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FERMILAB-PUB-07-641-E

Search for ZZ and Zγ∗ production in pp collisions at√

s = 1.96 TeV and limits on

anomalous ZZZ and ZZγ∗ couplings

V.M. Abazov36, B. Abbott76, M. Abolins66, B.S. Acharya29, M. Adams52, T. Adams50, E. Aguilo6, S.H. Ahn31,

M. Ahsan60, G.D. Alexeev36, G. Alkhazov40, A. Alton65,a, G. Alverson64, G.A. Alves2, M. Anastasoaie35,

L.S. Ancu35, T. Andeen54, S. Anderson46, B. Andrieu17, M.S. Anzelc54, Y. Arnoud14, M. Arov61, M. Arthaud18,

A. Askew50, B. Asman41, A.C.S. Assis Jesus3, O. Atramentov50, C. Autermann21, C. Avila8, C. Ay24, F. Badaud13,

A. Baden62, L. Bagby53, B. Baldin51, D.V. Bandurin60, S. Banerjee29, P. Banerjee29, E. Barberis64, A.-F. Barfuss15,

P. Bargassa81, P. Baringer59, J. Barreto2, J.F. Bartlett51, U. Bassler18, D. Bauer44, S. Beale6, A. Bean59,

M. Begalli3, M. Begel72, C. Belanger-Champagne41, L. Bellantoni51, A. Bellavance51, J.A. Benitez66, S.B. Beri27,

G. Bernardi17, R. Bernhard23, I. Bertram43, M. Besancon18, R. Beuselinck44, V.A. Bezzubov39, P.C. Bhat51,

V. Bhatnagar27, C. Biscarat20, G. Blazey53, F. Blekman44, S. Blessing50, D. Bloch19, K. Bloom68, A. Boehnlein51,

D. Boline63, T.A. Bolton60, G. Borissov43, T. Bose78, A. Brandt79, R. Brock66, G. Brooijmans71, A. Bross51,

D. Brown82, N.J. Buchanan50, D. Buchholz54, M. Buehler82, V. Buescher22, V. Bunichev38, S. Burdin43,b,

S. Burke46, T.H. Burnett83, C.P. Buszello44, J.M. Butler63, P. Calfayan25, S. Calvet16, J. Cammin72, W. Carvalho3,

B.C.K. Casey51, N.M. Cason56, H. Castilla-Valdez33, S. Chakrabarti18, D. Chakraborty53, K.M. Chan56, K. Chan6,

A. Chandra49, F. Charles19,‡, E. Cheu46, F. Chevallier14, D.K. Cho63, S. Choi32, B. Choudhary28, L. Christofek78,

T. Christoudias44,†, S. Cihangir51, D. Claes68, Y. Coadou6, M. Cooke81, W.E. Cooper51, M. Corcoran81,

F. Couderc18, M.-C. Cousinou15, S. Crepe-Renaudin14, D. Cutts78, M. Cwiok30, H. da Motta2, A. Das46,

G. Davies44, K. De79, S.J. de Jong35, E. De La Cruz-Burelo65, C. De Oliveira Martins3, J.D. Degenhardt65,

F. Deliot18, M. Demarteau51, R. Demina72, D. Denisov51, S.P. Denisov39, S. Desai51, H.T. Diehl51, M. Diesburg51,

A. Dominguez68, H. Dong73, L.V. Dudko38, L. Duflot16, S.R. Dugad29, D. Duggan50, A. Duperrin15, J. Dyer66,

A. Dyshkant53, M. Eads68, D. Edmunds66, J. Ellison49, V.D. Elvira51, Y. Enari78, S. Eno62, P. Ermolov38,

H. Evans55, A. Evdokimov74, V.N. Evdokimov39, A.V. Ferapontov60, T. Ferbel72, F. Fiedler24, F. Filthaut35,

W. Fisher51, H.E. Fisk51, M. Ford45, M. Fortner53, H. Fox23, S. Fu51, S. Fuess51, T. Gadfort83, C.F. Galea35,

E. Gallas51, E. Galyaev56, C. Garcia72, A. Garcia-Bellido83, V. Gavrilov37, P. Gay13, W. Geist19, D. Gele19,

C.E. Gerber52, Y. Gershtein50, D. Gillberg6, G. Ginther72, N. Gollub41, B. Gomez8, A. Goussiou56, P.D. Grannis73,

H. Greenlee51, Z.D. Greenwood61, E.M. Gregores4, G. Grenier20, Ph. Gris13, J.-F. Grivaz16, A. Grohsjean25,

S. Grunendahl51, M.W. Grunewald30, J. Guo73, F. Guo73, P. Gutierrez76, G. Gutierrez51, A. Haas71, N.J. Hadley62,

P. Haefner25, S. Hagopian50, J. Haley69, I. Hall66, R.E. Hall48, L. Han7, K. Hanagaki51, P. Hansson41, K. Harder45,

A. Harel72, R. Harrington64, J.M. Hauptman58, R. Hauser66, J. Hays44, T. Hebbeker21, D. Hedin53, J.G. Hegeman34,

J.M. Heinmiller52, A.P. Heinson49, U. Heintz63, C. Hensel59, K. Herner73, G. Hesketh64, M.D. Hildreth56,

