Study Z gamma events and limits on anomalous ZZ gamma and Z gamma gamma couplings in pp(-) collisions at root s=1.96 Tev

Study of

Z

Events and Limits on Anomalous

ZZ

and

Z

Couplings

in

p

_p

_{Collisions at}

s

p

1

_:

96 TeV

V. M. Abazov,35B. Abbott,72M. Abolins,63B. S. Acharya,29M. Adams,50T. Adams,48M. Agelou,18J.-L. Agram,19 S. H. Ahn,31M. Ahsan,57G. D. Alexeev,35G. Alkhazov,39A. Alton,62G. Alverson,61G. A. Alves,2M. Anastasoaie,34 T. Andeen,52S. Anderson,44B. Andrieu,17Y. Arnoud,14A. Askew,48B. A˚ sman,40A. C. S. Assis Jesus,3O. Atramentov,55 C. Autermann,21C. Avila,8F. Badaud,13A. Baden,59B. Baldin,49P. W. Balm,33S. Banerjee,29E. Barberis,61P. Bargassa,76 P. Baringer,56C. Barnes,42J. Barreto,2J. F. Bartlett,49U. Bassler,17D. Bauer,53A. Bean,56S. Beauceron,17M. Begel,68

A. Bellavance,65S. B. Beri,27G. Bernardi,17R. Bernhard,49,* I. Bertram,41M. Besanc¸on,18R. Beuselinck,42 V. A. Bezzubov,38P. C. Bhat,49V. Bhatnagar,27M. Binder,25C. Biscarat,41K. M. Black,60I. Blackler,42G. Blazey,51

F. Blekman,33S. Blessing,48D. Bloch,19U. Blumenschein,23A. Boehnlein,49O. Boeriu,54T. A. Bolton,57 F. Borcherding,49G. Borissov,41K. Bos,33T. Bose,67A. Brandt,74R. Brock,63G. Brooijmans,67A. Bross,49 N. J. Buchanan,48D. Buchholz,52M. Buehler,50V. Buescher,23S. Burdin,49T. H. Burnett,78E. Busato,17J. M. Butler,60

J. Bystricky,18S. Caron,33W. Carvalho,3B. C. K. Casey,73N. M. Cason,54H. Castilla-Valdez,32S. Chakrabarti,29 D. Chakraborty,51K. M. Chan,68A. Chandra,29D. Chapin,73F. Charles,19E. Cheu,44D. K. Cho,68S. Choi,47 B. Choudhary,28T. Christiansen,25L. Christofek,56D. Claes,65B. Cle´ment,19C. Cle´ment,40Y. Coadou,5M. Cooke,76

W. E. Cooper,49D. Coppage,56M. Corcoran,76A. Cothenet,15M.-C. Cousinou,15B. Cox,43S. Cre´pe´-Renaudin,14 M. Cristetiu,47D. Cutts,73H. da Motta,2B. Davies,41G. Davies,42G. A. Davis,52K. De,74P. de Jong,33S. J. de Jong,34

E. De La Cruz-Burelo,32C. De Oliveira Martins,3S. Dean,43J. D. Degenhardt,62F. De´liot,18M. Demarteau,49 R. Demina,68P. Demine,18D. Denisov,49S. P. Denisov,38S. Desai,69H. T. Diehl,49M. Diesburg,49M. Doidge,41 H. Dong,69S. Doulas,61L. V. Dudko,37L. Duflot,16S. R. Dugad,29A. Duperrin,15J. Dyer,63A. Dyshkant,51M. Eads,51

D. Edmunds,63T. Edwards,43J. Ellison,47J. Elmsheuser,25V. D. Elvira,49S. Eno,59P. Ermolov,37O. V. Eroshin,38 J. Estrada,49D. Evans,42H. Evans,67A. Evdokimov,36V. N. Evdokimov,38J. Fast,49S. N. Fatakia,60L. Feligioni,60 T. Ferbel,68F. Fiedler,25F. Filthaut,34W. Fisher,66H. E. Fisk,49I. Fleck,23M. Fortner,51H. Fox,23S. Fu,49S. Fuess,49

T. Gadfort,78C. F. Galea,34E. Gallas,49E. Galyaev,54C. Garcia,68A. Garcia-Bellido,78J. Gardner,56V. Gavrilov,36 P. Gay,13D. Gele´,19R. Gelhaus,47K. Genser,49C. E. Gerber,50Y. Gershtein,48G. Ginther,68T. Golling,22B. Go´mez,8 K. Gounder,49A. Goussiou,54P. D. Grannis,69S. Greder,3H. Greenlee,49Z. D. Greenwood,58E. M. Gregores,4Ph. Gris,13

