Article
Reference
Measurement of the tt Production Cross Section in pp Collisions at s√=1.96 TeV Using Dilepton Events
CDF Collaboration
CAMPANELLI, Mario (Collab.), et al.
Abstract
We report a measurement of the tt¯ production cross section using dilepton events with jets and missing transverse energy in pp collisions at a center-of-mass energy of 1.96 TeV. Using a 197±12 pb−1 data sample recorded by the upgraded Collider Detector at Fermilab, we use two complementary techniques to select candidate events. We compare the number of observed events and selected kinematical distributions with the predictions of the standard model and find good agreement. The combined result of the two techniques yields a tt¯
production cross section of 7.0+2.4−2.1(stat)+1.6−1.1(syst)±0.4(lum) pb.
CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Measurement of the tt Production Cross Section in pp Collisions at s√=1.96 TeV Using Dilepton Events. Physical Review Letters , 2004, vol. 93, no. 14, p. 142001
DOI : 10.1103/PhysRevLett.93.142001
Available at:
http://archive-ouverte.unige.ch/unige:38092
Disclaimer: layout of this document may differ from the published version.
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Measurement of the tt Production Cross Section in pp Collisions at
p s
1:96 TeV Using Dilepton Events
D. Acosta,15T. Affolder,8T. Akimoto,53M. G. Albrow,14D. Ambrose,42S. Amerio,41D. Amidei,32A. Anastassov,49 K. Anikeev,30A. Annovi,43J. Antos,1M. Aoki,53G. Apollinari,14T. Arisawa,55J-F. Arguin,31A. Artikov,12
W. Ashmanskas,2A. Attal,6F. Azfar,40P. Azzi-Bacchetta,41N. Bacchetta,41H. Bachacou,27W. Badgett,14 A. Barbaro-Galtieri,27G. J. Barker,24V. E. Barnes,45B. A. Barnett,23S. Baroiant,5M. Barone,16G. Bauer,30 F. Bedeschi,43S. Behari,23S. Belforte,52G. Bellettini,43J. Bellinger,57D. Benjamin,13A. Beretvas,14 A. Bhatti,47 M. Binkley,14D. Bisello,41M. Bishai,14R. E. Blair,2C. Blocker,4K. Bloom,32B. Blumenfeld,23A. Bocci,47A. Bodek,46 G. Bolla,45A. Bolshov,30P. S. L. Booth,28D. Bortoletto,45J. Boudreau,44S. Bourov,14C. Bromberg,33E. Brubaker,27
J. Budagov,12H. S. Budd,46K. Burkett,14G. Busetto,41P. Bussey,18K. L. Byrum,2S. Cabrera,13P. Calafiura,27 M. Campanelli,17M. Campbell,32A. Canepa,45M. Casarsa,52D. Carlsmith,57S. Carron,13R. Carosi,43A. Castro,3
P. Catastini,43D. Cauz,52A. Cerri,27C. Cerri,43L. Cerrito,22J. Chapman,32C. Chen,42Y. C. Chen,1M. Chertok,5 G. Chiarelli,43G. Chlachidze,12F. Chlebana,14I. Cho,26K. Cho,26D. Chokheli,12M. L. Chu,1S. Chuang,57J. Y. Chung,37
W-H. Chung,57Y. S. Chung,46C. I. Ciobano,22M. A. Ciocci,43A. G. Clark,17 D. Clark,4M. Coca,46A. Connolly,27 M. Convery,47J. Conway,49M. Cordelli,16G. Cortiana,41J. Cranshaw,51J. Cuevas,9R. Culbertson,14C. Currat,27
D. Cyr,57D. Dagenhart,4S. Da Ronco,41S. D’Auria,18P. de Barbaro,46S. De Cecco,48G. De Lentdecker,46 S. Dell’Agnello,16M. Dell’Orso,43S. Demers,46L. Demortier,47M. Deninno,3D. De Pedis,48P. F. Derwent,14 C. Dionisi,48J. R. Dittmann,14P. Doksus,22A. Dominguez,27S. Donati,43M. Donega,17M. D’Onofrio,17 T. Dorigo,41 V. Drollinger,35K. Ebina,55N. Eddy,22R. Ely,27R. Erbacher,14M. Erdmann,24D. Errede,22S. Errede,22R. Eusebi,46 H-C. Fang,27S. Farrington,28I. Fedorko,43R. G. Feild,58M. Feindt,24 J. P. Fernandez,45C. Ferretti,32R. D. Field,15 I. Fiori,43G. Flanagan,33B. Flaugher,14L. R. Flores-Castillo,44A. Foland,19S. Forrester,5G.W. Foster,14M. Franklin,19