R. Hirosky82, J.D. Hobbs73, B. Hoeneisen12, H. Hoeth26, M. Hohlfeld22, S.J. Hong31, S. Hossain76, P. Houben34,

Y. Hu73, Z. Hubacek10, V. Hynek9, I. Iashvili70, R. Illingworth51, A.S. Ito51, S. Jabeen63, M. Jaffre16, S. Jain76,

K. Jakobs23, C. Jarvis62, R. Jesik44, K. Johns46, C. Johnson71, M. Johnson51, A. Jonckheere51, P. Jonsson44,

A. Juste51, D. Kafer21, E. Kajfasz15, A.M. Kalinin36, J.R. Kalk66, J.M. Kalk61, S. Kappler21, D. Karmanov38,

P. Kasper51, I. Katsanos71, D. Kau50, R. Kaur27, V. Kaushik79, R. Kehoe80, S. Kermiche15, N. Khalatyan51,

A. Khanov77, A. Kharchilava70, Y.M. Kharzheev36, D. Khatidze71, H. Kim32, T.J. Kim31, M.H. Kirby54,

M. Kirsch21, B. Klima51, J.M. Kohli27, J.-P. Konrath23, M. Kopal76, V.M. Korablev39, A.V. Kozelov39, D. Krop55,

T. Kuhl24, A. Kumar70, S. Kunori62, A. Kupco11, T. Kurca20, J. Kvita9, F. Lacroix13, D. Lam56, S. Lammers71,

G. Landsberg78, P. Lebrun20, W.M. Lee51, A. Leflat38, F. Lehner42, J. Lellouch17, J. Leveque46, P. Lewis44, J. Li79,

Q.Z. Li51, L. Li49, S.M. Lietti5, J.G.R. Lima53, D. Lincoln51, J. Linnemann66, V.V. Lipaev39, R. Lipton51, Y. Liu7,†,

Z. Liu6, L. Lobo44, A. Lobodenko40, M. Lokajicek11, P. Love43, H.J. Lubatti83, A.L. Lyon51, A.K.A. Maciel2,

D. Mackin81, R.J. Madaras47, P. Mattig26, C. Magass21, A. Magerkurth65, P.K. Mal56, H.B. Malbouisson3,

S. Malik68, V.L. Malyshev36, H.S. Mao51, Y. Maravin60, B. Martin14, R. McCarthy73, A. Melnitchouk67,

A. Mendes15, L. Mendoza8, P.G. Mercadante5, M. Merkin38, K.W. Merritt51, J. Meyer22,d, A. Meyer21, T. Millet20,

J. Mitrevski71, J. Molina3, R.K. Mommsen45, N.K. Mondal29, R.W. Moore6, T. Moulik59, G.S. Muanza20,

M. Mulders51, M. Mulhearn71, O. Mundal22, L. Mundim3, E. Nagy15, M. Naimuddin51, M. Narain78,

N.A. Naumann35, H.A. Neal65, J.P. Negret8, P. Neustroev40, H. Nilsen23, H. Nogima3, A. Nomerotski51,

S.F. Novaes5, T. Nunnemann25, V. O’Dell51, D.C. O’Neil6, G. Obrant40, C. Ochando16, D. Onoprienko60,

2

N. Oshima51, J. Osta56, R. Otec10, G.J. Otero y Garzon51, M. Owen45, P. Padley81, M. Pangilinan78, N. Parashar57,

S.-J. Park72, S.K. Park31, J. Parsons71, R. Partridge78, N. Parua55, A. Patwa74, G. Pawloski81, B. Penning23,

M. Perfilov38, K. Peters45, Y. Peters26, P. Petroff16, M. Petteni44, R. Piegaia1, J. Piper66, M.-A. Pleier22,

P.L.M. Podesta-Lerma33,c, V.M. Podstavkov51, Y. Pogorelov56, M.-E. Pol2, P. Polozov37, B.G. Pope66,

A.V. Popov39, C. Potter6, W.L. Prado da Silva3, H.B. Prosper50, S. Protopopescu74, J. Qian65, A. Quadt22,d,

