, WZ, ZZ) with extra jets from initial and final state radiation (ISR, FSR). The expected contributions in a data set corresponding to an integrated luminosity of680 pb1 are presented in TableI. Standard modelttand diboson expectations are calculated according to their theoretical cross sections [15,16] and their accep- tances estimated from Monte Carlo simulations (see be- low). The quoted uncertainties include the uncertainties on the theoretical cross sections, integrated luminosity, and acceptances. Given the lack of precise theoretical predic- tions forWjets (4jets) and multijet production cross sections, the contributions from these backgrounds listed in TableI are obtained by constraining the total numbers to add up to the 450 events observed in data. The multijets to Wjets ratio in this sample is fixed at 10%, which is consistent with an estimate obtained in a previous CDF analysis [9]. The quoted uncertainties for these two back- grounds account for both the Poisson statistical uncertainty on the total number of observed events and the uncertain- ties on thettand diboson predictions.
The signature for top-quark-pair resonant production is a peak in the invariant mass spectrum of the tt pair (Mtt), while, in the mass range investigated here, all of the standard model backgrounds fall smoothly with increasing Mtt. The resolution on thettinvariant mass is limited by the uncertainty on the jet energy and by the unmeasured longitudinal momentum of the neutrino. The approach adopted here is to use a matrix-element technique [17] to reconstruct Mtt for an event. We integrate the matrix- element for standard modelttproduction over unmeasured quantities, convoluted with detector resolution functions for the jet energies. The matrix-element implemented for the event reconstruction is the standard model leading order tt production and decay, qqgg ! tt! WbWb!qq0b‘‘b, and it is used to derive the value ofMttin the same way for events in simulated samples and in data.
The a priori probability density for producing a tt parton-level final state,partfpg, relative to other~ ttfinal states, is the normalized differential cross section
TABLE I. Expected number of events from standard model processes in a data set corresponding to an integrated luminosity of680 pb1of data, assuming the null hypothesis. The observed number of events on data is 450.
Background Number of events
SMtt 19924
Diboson 141
Multijet 2210
W!‘‘ 4j 21530
231801-4
partfpg~ Y
i
d3p~i1
Z1
0
Z1
0
dzadzbfkzaflzbdklfpg; z~ aP; z~ bP;~
wherefk(fl) are the proton (antiproton) parton distribution functions,dkl thettdifferential cross section, P~ P~ the proton (antiproton) momentum, andfpg~ the set of six final state three-momenta. Indicesk,lcover the parton types in the proton and antiproton, respectively, and a sum over both indices is implied. Thea prioriprobability density for the parton-level final statefpg~ and the measured jet quan- titiesfjg~ is given by the product
part;jetsfpg;~ fjg ~ partfpg~ Y4
i1
Tj~ijp~i;
where Tjj~p~ denotes the parton to jet transfer function, i.e., the probability that a parton of momentump~ is mea- sured as a jet of momentum j. Transfer functions are~ parametrized in jj and momentum of the jet, and are derived from Monte Carlo simulated (PYTHIA) [18] events.
Different sets of transfer functions are adopted forbquarks and lighter quarks. No transfer function is used for the charged lepton given its negligible momentum uncertainty.
Also, the E6 T measurement is not used to constrain the neutrino transverse momentum.
Frompart;jetsfpg;~ fjg~ we buildPfpgjf~ jg, the probabil-~ ity density for the parton momentafpg, given the observed~ quantitiesfjg. From this distribution we derive probability~ distributions for the new variable,Mtt, which is a function of the parton-level quantities fpg,~ by calculating Pfxjfjg ~ RPfpgjf~ jg x~ Mttfpgdf~ pg. Having an~ event probability distribution for Mtt, we choose the mean as the reconstructed value for that event, since Monte Carlo simulations showed it to be the best single valueMttestimator. However, given that we do not know which jet matches which parton, we also sum over all possible permutations before extracting the mean. If the event contains jets identified as originating frombquarks we sum only over permutations withb-tagged jets assigned tob-quark partons.
To produce theMtttemplates, we apply the reconstruc- tion algorithm to each signal and background sample.
PYTHIA [18] was used to simulate both the Z0 vector resonant production (Mt175 GeV=c2) and tt events.
ALPGEN[19] was used for the simulation of the W boson plus parton production, with HERWIG [20] used to model parton showers. All generated events are passed through the CDF detector simulation. The multijet background template was obtained using CDF II data.