H. Frisch,11Y. Fujii,25I. Furic,30A. Gajjar,28A. Gallas,36J. Galyardt,10M. Gallinaro,47M. Garcia-Sciveres,27 A. F. Garfinkel,45C. Gay,58H. Gerberich,13D.W. Gerdes,32E. Gerchtein,10S. Giagu,48P. Giannetti,43A. Gibson,27
K. Gibson,10C. Ginsburg,57K. Giolo,45M. Giordani,52G. Giurgui,10V. Glagolev,12D. Glenzinski,14M. Gold,35 N. Goldschmidt,32D. Goldstein,6J. Goldstein,40G. Gomez,9G. Gomez-Ceballos,30M. Goncharov,50O. Gonza´lez,45
I. Gorelov,35A. T. Goshaw,13Y. Gotra,44K. Goulianos,47A. Gresele,3C. Grosso-Pilcher,11M. Guenther,45 J. Guimaraes de Costa,19C. Haber,27K. Hahn,42S. R. Hahn,14E. Halkiadakis,46R. Handler,57F. Happacher,16K. Hara,53
M. Hare,54R. F. Harr,56R. M. Harris,14F. Hartmann,24K. Hatakeyama,47J. Hauser,6C. Hays,13H. Hayward,28 E. Heider,54B. Heinemann,28J. Heinrich,42M. Hennecke,24M. Herndon,23C. Hill,8D. Hirschbuehl,24A. Hocker,46 K. D. Hoffman,11A. Holloway,19S. Hou,1M. A. Houlden,28B. T. Huffman,40Y. Huang,13R. E. Hughes,27J. Huston,33
K. Ikado,55J. Incandela,8G. Introzzi,43M. Iori,48Y. Ishizawa,53C. Issever,8A. Ivanov,46Y. Iwata,21B. Iyutin,30 E. James,14 D. Jang,49J. Jarrell,35D. Jeans,48H. Jensen,14E. J. Jeon,26M. Jones,45K. K. Joo,26S. Jun,10T. Junk,22 T. Kamon,50J. Kang,32M. Karagoz Unel,36P. E. Karchin,56S. Kartal,14Y. Kato,39Y. Kemp,24R. Kephart,14U. Kerzel,24
V. Khotilovich,50B. Kilminster,37D. H. Kim,26H. S. Kim,22J. E. Kim,26M. J. Kim,10M. S. Kim,26S. B. Kim,26 S. H. Kim,53T. H. Kim,30Y. K. Kim,11B. T. King,28M. Kirby,13L. Kirsch,4S. Klimenko,15B. Knuteson,30B. R. Ko,13
H. Kobayashi,53P. Koehn,37D. J. Kong,26K. Kondo,55J. Konigsberg,15K. Kordas,31A. Korn,30A. Korytov,15 K. Kotelnikov,34A.V. Kotwal,13A. Kovalev,42J. Kraus,22I. Kravchenko,30A. Kreymer,14 J. Kroll,42M. Kruse,13 V. Krutelyov,50S. E. Kuhlmann,2N. Kuznetsova,14A. T. Laasanen,45S. Lai,31S. Lami,47S. Lammel,14J. Lancaster,13 M. Lancaster,29R. Lander,5K. Lannon,37A. Lath,49G. Latino,35R. Lauhakangas,20I. Lazzizzera,41Y. Le,23C. Lecci,24 T. LeCompte,2J. Lee,26J. Lee,46S.W. Lee,50N. Leonardo,30S. Leone,43J. D. Lewis,14K. Li,58C. Lin,58C. S. Lin,14
M. Lindgren,6T. M. Liss,22D. O. Litvintsev,14T. Liu,14Y. Liu,17N. S. Lockyer,42A. Loginov,34M. Loreti,41 P. Loverre,48R-S. Lu,1D. Lucchesi,41P. Lukens,14L. Lyons,40J. Lys,27R. Lysak,1D. MacQueen,31R. Madrak,19 K. Maeshima,14P. Maksimovic,23L. Malferrari,3G. Manca,28R. Marginean,37M. Martin,23A. Martin,58V. Martin,36
M. Martı´nez,14T. Maruyama,11H. Matsunaga,53M. Mattson,56P. Mazzanti,3K. S. McFarland,46D. McGivern,29 P. M. McIntyre,50P. McNamara,49R. McNulty,28S. Menzemer,30A. Menzione,43P. Merkel,14C. Mesropian,47 A. Messina,48T. Miao,14N. Miladinovic,4L. Miller,19R. Miller,33J. S. Miller,32C. Mills,8R. Miquel,27S. Miscetti,16
G. Mitselmakher,15A. Miyamoto,25Y. Miyazaki,39N. Moggi,3B. Mohr,6R. Moore,14M. Morello,43T. Moulik,45 A. Mukherjee,14 M. Mulhearn,30T. Muller,24R. Mumford,23A. Munar,42P. Murat,14J. Nachtman,14S. Nahn,58 I. Nakamura,42I. Nakano,38A. Napier,54R. Napora,23D. Naumov,35V. Necula,15F. Niell,32J. Nielsen,27C. Nelson,14
T. Nelson, C. Neu, M. S. Neubauer, C. Newman-Holmes, A-S. Nicollerat, T. Nignamov, L. Nodulman, K. Oesterberg,20T. Ogawa,55S. Oh,13Y. D. Oh,26T. Ohsugi,21T. Okusawa,39R. Oldeman,48R. Orava,20W. Orejudos,27
C. Pagliarone,43F. Palmonari,43R. Paoletti,43V. Papadimitriou,51S. Pashapour,31J. Patrick,14G. Pauletta,52 M. Paulini,10T. Pauly,40C. Paus,30D. Pellett,5A. Penzo,52T. J. Phillips,13G. Piacentino,43J. Piedra,9K. T. Pitts,22 C. Plager,6A. Pomposˇ,45L. Pondrom,57G. Pope,44O. Poukhov,12F. Prakoshyn,12T. Pratt,28A. Pronko,15J. Proudfoot,2