B. Quinn67, A. Rakitine43, M.S. Rangel2, K. Ranjan28, P.N. Ratoff43, P. Renkel80, S. Reucroft64, P. Rich45,

M. Rijssenbeek73, I. Ripp-Baudot19, F. Rizatdinova77, S. Robinson44, R.F. Rodrigues3, M. Rominsky76, C. Royon18,

P. Rubinov51, R. Ruchti56, G. Safronov37, G. Sajot14, A. Sanchez-Hernandez33, M.P. Sanders17, A. Santoro3,

G. Savage51, L. Sawyer61, T. Scanlon44, D. Schaile25, R.D. Schamberger73, Y. Scheglov40, H. Schellman54,

P. Schieferdecker25, T. Schliephake26, C. Schwanenberger45, A. Schwartzman69, R. Schwienhorst66, J. Sekaric50,

H. Severini76, E. Shabalina52, M. Shamim60, V. Shary18, A.A. Shchukin39, R.K. Shivpuri28, V. Siccardi19,

V. Simak10, V. Sirotenko51, P. Skubic76, P. Slattery72, D. Smirnov56, J. Snow75, G.R. Snow68, S. Snyder74,

S. Soldner-Rembold45, L. Sonnenschein17, A. Sopczak43, M. Sosebee79, K. Soustruznik9, M. Souza2, B. Spurlock79,

J. Stark14, J. Steele61, V. Stolin37, D.A. Stoyanova39, J. Strandberg65, S. Strandberg41, M.A. Strang70,

M. Strauss76, E. Strauss73, R. Strohmer25, D. Strom54, L. Stutte51, S. Sumowidagdo50, P. Svoisky56, A. Sznajder3,

M. Talby15, P. Tamburello46, A. Tanasijczuk1, W. Taylor6, J. Temple46, B. Tiller25, F. Tissandier13, M. Titov18,

V.V. Tokmenin36, T. Toole62, I. Torchiani23, T. Trefzger24, D. Tsybychev73, B. Tuchming18, C. Tully69, P.M. Tuts71,

R. Unalan66, S. Uvarov40, L. Uvarov40, S. Uzunyan53, B. Vachon6, P.J. van den Berg34, R. Van Kooten55,

W.M. van Leeuwen34, N. Varelas52, E.W. Varnes46, I.A. Vasilyev39, M. Vaupel26, P. Verdier20, L.S. Vertogradov36,

M. Verzocchi51, F. Villeneuve-Seguier44, P. Vint44, P. Vokac10, E. Von Toerne60, M. Voutilainen68,e, R. Wagner69,

H.D. Wahl50, L. Wang62, M.H.L.S Wang51, J. Warchol56, G. Watts83, M. Wayne56, M. Weber51, G. Weber24,

A. Wenger23,f , N. Wermes22, M. Wetstein62, A. White79, D. Wicke26, G.W. Wilson59, S.J. Wimpenny49,

M. Wobisch61, D.R. Wood64, T.R. Wyatt45, Y. Xie78, S. Yacoob54, R. Yamada51, M. Yan62, T. Yasuda51,

Y.A. Yatsunenko36, K. Yip74, H.D. Yoo78, S.W. Youn54, J. Yu79, A. Zatserklyaniy53, C. Zeitnitz26, T. Zhao83,

B. Zhou65, J. Zhu73, M. Zielinski72, D. Zieminska55, A. Zieminski55,‡, L. Zivkovic71, V. Zutshi53, and E.G. Zverev38

(The DØ Collaboration)

1Universidad de Buenos Aires, Buenos Aires, Argentina2LAFEX, Centro Brasileiro de Pesquisas Fısicas, Rio de Janeiro, Brazil

3Universidade do Estado do Rio de Janeiro, Rio de Janeiro, Brazil4Universidade Federal do ABC, Santo Andre, Brazil

5Instituto de Fısica Teorica, Universidade Estadual Paulista, Sao Paulo, Brazil6University of Alberta, Edmonton, Alberta, Canada,Simon Fraser University, Burnaby, British Columbia,Canada, York University, Toronto, Ontario, Canada,and McGill University, Montreal, Quebec, Canada

7University of Science and Technology of China, Hefei, People’s Republic of China8Universidad de los Andes, Bogota, Colombia

9Center for Particle Physics, Charles University, Prague, Czech Republic10Czech Technical University, Prague, Czech Republic

11Center for Particle Physics, Institute of Physics,Academy of Sciences of the Czech Republic, Prague, Czech Republic

12Universidad San Francisco de Quito, Quito, Ecuador13Laboratoire de Physique Corpusculaire, IN2P3-CNRS,

Universite Blaise Pascal, Clermont-Ferrand, France14Laboratoire de Physique Subatomique et de Cosmologie,IN2P3-CNRS, Universite de Grenoble 1, Grenoble, France

15CPPM, IN2P3-CNRS, Universite de la Mediterranee, Marseille, France16Laboratoire de l’Accelerateur Lineaire, IN2P3-CNRS et Universite Paris-Sud, Orsay, France

17LPNHE, IN2P3-CNRS, Universites Paris VI and VII, Paris, France18DAPNIA/Service de Physique des Particules, CEA, Saclay, France

19IPHC, Universite Louis Pasteur et Universite de Haute Alsace, CNRS, IN2P3, Strasbourg, France20IPNL, Universite Lyon 1, CNRS/IN2P3, Villeurbanne, France and Universite de Lyon, Lyon, France

21III. Physikalisches Institut A, RWTH Aachen, Aachen, Germany22Physikalisches Institut, Universitat Bonn, Bonn, Germany