J.-F. Grivaz,16L. Groer,67S. Gru¨nendahl,49M. W. Gru¨newald,30S. N. Gurzhiev,38G. Gutierrez,49P. Gutierrez,72 A. Haas,67N. J. Hadley,59S. Hagopian,48I. Hall,72R. E. Hall,46C. Han,62L. Han,7K. Hanagaki,49K. Harder,57 R. Harrington,61J. M. Hauptman,55R. Hauser,63J. Hays,52T. Hebbeker,21D. Hedin,51J. M. Heinmiller,50A. P. Heinson,47

U. Heintz,60C. Hensel,56G. Hesketh,61M. D. Hildreth,54R. Hirosky,77J. D. Hobbs,69B. Hoeneisen,12M. Hohlfeld,24 S. J. Hong,31R. Hooper,73P. Houben,33Y. Hu,69J. Huang,53I. Iashvili,47R. Illingworth,49A. S. Ito,49S. Jabeen,56

M. Jaffre´,16S. Jain,72V. Jain,70K. Jakobs,23A. Jenkins,42R. Jesik,42K. Johns,44M. Johnson,49A. Jonckheere,49 P. Jonsson,42A. Juste,49D. Ka¨fer,21W. Kahl,57S. Kahn,70E. Kajfasz,15A. M. Kalinin,35J. Kalk,63D. Karmanov,37

J. Kasper,60D. Kau,48R. Kaur,27R. Kehoe,75S. Kermiche,15S. Kesisoglou,73A. Khanov,68A. Kharchilava,54 Y. M. Kharzheev,35H. Kim,74B. Klima,49M. Klute,22J. M. Kohli,27M. Kopal,72V. M. Korablev,38J. Kotcher,70 B. Kothari,67A. Koubarovsky,37A. V. Kozelov,38J. Kozminski,63A. Kryemadhi,77S. Krzywdzinski,49S. Kuleshov,36

Y. Kulik,49A. Kumar,28S. Kunori,59A. Kupco,11T. Kurcˇa,20J. Kvita,11S. Lager,40N. Lahrichi,18G. Landsberg,73 J. Lazoflores,48A.-C. Le Bihan,19P. Lebrun,20W. M. Lee,48A. Leflat,37F. Lehner,49,* C. Leonidopoulos,67_{J. Leveque,}44

P. Lewis,42J. Li,74Q. Z. Li,49J. G. R. Lima,51D. Lincoln,49S. L. Linn,48J. Linnemann,63V. V. Lipaev,38R. Lipton,49 L. Lobo,42A. Lobodenko,39M. Lokajicek,11A. Lounis,19P. Love,41H. J. Lubatti,78L. Lueking,49M. Lynker,54

A. L. Lyon,49A. K. A. Maciel,51R. J. Madaras,45P. Ma¨ttig,26C. Magass,21A. Magerkurth,62A.-M. Magnan,14 N. Makovec,16P. K. Mal,29H. B. Malbouisson,3S. Malik,58V. L. Malyshev,35H. S. Mao,6Y. Maravin,49M. Martens,49 S. E. K. Mattingly,73A. A. Mayorov,38R. McCarthy,69R. McCroskey,44D. Meder,24H. L. Melanson,49A. Melnitchouk,64 A. Mendes,15M. Merkin,37K. W. Merritt,49A. Meyer,21M. Michaut,18H. Miettinen,76J. Mitrevski,67N. Mokhov,49

J. Molina,3N. K. Mondal,29R. W. Moore,5G. S. Muanza,20M. Mulders,49Y. D. Mutaf,69E. Nagy,15M. Narain,60 N. A. Naumann,34H. A. Neal,62J. P. Negret,8S. Nelson,48P. Neustroev,39C. Noeding,23A. Nomerotski,49S. F. Novaes,4 T. Nunnemann,25E. Nurse,43V. O’Dell,49D. C. O’Neil,5V. Oguri,3N. Oliveira,3N. Oshima,49G. J. Otero y Garzo´n,50 P. Padley,76N. Parashar,58S. K. Park,31J. Parsons,67R. Partridge,73N. Parua,69A. Patwa,70P. M. Perea,47E. Perez,18