F. Ptohos,16G. Punzi,43J. Rademacker,40A. Rakitine,30S. Rappoccio,18F. Ratnikov,49H. Ray,32A. Reichold,40 B. Reisert,14V. Rekovic,35P. Renton,40M. Rescigno,48F. Rimondi,3K. Rinnert,24L. Ristori,43W. J. Robertson,13 A. Robson,40T. Rodrigo,9S. Rolli,54L. Rosenson,30R. Roser,14R. Rossin,41C. Rott,45J. Russ,10A. Ruiz,9D. Ryan,54
H. Saarikko,20A. Safonov,5R. St. Denis,18W. K. Sakumoto,46G. Salamanna,48D. Saltzberg,6C. Sanchez,37 A. Sansoni,16L Santi,52S. Sarkar,48K. Sato,53P. Savard,31A. Savoy-Navarro,14P. Schemitz,24 P. Schlabach,14 E. E. Schmidt,14M. P. Schmidt,58M. Schmitt,36L. Scodellaro,41I. Sfiligoi,16T. Shears,28A. Scribano,43F. Scuri,43 A. Sedov,45S. Seidel,35Y. Seiya,39F. Semeria,3L. Sexton-Kennedy,14M. D. Shapiro,27P. F. Shepard,44M. Shimojima,53
M. Shochet,11Y. Shon,57I. Shreyber,34A. Sidoti,43M. Siket,1A. Sill,51P. Sinervo,31A. Sisakyan,12A. Skiba,24 A. J. Slaughter,14K. Sliwa,54J. R. Smith,5F. D. Snider,14R. Snihur,31S.V. Somalwar,49J. Spalding,14 M. Spezziga,51 L. Spiegel,14F. Spinella,43M. Spiropulu,8P. Squillacioti,43H. Stadie,24A. Stefanini,43B. Stelzer,31O. Stelzer-Chilton,31 J. Strologas,35D. Stuart,8A. Sukhanov,15K. Sumorok,30H. Sun,54T. Suzuki,53A. Taffard,22R. Tafirout,31S. F. Takach,56 H. Takano,53R. Takashima,21Y. Takeuchi,53K. Takikawa,53M. Tanaka,2R. Takaka,38N. Tanimoto,38S. Tapprogge,20
M. Tecchio,32P. K. Teng,1K. Terashi,47R. J. Tesarek,14S. Tether,30J. Thom,14 A. S. Thompson,18E. Thomson,37 P. Tipton,46V. Tiwari,10S. Tkaczyk,14D. Toback,50K. Tollefson,33D. Tonelli,43M. Tonnesmann,33S. Torre,43
D. Torretta,14W. Trischuk,31J. Tseng,30R. Tsuchiya,55S. Tsuno,53D. Tsybychev,15N. Turini,43M. Turner,28 F. Ukegawa,53T. Unverhau,18S. Uozumi,53D. Usynin,42L. Vacavant,27A. Vaiciulis,46A. Varganov,32E. Vataga,43 S. Vejcik III,14G. Velev,14G. Veramendi,22T. Vickey,22R. Vidal,14I. Vila,9R. Vilar,9I. Volobouev,27M. von der Mey,6
R. G. Wagner,2R. L. Wagner,14W. Wagner,24R. Wallny,6T. Walter,24T. Yamashita,38K. Yamamoto,39Z. Wan,49 M. J. Wang,1S. M. Wang,15A. Warburton,31B. Ward,18S. Waschke,18D. Waters,29T. Watts,49M. Weber,27 W. C. Wester III,14 B. Whitehouse,54A. B. Wicklund,2E. Wicklund,14H. H. Williams,42P. Wilson,14B. L. Winer,37 P. Wittich,42S. Wolbers,14M. Wolter,54M. Worcester,6S. Worm,49T. Wright,32X. Wu,17F. Wu¨rthwein,7A. Wyatt,29 A. Yagil,14U. K. Yang,11W. Yao,27G. P. Yeh,14K. Yi,23J. Yoh,14P. Yoon,46K. Yorita,55T. Yoshida,39I. Yu,26S. Yu,42Z. Yu,58
J. C. Yun,14L. Zanello,48A. Zanetti,52I. Zaw,19F. Zetti,43J. Zhou,49A. Zsenei,17and S. Zucchelli3 (CDF Collaboration)
1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China
2Argonne National Laboratory, Argonne, Illinois 60439 USA
3Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy
4Brandeis University, Waltham, Massachusetts 02254 USA
5University of California at Davis, Davis, California 95616 USA
6University of California at Los Angeles, Los Angeles, California 90024 USA
7University of California at San Diego, La Jolla, California 92093 USA
8University of California at Santa Barbara, Santa Barbara, California 93106 USA
9Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
10Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 USA
11Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637 USA
12Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
13Duke University, Durham, North Carolina 27708 USA
14Fermi National Accelerator Laboratory, Batavia, Illinois 60510 USA
15University of Florida, Gainesville, Florida 32611 USA
16Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
17University of Geneva, CH-1211 Geneva 4, Switzerland
18Glasgow University, Glasgow G12 8QQ, United Kingdom
19Harvard University, Cambridge, Massachusetts 02138 USA
20The Helsinki Group: Helsinki Institute of Physics; and Division of High Energy Physics, Department of Physical Sciences, University of Helsinki, FIN-00044, Helsinki, Finland
21Hiroshima University, Higashi-Hiroshima 724, Japan
22University of Illinois, Urbana, Illinois 61801 USA
23The Johns Hopkins University, Baltimore, Maryland 21218 USA
142001-2 142001-2
24Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany
25High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305, Japan
26Center for High Energy Physics: Kyungpook National University, Taegu 702-701; Seoul National University, Seoul 151-742;
and SungKyunKwan University, Suwon 440-746; Korea
27Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720 USA
28University of Liverpool, Liverpool L69 7ZE, United Kingdom
29University College London, London WC1E 6BT, United Kingdom
30Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 USA
31Institute of Particle Physics, McGill University, Montre´al, Quebec H3A 2T8 Canada; and University of Toronto, Toronto, Ontario M5S 1A7 Canada
32University of Michigan, Ann Arbor, Michigan 48109 USA
33Michigan State University, East Lansing, Michigan 48824 USA
34Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia
35University of New Mexico, Albuquerque, New Mexico 87131 USA
36Northwestern University, Evanston, Illinois 60208 USA
37The Ohio State University, Columbus, Ohio 43210 USA
38Okayama University, Okayama 700-8530, Japan
39Osaka City University, Osaka 588, Japan
40University of Oxford, Oxford OX1 3RH, United Kingdom
41University of Padova, Istituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy
42University of Pennsylvania, Philadelphia, Pennsylvania 19104 USA
43Istituto Nazionale di Fisica Nucleare, University and Scuola Normale Superiore of Pisa, I-56100 Pisa, Italy
44University of Pittsburgh, Pittsburgh, Pennsylvania 15260 USA
45Purdue University, West Lafayette, Indiana 47907 USA
46University of Rochester, Rochester, New York 14627 USA
47The Rockefeller University, New York, New York 10021 USA
48Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, University di Roma ‘‘La Sapienza,’’ I-00185 Roma, Italy
49Rutgers University, Piscataway, New Jersey 08855 USA
50Texas A&M University, College Station, Texas 77843 USA
51Texas Tech University, Lubbock, Texas 79409 USA
52Istituto Nazionale di Fisica Nucleare, University of Trieste, Udine, Italy
53University of Tsukuba, Tsukuba, Ibaraki 305, Japan
54Tufts University, Medford, Massachusetts 02155 USA
55Waseda University, Tokyo 169, Japan
56Wayne State University, Detroit, Michigan 48201 USA
57University of Wisconsin, Madison, Wisconsin 53706 USA
58Yale University, New Haven, Connecticut 06520 USA (Received 26 April 2004; published 27 September 2004)
We report a measurement of the tt production cross section using dilepton events with jets and missing transverse energy in pp collisions at a center-of-mass energy of 1.96 TeV. Using a 197 12 pb1data sample recorded by the upgraded Collider Detector at Fermilab, we use two complemen- tary techniques to select candidate events. We compare the number of observed events and selected kinematical distributions with the predictions of the standard model and find good agreement.