23Physikalisches Institut, Universitat Freiburg, Freiburg, Germany24Institut fur Physik, Universitat Mainz, Mainz, Germany

3

25Ludwig-Maximilians-Universitat Munchen, Munchen, Germany26Fachbereich Physik, University of Wuppertal, Wuppertal, Germany

27Panjab University, Chandigarh, India28Delhi University, Delhi, India

29Tata Institute of Fundamental Research, Mumbai, India30University College Dublin, Dublin, Ireland

31Korea Detector Laboratory, Korea University, Seoul, Korea32SungKyunKwan University, Suwon, Korea

33CINVESTAV, Mexico City, Mexico34FOM-Institute NIKHEF and University of Amsterdam/NIKHEF, Amsterdam, The Netherlands

35Radboud University Nijmegen/NIKHEF, Nijmegen, The Netherlands36Joint Institute for Nuclear Research, Dubna, Russia

37Institute for Theoretical and Experimental Physics, Moscow, Russia38Moscow State University, Moscow, Russia

39Institute for High Energy Physics, Protvino, Russia40Petersburg Nuclear Physics Institute, St. Petersburg, Russia

41Lund University, Lund, Sweden, Royal Institute of Technology and Stockholm University,Stockholm, Sweden, and Uppsala University, Uppsala, Sweden42Physik Institut der Universitat Zurich, Zurich, Switzerland

43Lancaster University, Lancaster, United Kingdom44Imperial College, London, United Kingdom

45University of Manchester, Manchester, United Kingdom46University of Arizona, Tucson, Arizona 85721, USA

47Lawrence Berkeley National Laboratory and University of California, Berkeley, California 94720, USA48California State University, Fresno, California 93740, USA49University of California, Riverside, California 92521, USA50Florida State University, Tallahassee, Florida 32306, USA

51Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA52University of Illinois at Chicago, Chicago, Illinois 60607, USA

53Northern Illinois University, DeKalb, Illinois 60115, USA54Northwestern University, Evanston, Illinois 60208, USA55Indiana University, Bloomington, Indiana 47405, USA

56University of Notre Dame, Notre Dame, Indiana 46556, USA57Purdue University Calumet, Hammond, Indiana 46323, USA

58Iowa State University, Ames, Iowa 50011, USA59University of Kansas, Lawrence, Kansas 66045, USA

60Kansas State University, Manhattan, Kansas 66506, USA61Louisiana Tech University, Ruston, Louisiana 71272, USA

62University of Maryland, College Park, Maryland 20742, USA63Boston University, Boston, Massachusetts 02215, USA

64Northeastern University, Boston, Massachusetts 02115, USA65University of Michigan, Ann Arbor, Michigan 48109, USA

66Michigan State University, East Lansing, Michigan 48824, USA67University of Mississippi, University, Mississippi 38677, USA

68University of Nebraska, Lincoln, Nebraska 68588, USA69Princeton University, Princeton, New Jersey 08544, USA

70State University of New York, Buffalo, New York 14260, USA71Columbia University, New York, New York 10027, USA

72University of Rochester, Rochester, New York 14627, USA73State University of New York, Stony Brook, New York 11794, USA

74Brookhaven National Laboratory, Upton, New York 11973, USA75Langston University, Langston, Oklahoma 73050, USA

76University of Oklahoma, Norman, Oklahoma 73019, USA77Oklahoma State University, Stillwater, Oklahoma 74078, USA

78Brown University, Providence, Rhode Island 02912, USA79University of Texas, Arlington, Texas 76019, USA

80Southern Methodist University, Dallas, Texas 75275, USA81Rice University, Houston, Texas 77005, USA

82University of Virginia, Charlottesville, Virginia 22901, USA and83University of Washington, Seattle, Washington 98195, USA

(Dated: December 3, 2007)

We present a study of µµµµ, eeee, and µµee events using 1 fb−1 of data collected with theD0 detector at the Fermilab Tevatron pp Collider at

√s = 1.96 TeV. Requiring the lepton pair

4

masses to be greater than 30 GeV, we observe one event, consistent with the expected backgroundof 0.13± 0.03 events and with the predicted standard model ZZ and Zγ∗ production of 1.71± 0.15events. We set an upper limit on the ZZ and Zγ∗ cross section of 4.4 pb at the 95% C.L. We alsoderive limits on anomalous neutral trilinear ZZZ and ZZγ∗ gauge couplings. The one-parameter95% C.L. coupling limits with a form factor scale Λ = 1.2 TeV are −0.28 < fZ

40 < 0.28, −0.31 <fZ50 < 0.29, −0.26 < fγ

40< 0.26, and −0.30 < fγ

50< 0.28.