Y. Pogorelov,54B. G. Pope,63W. L. Prado da Silva,3H. B. Prosper,48S. Protopopescu,70J. Qian,62A. Quadt,22B. Quinn,64 K. J. Rani,29K. Ranjan,28P. A. Rapidis,49P. N. Ratoff,41N. W. Reay,57S. Reucroft,61M. Rijssenbeek,69I. Ripp-Baudot,19 F. Rizatdinova,57R. F. Rodrigues,3C. Royon,18P. Rubinov,49R. Ruchti,54V. I. Rud,37G. Sajot,14A. Sa´nchez-Herna´ndez,32 M. P. Sanders,59A. Santoro,3G. Savage,49L. Sawyer,58T. Scanlon,42D. Schaile,25R. D. Schamberger,69H. Schellman,52 P. Schieferdecker,25C. Schmitt,26A. Schwartzman,66R. Schwienhorst,63S. Sengupta,48H. Severini,72E. Shabalina,50

M. Shamim,57V. Shary,18A. A. Shchukin,38W. D. Shephard,54R. K. Shivpuri,28D. Shpakov,61R. A. Sidwell,57 V. Simak,10V. Sirotenko,49P. Skubic,72P. Slattery,68R. P. Smith,49K. Smolek,10G. R. Snow,65J. Snow,71S. Snyder,70

S. So¨ldner-Rembold,43X. Song,51L. Sonnenschein,17A. Sopczak,41M. Sosebee,74K. Soustruznik,9M. Souza,2 B. Spurlock,74N. R. Stanton,57J. Stark,14J. Steele,58K. Stevenson,53V. Stolin,36A. Stone,50D. A. Stoyanova,38 J. Strandberg,40M. A. Strang,74M. Strauss,72R. Stro¨hmer,25D. Strom,52M. Strovink,45L. Stutte,49S. Sumowidagdo,48 A. Sznajder,3M. Talby,15P. Tamburello,44W. Taylor,5P. Telford,43J. Temple,44E. Thomas,15B. Thooris,18M. Tomoto,49 T. Toole,59J. Torborg,54S. Towers,69T. Trefzger,24S. Trincaz-Duvoid,17B. Tuchming,18C. Tully,66A. S. Turcot,70 P. M. Tuts,67L. Uvarov,39S. Uvarov,39S. Uzunyan,51B. Vachon,5R. Van Kooten,53W. M. van Leeuwen,33N. Varelas,50

E. W. Varnes,44A. Vartapetian,74I. A. Vasilyev,38M. Vaupel,26P. Verdier,16L. S. Vertogradov,35M. Verzocchi,59 F. Villeneuve-Seguier,42J.-R. Vlimant,17E. Von Toerne,57M. Vreeswijk,33T. Vu Anh,16H. D. Wahl,48R. Walker,42 L. Wang,59Z.-M. Wang,69J. Warchol,54G. Watts,78M. Wayne,54M. Weber,49H. Weerts,63M. Wegner,21N. Wermes,22 A. White,74V. White,49D. Wicke,49D. A. Wijngaarden,34G. W. Wilson,56S. J. Wimpenny,47J. Wittlin,60M. Wobisch,49 J. Womersley,49D. R. Wood,61T. R. Wyatt,43Q. Xu,62N. Xuan,54S. Yacoob,52R. Yamada,49M. Yan,59T. Yasuda,49 Y. A. Yatsunenko,35Y. Yen,26K. Yip,70H. D. Yoo,73S. W. Youn,52J. Yu,74A. Yurkewicz,69A. Zabi,16A. Zatserklyaniy,51

M. Zdrazil,69C. Zeitnitz,24D. Zhang,49X. Zhang,72T. Zhao,78Z. Zhao,62B. Zhou,62J. Zhu,69M. Zielinski,68 D. Zieminska,53A. Zieminski,53R. Zitoun,69V. Zutshi,51and E. G. Zverev37

(D0 Collaboration)

1_{Universidad de Buenos Aires, Buenos Aires, Argentina} 2

LAFEX, Centro Brasileiro de Pesquisas Fı´sicas, Rio de Janeiro, Brazil

3_{Universidade do Estado do Rio de Janeiro, Rio de Janeiro, Brazil} 4

Instituto de Fı´sica Teo´rica, Universidade Estadual Paulista, Sa˜o Paulo, Brazil

5

University of Alberta, Edmonton, Alberta, Canada, Simon Fraser University, Burnaby, British Columbia, Canada, York University, Toronto, Ontario, Canada, and McGill University, Montreal, Quebec, Canada

6

Institute of High Energy Physics, Beijing, People’s Republic of China

7_{University of Science and Technology of China, Hefei, People’s Republic of China} 8

Universidad de los Andes, Bogota´, Colombia

9_{Center for Particle Physics, Charles University, Prague, Czech Republic} 10

Czech Technical University, Prague, Czech Republic

11_{Institute of Physics, Academy of Sciences, Center for Particle Physics, Prague, Czech Republic} 12