The combined result of the two techniques yields a tt production cross section of 7:02:42:1stat1:61:1syst 0:4lumpb.
DOI: 10.1103/PhysRevLett.93.142001 PACS numbers: 14.65.Ha, 12.38.Qk, 13.85.Qk
Since the discovery of the top quark [1], experimental attention has turned to the examination of its production and decay properties. Within the standard model (SM), the top quark production cross section is calculated with an uncertainty of&15%[2,3]. Furthermore, in the SM, the top quark decays to aW boson andbquark100%of the time. TheW subsequently decays to either a pair of quarks or a lepton-neutrino pair. Measuring the rate of the reaction pp!tt!b‘‘b‘ 0‘0 tests both the pro- duction and decay mechanisms of the top quark. A sig- nificant deviation from the SM prediction would indicate
either a novel production mechanism, e.g., a heavy reso- nance decaying intottpairs [4], or a novel decay mecha- nism, e.g., decay into supersymmetric particles [5]. The Collider Detector at Fermilab (CDF) and D0 collabora- tions previously measured thettproduction cross section in the dilepton channel during run I of the Fermilab Tevatron [6]. These and related measurements were con- sistent with SM expectations but suffered large uncer- tainties due to small event samples.
This Letter describes a measurement of the tt cross section in the dilepton channel using data from run II of
the Tevatron taken with the upgraded Collider Detector at Fermilab (CDF II). The data sample corresponds to an integrated luminosity of 19712 pb1 [7], 2 that used in run I. Moreover, we expect the higher center-of- mass energy of 1.96 TeV in run II to increase the produc- tion of tt events by 30% relative to the run I rate at 1.8 TeV [2,3]. The upgrades to the CDF II detector further increase thettyield with improved lepton acceptance. We perform two complementary analyses of the new data.
One, inspired by the technique used by CDF in run I, requires that both leptons be specifically identified as either electrons or muons (DIL analysis). The other tech- nique allows one of the leptons to be identified only as a high-pT, isolated track (LTRK analysis), thereby signifi- cantly increasing the lepton detection efficiency with some increase in expected background events.
The CDF II detector [8] is an azimuthally and forward- backward symmetric apparatus designed to study pp reactions at the Tevatron. The detector has a charged particle tracking system immersed in a 1.4 T magnetic field, aligned coaxially with the pp beams. A silicon microstrip detector provides tracking over the radial range 1.5 to 28 cm. A 3.1 m long open-cell drift chamber, the central outer tracker, covers the radial range from 40 to 137 cm. The fiducial region of the silicon detector extends to jj 2 [9], while the central outer tracker provides coverage forjj&1.
Segmented electromagnetic and hadronic sampling calorimeters surround the tracking system and measure the energy flow of interacting particles in the pseudora- pidity range jj<3:6. This analysis uses the new end plug detectors to identify electron candidates with1:2<
jj<2:0 in addition to the central detectors for lepton candidates withjj<1:1. A set of drift chambers located outside the central hadron calorimeters and another set behind a 60 cm iron shield detect energy deposition from muon candidates withjj 0:6. Additional drift cham- bers and scintillation counters detect muons in the region 0:6 jj 1:0. Gas Cherenkov counters located in the 3:7<jj<4:7region [10] measure the average number of inelasticppcollisions per bunch crossing and thereby determine the beam luminosity.
The b‘‘b‘ 0‘0 events under study produce two high-pT leptons, missing transverse energy (E6 T) [9]
from the undetected neutrinos, and two jets from the hadronization of the b quarks. Additional jets are often produced by initial-state and final-state radiation. A trig- ger system first identifies candidate events by finding either a central electron or muon candidate with ET >
18 GeV[11], or an end plug electron candidate withET >
20 GeV [9] in an event with E6 T>15 GeV. After full event reconstruction, the candidate event sample is fur- ther refined by selection criteria determined a priori to minimize the expected statistical and systematic uncer- tainties of the cross section result.
Both analyses require two oppositely charged leptons with ET >20 GeV[11]. One lepton, the ‘‘tight’’ lepton, must pass strict lepton identification requirements and be isolated. A lepton is isolated if the totalET within a cone
R
2 2
p 0:4, minus the lepton ET, is
<10% of the lepton ET [11]. Tight electrons have a well-measured track pointing at an energy deposition in the calorimeter. For electrons with jj>1:2, this track association uses a calorimeter-seeded silicon tracking algorithm [12]. In addition, the candidate’s electromag- netic shower profile must be consistent with that expected for electrons. Tight muons must have a well-measured track linked to hits in the muon chambers and energy deposition in the calorimeters consistent with that ex- pected for muons.