PACS numbers: 12.15.Ji, 13.40.Em, 13.85.Qk

The standard model (SM) makes precise predictionsfor the couplings between gauge bosons based on thenon-Abelian symmetries of the model. These predictionscan be tested by studying di-boson production (WW ,WZ, ZZ, Zγ, and Wγ) at particle colliders. ZZ produc-tion is predicted to have the smallest cross section amongthe di-boson processes. Because the decay channels withthe smallest expected background also have the smallestbranching fractions, it has not been observed at a hadroncollider. Nevertheless, the final states with small back-grounds may provide the best opportunity to observe theeffects of physics beyond the SM. In addition to the pro-duction of new particles that could decay into ZZ, vari-ous extensions of the SM predict large anomalous valuesof the trilinear couplings [1] ZZZ and ZZγ∗ that wouldresult in higher cross sections than the SM prediction.The direct ZZγ∗ and ZZZ couplings are zero in the SM.Consequently, an observation of an enhancement of thecross section would indicate physics beyond the SM.

Previous studies of ZZ production were made at theLEP electron-positron collider. The combined LEP re-sults are available in Ref. [2]. All results are consistentwith SM predictions. The CDF Collaboration searchedfor ZZ and WZ production in pp collisions at center-of-mass energy

√s = 1.96 TeV with the result that

σ(ZZ) + σ(WZ) < 15.2 pb at 95% C.L. [3] The pre-dicted ZZ production cross section in the SM is 1.6±0.1pb at the Tevatron Collider energy [1, 4].

In this Letter, we present a search for ZZ and Zγ∗ pro-duction and a search for anomalous trilinear ZZγ∗ andZZZ couplings at the Fermilab Tevatron Collider withthe D0 detector. We follow the framework of Ref. [1],where general ZZV (V = Z, γ) gauge and Lorentz-invariant interactions are considered. Such ZZV cou-plings can be parameterized by two CP-violating (fV

4 )and two CP-conserving (fV

5 ) complex parameters. Ad-ditional anomalous couplings [5] can contribute whenthe Z bosons are off-shell. However, these couplingsare highly suppressed near the Z boson resonance. Wenote that the ZZV couplings are distinct [1] from theZγV couplings probed in Zγ production in e+e− andhadronic collisions. Partial-wave unitarity is ensured byusing a form-factor parameterization that causes the cou-pling to vanish at high parton center-of-mass energy

√s:

fVi = fV

i0/(1+ s/Λ2)n. Here, Λ is a form-factor scale, fVi0

are the low-energy approximations of the couplings, andn is the form-factor power. In accordance with Ref. [1],

we set n = 3 for all cases. The form-factor scale Λ isselected so that limits are within the values provided byunitarity at Tevatron Collider energies.

We search for ZZ and Zγ∗ production with a finalstate signature that consists of four leptons: two pairs ofeither electrons or muons. The electron and muon pairscan be produced either by the decay of an on-shell Z bo-son or via a virtual Z boson or photon. We studied threefinal states: four muon (µµµµ), four electron (eeee), andtwo muons and two electrons (µµee).

Data used in this analysis [6] were collected with theD0 detector at the Fermilab Tevatron pp collider at√

s = 1.96 TeV between October 2002 and February2006. The integrated luminosities [7] of the three chan-nels are approximately 1 fb−1 each and are shown in Ta-ble I.

The D0 detector [8] is a multi-purpose detector de-signed to operate at high luminosity. The main com-ponents of the detector are an inner tracker, a liquid-argon/uranium calorimeter, and a muon system. The in-ner tracker consists of a silicon microstrip tracker (SMT)and a central fiber tracker (CFT) operating in a 2 Teslasolenoidal magnetic field. The SMT has coverage topseudorapidity [9] |η| ≈ 3.0, while the CFT has cov-erage to |η| ≈ 1.8. The calorimeter system is outsidethe solenoid and has three cryostats, one for each of thetwo endcap calorimeters (EC) and one for the centralcalorimeter (CC). The CC covers the region |η| < 1.1,while endcap calorimeters extend the coverage to |η| ≈ 4.The calorimeters are segmented along the shower direc-tion with four layers forming the electromagnetic section(EM) and an additional three to five layers forming thehadronic section. This allows for electron-pion separationbased on longitudinal and transverse shower shape. Themuon system is outside of the calorimeters and consistsof 1.8 Tesla iron toroid magnets with tracking chambersand scintillator counters mounted both inside and outsidethe toroid iron. The muon system extends to |η| ≈2.

The D0 detector utilizes a three-level (L1, L2, and L3)trigger system. Electrons candidates are required to havedeposited energy in the EM section of the calorimeter atL1 and L2, and then to satisfy selection criteria on showershape and the fraction of energy in the EM calorimeterat L3. Muon candidates are required to have hits in themuon scintillation counters and a match with a track inthe L1 track system. In a fraction of the data, muons arealso required to have hits in the chambers of the muon

5

system. At L2, muon candidates are required to havetrack segments in the muon tracking detectors, and aminimum transverse momentum (pT ) is required. At L3,some muons are also required to have a match with atrack from the inner tracker.

Combinations of single-electron and di-electron, as wellas single-muon triggers are used in this analysis. Thesetriggers have varying pT and quality requirements on theleptons. The trigger efficiency for events with four high-pT leptons that satisfy all other selection requirementsis approximately 99%. In order to increase the collec-tion efficiency, we do not require pairs of leptons to haveopposite electric charge.