Universidad San Francisco de Quito, Quito, Ecuador

13

Laboratoire de Physique Corpusculaire, IN2P3-CNRS, Universite´ Blaise Pascal, Clermont-Ferrand, France

14_{Laboratoire de Physique Subatomique et de Cosmologie, IN2P3-CNRS, Universite de Grenoble 1, Grenoble, France} 15

CPPM, IN2P3-CNRS, Universite´ de la Me´diterrane´e, Marseille, France

16_{Laboratoire de l’Acce´le´rateur Line´aire, IN2P3-CNRS, Orsay, France} 17

LPNHE, IN2P3-CNRS, Universite´s Paris VI and VII, Paris, France

18_{DAPNIA/Service de Physique des Particules, CEA, Saclay, France} 19

IReS, IN2P3-CNRS, Universite´ Louis Pasteur, Strasbourg, France, and Universite´ de Haute Alsace, Mulhouse, France

20_{Institut de Physique Nucle´aire de Lyon, IN2P3-CNRS, Universite´ Claude Bernard, Villeurbanne, France} 21_{III. Physikalisches Institut A, RWTH Aachen, Aachen, Germany}

22

Physikalisches Institut, Universita¨t Bonn, Bonn, Germany

23_{Physikalisches Institut, Universita¨t Freiburg, Freiburg, Germany} 24

Institut fu¨r Physik, Universita¨t Mainz, Mainz, Germany

25_{Ludwig-Maximilians-Universita¨t Mu¨nchen, Mu¨nchen, Germany} 26

Fachbereich Physik, University of Wuppertal, Wuppertal, Germany

27_{Panjab University, Chandigarh, India} 28

Delhi University, Delhi, India

29

Tata Institute of Fundamental Research, Mumbai, India

30_{University College Dublin, Dublin, Ireland} 31

32

CINVESTAV, Mexico City, Mexico

33_{FOM-Institute NIKHEF and University of Amsterdam/NIKHEF, Amsterdam, The Netherlands} 34

Radboud University Nijmegen/NIKHEF, Nijmegen, The Netherlands

35_{Joint Institute for Nuclear Research, Dubna, Russia} 36

Institute for Theoretical and Experimental Physics, Moscow, Russia

37_{Moscow State University, Moscow, Russia} 38

Institute for High Energy Physics, Protvino, Russia

39_{Petersburg Nuclear Physics Institute, St. Petersburg, Russia} 40

Lund University, Lund, Sweden, Royal Institute of Technology and Stockholm University, Stockholm, Sweden, and Uppsala University, Uppsala, Sweden

41_{Lancaster University, Lancaster, United Kingdom} 42

Imperial College, London, United Kingdom

43_{University of Manchester, Manchester, United Kingdom} 44

University of Arizona, Tucson, Arizona 85721, USA

45_{Lawrence Berkeley National Laboratory and University of California, Berkeley, California 94720, USA} 46

California State University, Fresno, California 93740, USA

47_{University of California, Riverside, California 92521, USA} 48_{Florida State University, Tallahassee, Florida 32306, USA} 49

Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA

50_{University of Illinois at Chicago, Chicago, Illinois 60607, USA} 51

Northern Illinois University, DeKalb, Illinois 60115, USA

52_{Northwestern University, Evanston, Illinois 60208, USA} 53

Indiana University, Bloomington, Indiana 47405, USA

54_{University of Notre Dame, Notre Dame, Indiana 46556, USA} 55

Iowa State University, Ames, Iowa 50011, USA

56

University of Kansas, Lawrence, Kansas 66045, USA

57_{Kansas State University, Manhattan, Kansas 66506, USA} 58

Louisiana Tech University, Ruston, Louisiana 71272, USA

59_{University of Maryland, College Park, Maryland 20742, USA} 60

Boston University, Boston, Massachusetts 02215, USA

61_{Northeastern University, Boston, Massachusetts 02115, USA} 62

University of Michigan, Ann Arbor, Michigan 48109, USA

63_{Michigan State University, East Lansing, Michigan 48824, USA} 64_{University of Mississippi, University, Mississippi 38677, USA}

65

University of Nebraska, Lincoln, Nebraska 68588, USA

66_{Princeton University, Princeton, New Jersey 08544, USA} 67

Columbia University, New York, New York 10027, USA

68_{University of Rochester, Rochester, New York 14627, USA} 69

State University of New York, Stony Brook, New York 11794, USA

70_{Brookhaven National Laboratory, Upton, New York 11973, USA} 71_{Langston University, Langston, Oklahoma 73050, USA} 72_{University of Oklahoma, Norman, Oklahoma 73019, USA}