The other lepton, the ‘‘loose’’ lepton, is identified dif- ferently by the two analyses. The DIL analysis requires the loose lepton to be an electron or muon selected as above, with the exceptions that it need not be isolated and muon identification requirements are relaxed. The LTRK analysis defines a loose lepton as a well-measured, iso- lated track withpT>20 GeV=c in the range ofjj<1 where the isolation requirement is the tracking analog of the calorimetric isolation employed for tight leptons.
These selections add acceptance for dilepton events where electrons or muons pass through gaps in the calorimetry or muon systems. They also contribute acceptance for single prong hadronic decays of thelepton from W! . Consequently, the LTRK analysis derives 20% of its acceptance from taus, compared with 12% for the DIL analysis.
Candidate events must have E6 T>25 GeV. To reduce the occurrence of false E6 T due to mismeasured jets, we require that theE6 T vector point away from any jet. Each analysis takes additional steps to further suppress falseE6 T arising from mismeasurement of their respective loose leptons. The DIL analysis requires that theE6 Tvector be at least 20 from the closest lepton. The LTRK analysis corrects the E6 T for all loose leptons whenever the asso- ciated calorimeterET is<70%of the trackpT. It further rejects events for which theE6 Tvector lies within5of the loose lepton axis. In both analyses, these additional topo- logical cuts are not applied in events withE6 T>50 GeV.
The DIL (LTRK) analysis counts jets with ET>
1520 GeVdetected injj<2:52:0, where we define a jet as a fixed-cone cluster with a cone size ofR0:4.
We correct jet ET measurements for the effects of calo- rimeter nonuniformity and absolute energy scale [13].
After removal of cosmic-ray muons and photon- conversion electrons, the dominant backgrounds to dilep- tonttevents are Drell-Yan (qq !Z=?!‘‘) produc- tion,‘‘fake’’ leptons inW!‘jetevents where a jet is falsely reconstructed as a lepton candidate, and diboson (WW, WZ, and ZZ) production. Drell-Yan events typi- cally have littleE6 T. Thus, for events with dilepton invari-
142001-4 142001-4
ant mass within15 GeV=c2of theZboson resonance, the DIL analysis imposes a cut on the ratio ofE6 T to the sum of the jetET’s projected along theE6 T vector, whereas the LTRK analysis tightens its E6 T requirement to E6 T >
40 GeV. To estimate residual Drell-Yan sample contami- nation we utilize both a PYTHIA[14] Monte Carlo calcu- lation with detector simulation and the data itself. We select Z boson candidates in the mass range of 76–106 GeV=c2 and count the number of events passing nominal and Drell-Yan-specific selection criteria. After subtraction of expected non-Drell-Yan contributions, these two numbers provide the normalization for the distribution of expected contributions inside and outside theZboson mass window. This distribution is obtained as a function of jet multiplicity using a sample of simulated events.
We estimate the fake lepton background contribution by applying a fake lepton rate to a data sample ofW !
‘jetevents. We determine this fake rate using a large sample of events triggered by at least one jet withET >
50 GeVafter removing sources of real leptons such asW and Z decays. To check the accuracy and robustness of this estimate we apply our fake lepton rate to different samples with varied physics content: jet data with 20, 70 and 100 GeV trigger thresholds, an inclusive photon sam- ple, and an inclusive electron sample. The observed num- bers of fake leptons agree with our fake rate predictions within statistical uncertainties (e.g., 74 observed vs70 14 predicted for LTRK). An additional check is per- formed on the like-sign subset of the dilepton sample itself, which is dominantlyW!‘jetevents with one fake lepton. We compare the number of observed like-sign events to the like-sign fake background predictions and find good agreement (e.g., five observed vs 6:31:4 predicted for DIL).
We determine geometric and kinematic acceptance for the diboson backgrounds using PYTHIA and
ALPGEN+HERWIG Monte Carlo calculations [15,16] fol- lowed by a simulation of the CDF II detector. We use the CTEQ5L parton distribution functions [17] to model the momentum distribution of the initial-state partons.
We normalize the total number of expected events for
these processes to their theoretical cross sections:13:3 pb for WW, 4:0 pb for WZ and 1:5 pb for ZZ [18]. We estimate the uncertainty in these background predictions by comparing different Monte Carlo calculations for the same diboson process. Similarly, we obtain the accep- tance for tt using a PYTHIA Monte Carlo calculation assumingmtop175 GeV=c2andBRW!‘ 10:8%.
We present the predicted and observed numbers of oppositely charged dilepton events versus jet multiplicity in Table I. Good agreement is seen for the background- dominated zero and one-jet events, establishing confi- dence in the background estimates detailed above. We measure the tt production cross section using events with two or more jets. The DIL analysis enhances its signal sensitivity by requiring that HT, the scalar sum of the leptonpT, jetET, andE6 T, be>200 GeV.