For the µµµµ channel, each muon is required to satisfyquality criteria based on scintillator and wire chamber in-formation from the muon system and to have a matchingtrack in the inner tracker. The track matched to themuon must have pT > 15 GeV. Additionally, muons thatare only identified in the muon detector layers before thetoroid are required to have less than 2.5 GeV of totaltransverse energy deposited in the calorimeter in an an-nulus between R = 0.1 and R = 0.4 centered around thetrack matched to the muon [10]. Finally, the distance inthe transverse plane of closest approach to the beamspotfor the track matched to the muon must be less than0.02 cm for tracks with SMT hits and less than 0.2 cm fortracks without SMT hits. This reduces the backgroundfrom muons that do not originate from the primary ver-tex such as those from b decays and cosmic rays. Themaximum distance between the muon track vertices forall muon pairs along the beam axis is required to be lessthan 3 cm. This reduces backgrounds from pairs of cos-mic ray muons and from beam halo. Since the charge ofthe muons is not considered, three possible Z/γ∗ com-binations can be formed for each µµµµ event. Selectedevents are required to have the invariant masses of bothmuon pairs above 30 GeV for at least one of the combi-nations.

For the eeee channel, each electron is required to havetransverse energy (ET ) greater than 15 GeV, and to have|η| < 1.1 or 1.5 < |η| < 3.2, and to be isolated from otherenergy clusters. Electrons are also required to satisfyidentification criteria based on multivariate discrimina-tors derived from calorimeter shower shape and trackingvariables. Since the calorimeter fiducial region covers re-gions where there is little or no tracking coverage, onlythree of the four electrons are required to have an asso-ciated inner track. Electrons without a track match arerequired to satisfy more stringent shower shape require-ments. Similar to the µµµµ channel, events are requiredto have the invariant masses of both electron pairs tobe above 30 GeV for at least one of the three possiblecombinations.

For the µµee channel, muons are required to satisfy thesame single muon selection criteria as in the µµµµ chan-nel and electrons the same transverse energy and η cri-

teria as in the eeee channel. Both electrons are requiredto satisfy the multivariate discriminator based on showershape and parameters of the matching inner track. In ad-dition, electrons and muons are required to be separatedby R > 0.2 to reduce backgrounds from muons that haveradiated photons spatially coincident with a track. Theinvariant masses of the muon pair and the electron pairare required to be greater than 30 GeV.

We determine the total acceptance using a combina-tion of information from MC and the data. We deter-mine single electron and muon identification efficienciesdirectly from data [11], which contains high pT electronsand muons from more than 100,000 single Z decays ineach channel. Efficiencies are parameterized as functionsof the relevant variables for the tracking, calorimeter andmuon systems such as the position of the interactionalong the beam direction and the η of the electron ormuon. The acceptance for all channels is then determinedusing ZZ and Zγ∗ events generated with pythia [12]and a parameterized simulation of the detector. The mo-menta of the muons and electrons are fluctuated in theMonte Carlo (MC) using angular and energy resolutionsdetermined from the data, and the measured single elec-tron and muon identification efficiencies are taken intoaccount in calculating the acceptance. The number ofobserved events, the acceptance, and the expected signalcalculated assuming the cross section is 1.6 pb are listedin Table I for each channel. Systematic uncertainties inacceptance due to momentum and energy scale calibra-tions, angular resolution, lepton identification variationwith luminosity as well as other effects were included.Taking into account correlations between these uncer-tainties, a total of 1.71 ± 0.15 ZZ and Zγ∗ events isexpected.

The main background sources are “Z + multi-jet”events, events where a top-antitop (tt) quark pair isproduced, and “combinatoric events”, which are four-lepton events that survive because mis-paired leptonscause them to satisfy the dilepton invariant mass selec-tion criteria when they would not have otherwise.

For a “Z + multi-jet” event to be a ZZ candidate,the jets are either mis-identified as electrons or muons orcontain real electrons or muons from in-flight decays ofpions, kaons, or heavy quarks. Though Z + jets (and γ∗

+ jets) events are the primary source of this background,it also includes those events where the Z boson or γ∗ isreconstructed from one or more “fake” leptons. The “Z+ multi-jet” event background is measured by first de-termining the probability for a jet in data to produce anelectron or muon that satisfies the identification criteria.The probability for a jet to mimic a muon was measuredin two-jet events selected via jet triggers. Muons thatsatisfy the selection criteria, have pT > 15 GeV, and arefound within R < 0.2 of the lower-ET jet are considered“fake”. The probability for a jet to mimic a muon is pa-rameterized in terms of the tag-jet ET . It is 10−4 for jets

6

Decay Channel Luminosity (pb−1) Acceptance Expected Signal Observed Eventsµµµµ 944 ± 58 0.27 ± 0.02 0.46 ± 0.05 0eeee 1070 ± 65 0.23 ± 0.01 0.44 ± 0.03 0µµee 1020 ± 62 0.22 ± 0.02 0.81 ± 0.09 1