73_{Brown University, Providence, Rhode Island 02912, USA} 74

University of Texas, Arlington, Texas 76019, USA

75_{Southern Methodist University, Dallas, Texas 75275, USA} 76

Rice University, Houston, Texas 77005, USA

77_{University of Virginia, Charlottesville, Virginia 22901, USA} 78

University of Washington, Seattle, Washington 98195, USA

(Received 18 February 2005; published 27 July 2005)

We present a measurement of theZproduction cross section and limits on anomalousZZandZ

couplings for form-factor scales of750 and 1000 GeV. The measurement is based on 138 (152) candidates in theee() final state using320290pb1 _{ofpp collisions at}

s p

1:96 TeV. The 95% C.L. limits on real and imaginary parts of individual anomalous couplings are _jhZ

10;30j<0:23,

jhZ

20;40j<0:020,jh

10;30j<0:23, andjh

20;40j<0:019for1000 GeV.

DOI:10.1103/PhysRevLett.95.051802 PACS numbers: 12.15.Ji, 13.38.Dg, 13.40.Em, 13.85.Qk

Studies of events containing pairs of vector bosons provide important tests of the standard model (SM) of

photons do not interact with Z bosons at lowest order. Evidence for such an interaction would indicate new phys-ics [1].

We present a new study ofZproduction inpp colli-sions usingZboson decays toe_{eand, where the} dilepton system can be produced by either an on-shell Z boson, or a virtualZboson or(the Drell-Yan process). The dilepton plus photon final state,‘_{‘, can be} pro-duced in the SM through either of two processes. The photon may be emitted through initial state radiation (ISR) from one of the partons in theporp, or produced as final state radiation (FSR) from one of the final state leptons. We collectively refer to these processes as Z production.

The SMZprocesses produce photons with a rapidly falling transverse energy,E_T. In contrast, anomalousZZ andZcouplings, which appear in extensions of the SM, can cause production of photons with high E_T and can increase the ‘_{‘ cross section compared to the SM} prediction. Below we describe a search for this anomalous production within the framework of Ref. [2]. This formal-ism assumes only that the ZV(VZ; ) couplings are Lorentz and gauge invariant. The most generalZV cou-pling is parametrized by twoCP-violating (hV

1 andhV2) and

two CP-conserving (hV

3 and hV4) complex coupling

pa-rameters. Partial wave unitarity is ensured at high energies by using a form-factor ansatzhV

i hVi0=1s=^ 2ni(i

1;. . .;4), where ^ s p

is the parton center-of-mass energy,

is the form-factor scale, andni is the form-factor power.

We set the form-factor powers n1n33 and n2

n44, in accordance with Ref. [2].

Previous studies ofZboson and photon production have been made by the CDF [3] and D0 [4] Collaborations using pp collisions, and by the DELPHI [5], L3 [6], and OPAL [7] Collaborations using e_{e collisions. The combined} LEP results are available in Ref. [8].

The data for this analysis were collected by the D0 Run II detector at the Fermilab Tevatron Collider with pp center-of-mass energy p_s

1:96 TeV between April 2002 and June 2004. The integrated luminosities used for this analysis are320 pb1 _{for the electron final}

state and290 pb1 _{for the muon final state.}

The D0 detector [9] consists of an inner tracker, sur-rounded by liquid-argon/uranium calorimeters, and a muon spectrometer. The detector subsystems provide measure-ments over the full range of azimuthal angle [10] and over different, overlapping regions of detector pseudora-pidity. The inner tracker consists of a silicon microstrip tracker (SMT) and a central fiber tracker (CFT), both located within a 2 T superconducting solenoidal magnet. The CFT and the SMT have coverage out tojj&1:8and j_j&3:0, respectively. The calorimeter is divided into a central calorimeter (CC) covering the rangej_j<1:1and two end calorimeters (EC) housed in separate cryostats, which extend coverage to j_j 4. The calorimeters are

longitudinally segmented into electromagnetic (EM) and hadronic sections. The muon system lies outside the calo-rimeters and consists of tracking detectors, scintillation trigger counters, and a 1.8 T toroid magnet. It has coverage up to _jj 2:0. Luminosity is measured using plastic scintillator arrays located in front of the EC cryostats, covering2:7<_jj<4:4.