Systematic uncertainties include uncertainties on the acceptance times efficiency a and the background estimates. The dominant uncertainty on a is due to uncertainties on the jet energy scale and lepton identifi- cation/isolation efficiencies. The background uncertainty is dominated by the statistical uncertainty in the Drell- Yan contribution arising from the limited number of Z events with high E6 T. Table II lists all systematic uncertainties.
Using Table I, the expected signal-to-background ratios are 3.1 for the DIL analysis and 1.7 for the LTRK analy- sis. The products aBRtt!b‘‘b‘ 0‘0 are
TABLE I. Expected background andttcontributions (mtop175 GeV=c2,6:7 pb) compared with observed data.
LTRK DIL
Njet0 Njet1 Njet2 Njet0 Njet1 Njet2 HT>200 GeV
WW; WZ; ZZ 21:85:2 6:31:5 1:20:3 11:43:3 3:20:9 1:10:3 0:70:2
Drell-Yan 26:59:8 16:46:0 4:21:6 4:41:9 2:91:1 1:30:5 0:90:5
Fakes 16:52:4 5:01:0 1:50:5 3:01:2 2:41:0 1:50:6 1:10:5
Total background 64:811:3 27:76:3 6:91:7 18:84:0 8:51:8 3:90:9 2:70:7
Expectedtt 0:30:2 3:40:6 11:51:5 0:10:0 1:30:2 8:51:2 8:21:1
Total 65:111:3 31:16:3 18:42:3 18:94:0 9:81:9 12:41:6 10:91:4
Observed 73 26 19 16 9 14 13
TABLE II. Summary of systematic uncertainties.
Signal and background uncertainties LTRK DIL
Lepton(track) ID 5%(6%) 5%
Jet energy scale — signal 6% 5%
Jet energy scale —background 10% 18– 29%
Initial/final state radiation 7% 2%
Parton distribution functions 6% 6%
Monte Carlo generators 5% 6%
WW; WZ; ZZestimate 20% 20%
Drell-Yan estimate 30% 51%
Fake estimate 12% 41%
0:620:09%and0:880:12%, respectively. The to- tal integrated luminosity is R
Ldt19712 pb1. Hence, the measured cross sections,NobsNbkg=a BRtt!b‘‘b‘ 0‘0 R
Ldt, are 8:43:22:71:51:1 0:5 pb for the DIL and 7:02:72:31:51:30:4 pb for the LTRK analysis, where the first two uncertainties are statistical and systematic and the third is due to the luminosity determination. We combine these results by dividing the analyses’ expected signal and background into three disjoint regions (DIL only, LTRK only, and the overlap). Eleven events are shared between DIL and LTRK. Using the combined aBRtt! b‘‘b‘ 0‘0 of 1.03% and accounting for common systematic uncertainties, a joint Poisson likelihood is maximized yielding [19]
tt7:02:42:1stat1:61:1syst 0:4lumpb:
We have performed several cross checks. The tech- niques reproduce the expected W and Z production cross sections (e.g., 2525 pb measured vs 252 0:9 pb expected for LTRK etrackZ candidates). We compare the number of events with identified bottom quark jets in the signal sample to expectations and find agreement within uncertainties (e.g., seven observed vs 5:91:8expected for DIL). The measured ttcross sec- tion is stable within its uncertainty to variations of the loose and tight leptonpT andET cuts. When we restrict the analysis to two tight isolated leptons, an expected signal-to-background ratio of 3.4 is achieved with a BRtt!b‘‘b‘ 0‘0 0:340:05%. We observe seven candidates with a predicted background of 1:30:5 events, yielding a cross section of 8:54:53:5stat1:81:4syst 0:5lum pb, in good agreement with the larger samples.
We present key kinematical distributions of the signal sample and find good agreement with the SM, assuming mtop175 GeV=c2. For example, using events from the
LTRK analysis, Fig. 1 shows a distribution of the previ- ously definedHTvariable. A Kolmogorov-Smirnov test of this distribution yields ap-value of 75%.
In the DIL analysis, both leptons are always identified as either an electron or a muon. In run I, seven of the nine observed events weree!, and these events populated the tail of the expectedE6 T distribution. The expected num- bers of ee,!!, ande!events for the run II DIL analysis, scaled to the 13 total observed events, are3:30:5,2:8 0:5, and 6:80:8, respectively. One ee, three !!, and ninee!events are observed in the data; theE6 T for these events is shown in Fig. 2. A Kolmogorov-Smirnov test of this distribution yields ap-value of 49%.
In summary, we have measured the tt production cross section in the dilepton channel to be 7:02:42:1stat1:61:1syst 0:4lum pb for mtop 175 GeV=c2 [20] using data from the first two years of running of the upgraded Tevatron Collider and CDF II detector. We observe good agreement between the data and the SM prediction in event yield and key kinematic distributions. The measured ttcross section agrees well with the full next-to-leading order SM prediction of 6:70:70:9 pb[2].