TABLE I: The integrated luminosity, acceptance (including lepton identification efficiencies), expected number of signal can-didates, and the number of observed events in the three decay channels for the ZZ and Zγ∗ cross section analysis. A total of1.71 ± 0.15 ZZ and Zγ∗ events is expected.

with ET ≈ 15 GeV and increases approximately linearlywith ET to 10−2 for jets with ET ≈ 150 GeV. The prob-ability for a jet to mimic an electron was measured usinga procedure similar to that described for muons. It isparameterized in terms of η and ET to account for dif-ferences in the EC and CC and is typically 10−4 (10−2)per jet for those electrons with (without) a track match.A systematic uncertainty of 30% is assigned to accountfor variations in the probabilities as a result of changesof the lepton identification criteria that were performedas cross checks. The probabilities for jets to be misiden-tified as muons are then applied to the jets in our µµ +jets data to determine the background to µµµµ. Simi-larly appropriate probabilities are applied to ℓℓℓ + jet(s)events to determine the background to eeee and µµee.There, because we started from a three lepton sample,we correctly account for events with two real leptons, aphoton misidentified as an electron, and a jet misidenti-fied as either an electron or muon.

The background from tt events is determined frompythia MC using a detailed geant-based [13] detec-tor simulation program and the same reconstruction pro-gram that was used for the data.

The combinatoric background occurs only in the four-electron and four-muon decay channels. While it couldbe reduced by ∼ 1/3 by requiring leptons which form Zbosons to have opposite signs, that would have resultedin a loss of signal efficiency for the high-pT leptons thatcould result from anomalous couplings. This backgroundwas estimated using MC simulation. It is 0.016 ± 0.003(0.015 ± 0.003) events in the µµµµ (eeee) channel; theuncertainty in the background comes from uncertaintyin the lepton pT resolution.

Table II displays the contributions of all non-negligiblebackgrounds and a summary for each decay channel. Thetotal expected number of candidates from backgroundsources is 0.13 ± 0.03 events.

One event is observed in data, consistent with theSM prediction of 1.71 ± 0.15 events plus background of0.13 ± 0.03 events. We set an upper limit of 4.4 pb atthe 95% C.L. on the cross section for pp → ZZ + X andZγ∗ +X , where dilepton pair masses are greater than 30GeV.

Because we do not observe an excess of events, we setlimits on anomalous trilinear couplings by comparing thenumber of observed candidates with the predicted back-

Background µµµµ eeee µµee

Z + multi-jet 0.004 ± 0.001 0.065 ± 0.021 0.007 ± 0.002tt 0.010 ± 0.005 - 0.006 ± 0.003

Combinatoric 0.016 ± 0.003 0.015 ± 0.003 -Beam Halo 0.003 ± 0.001 - -

Total 0.033 ± 0.006 0.080 ± 0.021 0.013 ± 0.004

TABLE II: Contributions of non-negligible backgrounds tothe expected number of candidates in the three decay channelsfor the cross section analysis. The total expected backgroundis 0.13 ± 0.03 events.

ground and the expected sum of ZZ events from a MCprogram [1] using anomalous ZZγ∗ and ZZZ couplings.We produced grids of MC samples by varying two of theanomalous couplings at a time. There are typically 5000events generated at each grid point. These events areprocessed through the same detector and reconstructionsimulation as was used in the cross section analysis. Be-cause on-shell Z bosons dominate the contributions ofthe ZZV couplings in the MC and because off-shell Zbosons are enhanced by couplings [5] not implementedin the MC, we required the dilepton invariant mass tobe > 50 (70) GeV for dimuons (dielectrons). This selec-tion criterion is set by the resolution of the dilepton massmeasurement and it removed the single µµee candidateevent.

The number of events expected for each choice ofanomalous couplings was used to form a likelihood [6]for that point. Poisson probabilities were used for theexpected signal plus background. These are convo-luted with Gaussian uncertainties on the acceptance,background, and luminosity. One-dimensional (1D) andtwo-dimensional (2D) limits on anomalous couplings areformed by finding the coupling values with likelihood of1.92 and 3.00 units greater than the minimum. Limitsare determined using Λ = 1.2 TeV. The 95% C.L. 1Dlimits are −0.28 < fZ

40 < 0.28, −0.26 < fγ40 < 0.26,

−0.31 < fZ50 < 0.29, and −0.30 < fγ

50 < 0.28. The 95%C.L. 2D contours fγ

40 vs. fZ40, fγ

40 vs. fγ50, fZ

40 vs. fZ50,

and fγ50 vs. fZ

50 are shown in Figure 1. In the four 2Dcases the other two couplings are assumed to be zero.