To allow direct comparison with the Tevatron run I results [6], we choose a resonance width Z0 0:012MZ0; however, the measured signal cross section upper limits are insensitive to width values up to Z0 0:05MZ0. The algorithm shows an intrinsic resolution on Mttof about25 GeV=c2when applied to standard modeltt
events, in case of correct partons to jet association. When applied to resonance samples, the mass is still correctly reconstructed in the case of proper parton-jet assignment;
however, incorrect parton-jet assignment leads to a low tail in the reconstructed mass distribution as shown in the inset of Fig. 1. The spectra plotted are restricted to the search region (Mtt>400 GeV=c2) in which the standard model sources fall off exponentially inMtt.
To derive the posterior probability for pp ! Z0BRZ0!tt (we will refer to it simply as ‘‘signal cross section’’) given the observed Mtt spectrum, we build the likelihood Lnj; ~~ Q
i2fbinsgeinii=ni!. This is the prior probability of observingn, where~ niis the number of observed events in Mtt mass bin i, isAsTsi P
jNjTji is the number of expected events in the same bin, s is the assumed signal cross section, As is the acceptance,Njis the contribution from thejth background, andTsi,Tjiis the content of theith bin of the normalized signal and background templates, respectively. The contri- butions from standard modelttand dibosons are weighted as in Table I, while the contributions due toWjets and multijets are rescaled to account for the presence of signal and still match the total number of events observed (NWjetsNmultijet NdataNsignalNttNdiboson). If one uses a flat prior distribution for the signal cross section and integrates over the nuisance parameters~(background
2] [GeV/c
t
Mt
300 400 500 600 700 800 900 1000 1100 1200
tt1/N dN/dM
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14
t SM t diboson
2] [GeV/c t Mt 300 400 500 600 700 800 900 1000 1100 1200
tt1/N dN/dM
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09
0.1 2
=500 GeV/c MZ’
=600 GeV/c2
MZ’
=700 GeV/c2
MZ’
=800 GeV/c2
MZ’
=900 GeV/c2
MZ’
FIG. 1 (color online). ReconstructedMttdistributions for two simulated standard model backgrounds. We plot the samples with the ‘‘hardest’’ (tt) and the ‘‘softest’’ (diboson) spectrum (normalized to unity); all the others lie in between. The inset shows the reconstructedMttdistributions for five signal samples.
contributions and signal acceptance) then Bayes’s theorem gives the posterior probability densityPjn. This is used~ to define upper limits at any given confidence level (C.L.), together with the most likely value, which is regarded as the measured cross section. This procedure is repeated for 10 resonance masses from 450 GeV=c2 to 900 GeV=c2, and 95% C.L. upper limits are established.
As mentioned previously, uncertainties on template weighting are incorporated in the prior probability by means of the prior densities of the nuisance parameters.
In this way the marginalized posterior probability density for the signal cross section includes the acceptance and cross section uncertainties. However, the relative contribu- tion of multijet andWjet events has been kept fixed at 10%. To evaluate the impact of the uncertainty on this ratio, the multijet component has been either set to zero or increased by a factor of 3, in each case yielding negli- gible change on the cross section posterior distribution.
This is because after the event selection theWjetand multijetMttspectra are very similar above400 GeV=c2, as shown in Fig.1. We assume a 10% acceptance uncertainty for all the resonant ttsignals, consistent with the uncer- tainty from the SMtt.
Other sources of systematic uncertainties can affect both the acceptances and the templates. These ‘‘shape system- atics’’ are due to the imperfect knowledge in the modeling of (1) jet energy scale, (2) ISR and FSR, (3) theQ2 scale forWjets production, and (4) parton distribution func- tions (PDF). To account for these uncertainties, we con- volute the cross section posterior probability with a Gaussian, whose width is estimated from a new set of templates and acceptances corresponding to a1change for each systematic. For a resonance of mass MZ0 700 GeV=c2 the uncertainties due to sources (1), (2), and (3) translate into a relative increase of the expected upper limits of 5%, 5%, 2%, and 2%, respectively, for an overall impact of about 10%. The uncertainty due to the choice of the PDFs turned out to be negligible.