The data were collected with a three-level trigger system (L1, L2, and L3). We require that the events in the electron decay channel satisfy one of the high-ET single electron

triggers, while the events in the muon decay channel must fire one of the high-p_Tsingle or dimuon triggers. At L1 the single electron triggers select events based on the energy deposited in the EM section of the calorimeter; typical L1 requirements are greater than 10 –15 GeV. At L3, addi-tional requirements are imposed on the fraction of energy deposited in the EM calorimeter and the shape of the energy deposition. The efficiency of the electron trigger requirement is about 80% for an electron with E_T 25 GeV and more than 98% for E_T>30 GeV. The muon trigger requires hits in the muon system scintillators at L1, and in portions of the data set also requires spatially matched hits in the muon tracking detectors. At L2, muon track segments are reconstructed andp_T requirements are imposed. At L3, some of the triggers used in this analysis require events to have a track reconstructed in the inner tracker with p_T greater than 10 GeV. The logical OR of single and dimuon triggers has an efficiency of 92% for muons fromZboson decay.

Electrons are reconstructed as clusters of energy in the calorimeter. These clusters are required to have 90% of their energy deposited in the EM calorimeter (in either the central calorimeterj_j<1:1or the end calorimeter1:5< j_j<2:5). We require that the longitudinal and transverse shower shape of the cluster is consistent with that expected from an electron and that the cluster is isolated from other activity in the calorimeter. Electron candidates in the cen-tral calorimeter are required to have spatially matched tracks. At least one electron candidate must be identified in the CC region, and at least one is required to havep_T> 25 GeV=c. Muons are identified by a central track matched to segments in the muon system. The muon must be within j_j<2:0. To reduce potential contamination from muons produced in b-quark decays of bb events, we impose isolation requirements on the muon candidates in both the calorimeter and the central tracker. To remove the background from cosmic ray muons, muon tracks must originate from the beam region and more than 0.05 radians from exactly back to back.

Zboson candidates are reconstructed by requiring a pair of high-p_T(p_T>15 GeV=c) electrons or muons that form an invariant mass above30 GeV=c2_.

In addition to aZboson candidate, we require events to have a photon candidate, with a separation from both leptons of R _‘2

‘2

q

with E_T>8 GeV. Photons are reconstructed as energy clusters in the central calorimeter. The transverse shower shape of the cluster must be consistent with that expected from a photon. We also require a photon candidate to deposit at least 90% of its energy in the EM calorimeter and to be isolated from other activity in the calorimeter and the tracker.

Muon and electron detection efficiencies for the above requirements are determined using a sample of Z_!‘‘ candidates. In the electron channel the combined trigger and reconstruction efficiency is measured to be734%. In the muon channel it is measured to be814%. The photon identification efficiency is measured as a function of E_T using a Monte Carlo simulation. A systematic un-certainty of 4% is assessed from the difference between the simulated electrons and electron candidates in Z_!ee data, and the difference between simulated electrons and photons. The photon identification efficiency is E_T-dependent and rises from about 75% at 8 GeV to about 90% above 27 GeV.

Background from processes with photons and leptons from misidentified jets is found to be negligible. Contributions fromZ!_{production with leptonic} decays of the tau are less than 1% of the sample and thus neglected. The only significant source of background toZ production is from Zjets processes in which a jet is misidentified as a photon. We estimate theZjets back-ground by folding the jet-E_T spectrum inZjets candi-dates with the probability for a jet to be misidentified as a photon. The probability is measured as a function of the photon candidate’sET using a sample of events dominated

by QCD multijet processes. The misidentification proba-bility is about5103_{and decreases withE}

T. We correct

the misidentification probability for direct photon produc-tion (jets) by fitting the photon candidateET

distribu-tion to the funcdistribu-tional form derived in [11]. For low ET

(E_T <75 GeV) this contribution is measured to be 9% of the probability, and we take this number as a systematic uncertainty.

We use an event generator employing leading order (LO) QCD calculations and first order electroweak radia-tion with a detector simularadia-tion tuned withZboson candi-dates to calculate the acceptances for the data and expected rates from both the SM and anomalousZproduction [2]. We use the CTEQ6L [12] parton distribution function (PDF) set. We estimate the uncertainty due to the PDF choice to be 3.3% using the prescription in Ref. [13]. Using a next to leading order (NLO) Z Monte Carlo [14] generator, we calculate an E_T-dependent K factor to pa-rametrize the effect ofE_T-dependent NLO corrections in the LO Monte Carlo sample. The value of theKfactor is approximately 1.15 forE_T 10 GeV, and increases to

1:3forE_T 100 GeV. The uncertainty due to the choice ofKfactor (flat with a value of 1.34 vsE_T dependent) is found to be negligible.

We observe 138 events in the electron channel, to be compared to the SM estimate of95:34:9e_eevents and 23:62:3background events. In the muon channel, we observe 152 events while SM estimates are126:07:8

events and22:43:0background events. The uncertainty in the SM signal is dominated by the uncertainty in the lepton and photon reconstruction efficiencies, and that in the background estimation is dominated by the uncertainty in the jet misidentification probability.