We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contribu- tions. 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 Bundesministerium fuer Bildung und Forschung, Germany; the Korean Science and Engineering Foundation and the Korean Research Foundation; the Particle Physics and Astronomy Research Council and Scalar Sum of Transverse Energy (GeV)
0 100 200 300 400 500 600
Events / 50 GeV
0 1 2 3 4 5 6 7 8 9
10 WW + WZ + ZZ
+ Drell-Yan + fakes
= 7.0 pb) σ ( t + t
-1) CDF II Data (197 pb
Scalar Sum of Transverse Energy (GeV)
0 100 200 300 400 500 600
Events / 50 GeV
0 1 2 3 4 5 6 7 8 9 10
FIG. 1. HT(defined in text) for events from the LTRK analy- sis with2 jets.
Missing Transverse Energy (GeV)
0 20 40 60 80 100 120 140 160 180 200
Events / 25 GeV
0 1 2 3 4 5 6
µ 2 eµµ 1
µ 3 e 1 ee 3 eµ
µ µ 1
µ µ eµ
WW + WZ + ZZ + Drell-Yan + fakes
= 8.4 pb) σ ( t + t
-1) CDF II Data (197 pb
Missing Transverse Energy (GeV)
0 20 40 60 80 100 120 140 160 180 200
Events / 25 GeV
0 1 2 3 4 5 6
FIG. 2. E6 T for events from the DIL analysis with HT>
200 GeVand2 jets.
142001-6 142001-6
the Royal Society, UK; the Russian Foundation for Basic Research; the Comision Interministerial de Ciencia y Tecnologia, Spain; in part by the European Community’s Human Potential Programme under con- tract No. HPRN-CT-20002, Probe for New Physics; by the Research Fund of Istanbul University Project No. 1755/
21122001; and by the Research Corporation.
[1] CDF Collaboration, F. Abe et al., Phys. Rev. Lett. 74, 2626 (1995); D0 Collaboration, S. Abachi et al., Phys.
Rev. Lett.74, 2632 (1995).
[2] R. Bonciani, S. Catani, M. L. Mangano, and P. Nason, Nucl. Phys. B529, 424 (1998); M. Cacciariet al., J. High Energy Phys. 04 (2004) 068.
[3] N. Kidonakis and R. Vogt, Phys. Rev. D 68, 114014 (2003).
[4] C. T. Hill and S. J. Parke, Phys. Rev. D49, 4454 (1994).
[5] H. P. Nilles, Phys. Rep. 110, 1 (1984); H. E. Haber and G. L. Kane, Phys. Rep.117, 75 (1985).
[6] D0 Collaboration, B. Abbottet al., Phys. Rev. Lett.79, 1203 (1997); CDF Collaboration, F. Abeet al., Phys. Rev.
Lett.80, 2779 (1998).
[7] S. Klimenko et al., Fermilab Report No. FERMILAB- FN-0741 2003 (unpublished); D. Acosta et al., Nucl.
Instrum. Methods Phys. Res., Sect. A494, 57 (2002).
[8] CDF II Collaboration, Fermilab Report No.
FERMILAB-PUB-96/390-E 1996 (unpublished).
[9] We use a cylindrical coordinate system about the beam axis in which " is the polar angle, is the azimuthal angle, and ln tan"=2. ETE sin" and pT psin" whereE is energy measured by the calorimeter andpis momentum measured by the spectrometer.E6 T 2P
iEiTni, whereni is the unit vector in the azimuthal plane that points from the beam line to theith calorime- ter tower.
[10] D. Acostaet al., Nucl. Instrum. Methods Phys. Res., Sect.
A461, 540 (2001).
[11] For muons, transverse momentum (not energy) is used.
[12] C. Issever, AIP Conf. Proc.670, 371 (2003).
[13] The absolute energy correction ranges from1:3at low energies to1:1at high energies.
[14] T. Sjo¨strand et al., Comput. Phys. Commun. 135, 238 (2001).
[15] M. L. Mangano et al., J. High Energy Phys. 07 (2003) 001.
[16] G. Marchesini et al., Comput. Phys. Commun. 67, 465 (1992); G. Corcellaet al., J. High Energy Phys. 01 (2001) 010.
[17] H. L. Laiet al., Eur. Phys. J. C12, 375 (2000).
[18] J. M. Campbell and R. K. Ellis, Phys. Rev. D60, 113006 (1999).
[19] The individual cross sections (DIL only, LTRK only, and overlap) range from 4–10 pbwhich aggregately yield the combined result of 7.0 pb.
[20] The dependence of the measured cross section onmtopis 0:04 pbperGeV=c2greater (less) than the assumed value of175 GeV=c2.