In the data sample used for this analysis, 450 events passed our event selection requirements, and 302 of them are found in the search regionMtt>400 GeV=c2. TheMtt
data spectrum is shown in Fig.2, together with the back- ground expectations (above 400 GeV=c2) based on Monte Carlo studies. In order to establish an a priori sensitivity of the reconstruction algorithm we generated 1000 simulated experiments in the null hypothesis and extracted, for each mass point, the 95% C.L. expected upper limit, defined as the median of the upper limits distribution. We also calculated the central 1 and 2 frequentist coverage bands.
In the data, the posterior probability distributions for the signal cross section show no evidence of resonantttpro- duction. The predicted and observed upper limits at 95%
C.L. are shown in Fig. 3, together with the theoretical prediction for the cross section as a function of mass in
the leptophobic Z0 model (with Z0 0:012MZ0) [5].
Based on this model, we exclude a leptophobic top-color resonance candidate with a mass of725 GeV=c2 or less.
In conclusion, we performed a search for a narrow heavy resonance decaying into tt in the leptonjets channel using 680 pb1 of CDF run 2 data. We set upper limits on the production cross section times branching ratio at the
2] [GeV/c
t
Mt
300 400 500 600 700 800 900 1000 1100 1200
2events/20GeV/c
10 20 30 40 50 60 70 80 90
CDF data
≥ 4j W+
multi-jet , 6.7 pb t SM t diboson
2] [GeV/c t Mt 300 400 500 600 700 800 900 100011001200 2events/20GeV/c
10-2 10-1 1 10 102
FIG. 2 (color online). The reconstructed Mtt spectrum (data) and the standard model prediction in the search region above 400 GeV=c2. The inset shows the same distributions in logarith- mic scale.
2] [GeV/c MZ’
450 500 550 600 650 700 750 800 850 900 ) [pb]t t→ BR(Z’ ⋅ Z’σ
0 1 2 3 4 5 6 7 8
9 Simulated 95% CL upper limits:
Median
Central 68% coverage band Central 95% coverage band Observed 95% CL upper limits:
data Theory:
=1.2% MZ’
ΓZ’
Leptophobic Z’,
FIG. 3 (color online). Predicted and experimental 95% C.L.
upper limits using data corresponding to680 pb1of integrated luminosity, together with the leptophobic top-color Z0 cross section prediction. Dark and light areas define the 1 and2 ranges for the expected limits. The arrow marks the mass upper limit.
231801-6
95% C.L. For one leptophobic top-color production mechanism we exclude masses up to725 GeV=c2, extend- ing significantly the run I limit of560 GeV=c2 [6].
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 Foundation; the A. P. Sloan Foundation; the Bundes- ministerium fu¨r Bildung und Forschung, Germany; the Korean Science and Engineering Foundation and the Korean Research Foundation; the Science and Technology Facilities Council and the Royal Society, United Kingdom; the Institut National de Physique Nucleaire et Physique des Particules/CNRS; the Russian Foundation for Basic Research; the Comisio´n Inter- ministerial de Ciencia y Tecnologı´a, Spain; the European Community’s Human Potential Programme; the Slovak R&D Agency; and the Academy of Finland.
aWith visitors from
bUniversity of Athens, 15784 Athens, Greece.
cChinese Academy of Sciences, Beijing 100864, China.
dUniversity of Bristol, Bristol BS8 1TL, United Kingdom.
eUniversity Libre de Bruxelles, B-1050 Brussels, Belgium.
fUniversity of California, Irvine, Irvine, CA 92697, USA.
gUniversity of California Santa Cruz, Santa Cruz, CA 95064, USA.
hCornell University, Ithaca, NY 14853, USA.
iUniversity of Cyprus, Nicosia CY-1678, Cyprus.
jUniversity College Dublin, Dublin 4, Ireland.
kUniversity of Edinburgh, Edinburgh EH9 3JZ, United Kingdom.
lUniversity of Heidelberg, D-69120 Heidelberg, Germany.
mUniversidad Iberoamericana, Mexico D.F., Mexico.
nUniversity of Manchester, Manchester M13 9PL, United Kingdom.
oNagasaki Institute of Applied Science, Nagasaki, Japan.
pUniversity de Oviedo, E-33007 Oviedo, Spain.
qQueen Mary’s College, University of London, London, E1 4NS, United Kingdom.
rTexas Tech University, Lubbock, TX 79409, USA.
sIFIC(CSIC–Universitat de Valencia), 46071 Valencia, Spain.
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