The ET spectrum for photon candidates is shown in

Fig. 1 with the estimation of the total SM prediction and its QCD background component overlaid. The highest transverse energy photon in the electron channel is 105 GeV, while the highest transverse energy photon in the muon channel is 166 GeV. In Fig. 2 we plot the three-body mass (M_‘‘) against the dilepton mass (M_‘‘) for each event in the data. The dilepton and three-body mass dis-tributions are given in Fig. 3. The ISR events with a dilepton system produced by an on-shellZboson populate a vertical band at M_‘‘ around Z boson mass, M_Z, and M_‘‘> M_Z. The on-shell Z boson FSR events cluster along a horizontal band at M‘‘MZ and have M‘‘<

M_Z. The Drell-Yan events populate the diagonal band with M_‘‘M_‘‘ extending from the lower left to the upper right corner of the plot.

For events satisfying the phase space requirements,

R_{‘>0:7, E}

T>8 GeV, and M‘‘>30 GeV=c2, the

combined cross section times branching ratio is measured to be 4:20:4statsyst 0:3luminpb, where the first uncertainty includes contributions from statistics and

(GeV)

T γ

E

0 100 200 300

Events

-2

10

-1

10 1 10

2

10

3

10

(GeV)

T γ

E

0 100 200 300

γ

ll QCD

+ QCD

γ

-l + SM l

=0.24) + QCD 4

=-2.1, h 3 h

γ

(ZZ

γ

-l + l

DØ

FIG. 1 (color online). Photon candidate ET spectrum for‘‘

data (solid circles), QCD multijet background (shaded histo-gram), the Monte Carlo simulation of the ‘_{‘ production}

all systematic effects except the luminosity, and the second is due to the luminosity measurement uncertainty [15]. This value is in agreement with the expected value of

3:90:1

0:2 pbfrom NLO theory calculations [14]. The largest

systematic uncertainty is due to photon identification, while lepton identification, PDF uncertainty, and back-ground model are of similar magnitude.

Given the separation exhibited in Fig. 2, we can measure a cross section of ISR-enhancedZproduction. By min-imizing the effects of final state radiation, one is able to examine the contribution from initial state radiation in more detail. With the additional requirements that the dilepton mass and three-body mass exceed 65 and

100 GeV=c2_{, respectively, the SM Monte Carlo simulation}

indicates that 80% of the remaining events are due to initial state radiation. For this restricted sample we observe 55 and 62 events in the electron and muon channels, respec-tively. The expectation for signal events is 31.1 and 37.9, while background expectations are 18.6 and 14.7 events in the electron and muon channels, respectively. The cross section times branching ratio is measured to be 1:07

0:15statsyst 0:07lumin pb, in agreement with the expected0:940₀:_:02₀₅ pb[14].

Given the good agreement observed between the data and the SM prediction, we extract upper limits on anoma-lous couplings [16]. We generate Monte Carlo events in a two-dimensional grid of CP-violating anomalous cou-plings (hV

10 and hV20) and do the same for CP-conserving

(hV

30andhV40) anomalous couplings. We calculate the

like-lihood of the agreement between theE_T distribution of the

290 data events (shown in Fig. 1) to the estimated back-ground and Monte Carlo simulation for each point of the grid. Assuming Poisson statistics for the data and Gaussian systematic uncertainties, we extract the 95% C.L. limits on each of the anomalous couplings while assuming the others are zero. The limits on CP-violating and CP-conserving anomalous couplings are nearly identical. We also find the limits on real and imaginary parts of the couplings to be similar as well. We present the limits on both real and imaginary parts of the CP-conserving and CP-violating couplings in Table I. The two-dimensional limit contours on individual CP-conserving couplings are shown in Fig. 4.

TABLE I. Summary of the 95% C.L. upper limits on the anomalous couplings. Limits are set by allowing only the real or imaginary part of one coupling to vary; all others are fixed to their standard model values. As indicated, we find upper limits on CP-conserving and CP-violating parameters to be nearly identical. We also find that nearly identical limits apply to the real or imaginary parts of all couplings.