In summary, we report results from a search for ZZand Zγ∗ production using the eeee, µµµµ and µµee de-cay signatures. We analyzed 1 fb−1 of data collected withthe D0 detector at the Fermilab Tevatron pp Collider at

7

γ40f

-0.4 -0.2 0 0.2 0.4

Z 40f

-0.4

-0.2

0

0.2

0.4(a)

-1D0, 1 fb

γ40f

-0.4 -0.2 0 0.2 0.4

γ 50f

-0.4

-0.2

0

0.2

0.4(b)

-1D0, 1 fb

Z40f

-0.4 -0.2 0 0.2 0.4

Z 50f

-0.4

-0.2

0

0.2

0.4(c)

-1D0, 1 fb

γ50f

-0.4 -0.2 0 0.2 0.4

Z 50f

-0.4

-0.2

0

0.2

0.4(d)

-1D0, 1 fb

FIG. 1: Limits on anomalous couplings for Λ = 1.2 TeV: (a)fγ40

vs. fZ40, (b) fγ

40vs. fγ

50, (c) fZ

40 vs. fZ50, and (d) fγ

50

vs. fZ50, assuming in each case that the other two couplings

are zero. The inner and outer curves are the 95% C.L. two-degree of freedom exclusion contour and the constraint fromthe unitarity condition, respectively. The inner crosshairs arethe 95% C.L. one-degree of freedom exclusion limits.

√s = 1.96 TeV. Requiring each lepton to have transverse

momentum greater than 15 GeV, and the dilepton pairmasses to be greater than 30 GeV, we observe one eventwith an expected SM background of 0.13 ± 0.03 events.The one observed event is consistent both with back-ground and with predicted SM ZZ and Zγ∗ productionof 1.71± 0.15 events. We set an upper limit of 4.4 pb atthe 95% C.L. on the cross section for pp → ZZ + X andZγ∗ + X , where dilepton pair masses are greater than30 GeV. This is the most restrictive cross section limitfor ZZ production at the Tevatron. We set limits onanomalous neutral trilinear ZZZ and ZZγ∗ gauge cou-plings. These represent the first bounds on these anoma-lous couplings from the Tevatron. Limits on fZ

40, fZ50, and

fγ50 are more restrictive than those of the combined LEP

experiments [2].We thank the staffs at Fermilab and collaborating in-

stitutions, and acknowledge support from the DOE andNSF (USA); CEA and CNRS/IN2P3 (France); FASI,Rosatom and RFBR (Russia); CAPES, CNPq, FAPERJ,

FAPESP and FUNDUNESP (Brazil); DAE and DST(India); Colciencias (Colombia); CONACyT (Mexico);KRF and KOSEF (Korea); CONICET and UBACyT(Argentina); FOM (The Netherlands); Science and Tech-nology Facilities Council (United Kingdom); MSMT andGACR (Czech Republic); CRC Program, CFI, NSERCand WestGrid Project (Canada); BMBF and DFG (Ger-many); SFI (Ireland); The Swedish Research Council(Sweden); CAS and CNSF (China); Alexander von Hum-boldt Foundation; and the Marie Curie Program.

[a] Visitor from Augustana College, Sioux Falls, SD, USA.[b] Visitor from The University of Liverpool, Liverpool, UK.[c] Visitor from ICN-UNAM, Mexico City, Mexico.[d] Visitor from II. Physikalisches Institut, Georg-August-

University Gottingen, Germany.[e] Visitor from Helsinki Institute of Physics, Helsinki, Fin-

land.[f] Visitor from Universitat Zurich, Zurich, Switzerland.[†] Fermilab International Fellow.[‡] Deceased.

[1] U. Baur and D. Rainwater, Phys. Rev. D 62, 113011(2000).

[2] The LEP Collaborations: ALEPH, DELPHI, L3, andOPAL, “A Combination of Preliminary ElectroweakMeasurements and Constraints on the Standard Model”,arXiv:hep-ex/0612034.

[3] CDF Collaboration, D. Acosta et al., Phys. Rev. D 71,091105(R) (2005).

[4] J. M. Campbell and R. K. Ellis, Phys. Rev. D 60, 072002(1999).

[5] G. J. Gounaris, J. Layssac, and F. M. Renard, Phys. Rev.D 62, 073012 (2000).

[6] C. Jarvis, Ph. D thesis, University of Maryland, 2007(unpublished).

[7] T. Andeen et al., FERMILAB-TM-2365 (2007).[8] DØ Collaboration, V. Abazov et al., Nucl. Instrum.

Methods A 565, 463 (2006).[9] The D0 coordinate system is cylindrical with the z-axis

along the proton beamline and the polar and azimuthalangles denoted as θ and φ respectively. The pseudorapid-ity is defined as η = − ln tan(θ/2).

[10] The variable R between two objects i and j is defined as

R =p

(ηi − ηj)2 + (φi − φj)2.[11] DØ Collaboration, V. Abazov et al., Phys. Rev. D 76,

052006 (2007).[12] T. Sjostrand et al., Comp. Phys. Commun. 135, 238

(2001).[13] R. Brun and F. Carminati, CERN Program Library Long

Writeup W5013, 1993 (unpublished).