Coupling 750 GeV 1 TeV

jRehZ

10;30j,jImhZ10;30j 0.24 0.23

jRehZ

20;40j,jImhZ20;40j 0.027 0.020

jReh_10;30j,_jImh_10;30j 0.29 0.23

jReh_20;40j,_jImh_20;40j 0.030 0.019

) 2 (GeV/c l l M

0 100 200 300

2

Events / 10 GeV/c

-3

10

-2

10

-1

10 1 10

2

10

) 2 (GeV/c l l M

0 100 200 300

γ ll QCD

+ QCD γ

-l

+

SM l

(a)

_DØ

) 2 (GeV/c γ

l l M

0 100 200 300 400 500

2

Events / 10 GeV/c

-3

10

-2

10

-1

10 1 10

2

10

) 2 (GeV/c γ

l l M

0 100 200 300 400 500

γ ll QCD

+ QCD γ

-l

+

SM l

(b)

_DØ

FIG. 3 (color online). (a) Dilepton mass and (b) dileptonphoton mass of ‘‘ data (solid circles), QCD multijet background (shaded histogram), and the standard model plus background (histogram). The shaded band is the systematic uncertainty on the SM plus background. The Monte Carlo dis-tribution is normalized to the luminosity.

)

2

(GeV/c

l l

M

10

)

2

(GeV/c

l l

M

2

10

∅

D

γ

ee

γ µ µ

DØ

FIG. 2 (color online). Dileptonphoton vs dilepton mass of

In conclusion, we have studied a sample of 290 ‘‘ events, consistent with Z production. This sample ex-ceeds that previously collected by D0 by an order of magnitude. This is due to 3 times more integrated lumi-nosity, an increased production cross section associated with the 10% higher center-of-mass energy, and significant improvements in particle detection efficiency achieved with the D0 Run II upgrade. The ‘‘ cross section is measured to be 4:20:4statsyst 0:3luminpb. After additional selection requirements, most of the final state radiation is removed, leaving the sample dominated by initial state radiation. The cross section for this ISR-enhanced Z production is measured to be 1:07

0:15statsyst 0:07lumin pb. These values are con-sistent with the SM expectations. We observe no significant deviation from the SM expectation in the total cross section or photon E_T distribution, and thus extract limits on anomalous Z couplings. The one-dimensional limits at 95% C.L. for bothCP-conserving andCP-violating cou-plings (both real and imaginary parts) are_jhZ

10;30j<0:23,

jhZ

20;40j<0:020, jh

10;30j<0:23, and jh

20;40j<0:019 for

1 TeV. These limits are substantially more restrictive than previous results that have been presented using this formalism [4]. Accounting for the different formalism used at LEP, our limits onhV

20 and hV40 are more than twice as

restrictive as the combined results of the four LEP experi-ments [17].

We thank the staffs at Fermilab and collaborating insti-tutions, and acknowledge support from the Department of Energy and National Science Foundation (USA), Commissariat a´ l’Energie Atomique and CNRS/Institut National de Physique Nucle´aire et de Physique des Particules (France), Ministry of Education and Science, Agency for Atomic Energy, and RF President Grants Program (Russia), CAPES, CNPq, FAPERJ, FAPESP, and FUNDUNESP (Brazil), Departments of Atomic Energy and Science and Technology (India), Colciencias

(Colombia), CONACyT (Mexico), KRF (Korea), CONICET and UBACyT (Argentina), The Foundation for Fundamental Research on Matter (The Netherlands), PPARC (United Kingdom), Ministry of Education (Czech Republic), Canada Research Chairs Program, CFI, Natural Sciences and Engineering Research Council, and WestGrid Project (Canada), BMBF and DFG (Germany), Science Foundation Ireland, A. P. Sloan Foundation, Research Corporation, Texas Advanced Research Program, Alexander von Humboldt Foundation, and the Marie Curie Program.

*Visitor from University of Zurich, Zurich, Switzerland. [1] G. J. Gounaris, J. Layssac, and F. M. Renard, Phys. Rev. D

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[6] M. Acciarriet al.(L3 Collaboration), Phys. Lett. B436, 187 (1998); P. Achard et al. (L3 Collaboration), Phys. Lett. B597, 119 (2004).

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[10] We use a cylindrical coordinate system about the beam pipe in which positivezis along the proton direction,is the polar angle, is the azimuthal angle, and pseudor-apidity () is defined aslntan=2.

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[15] T. Edwards et al., Report No. FERMILAB-TM-2278-E, 2004.

[16] We set limits on anomalous couplings using the 290Z

candidates that satisfied the following phase space require-ments: R_‘>0:7, E_T>8 GeV, and M‘‘> 30 GeV=c2_.

[17] S. Eidelman et al., Phys. Lett. B 592, 1 (2004); LEP electroweak working group, http://www.cern.ch/ LEPEWWG/lepww/tgc.

30 Z

h

-0.4 -0.2 0.0 0.2 0.4

40

Z

h

-0.04 -0.02 0.00 0.02

0.04 DØ

(a)

30 γ

h

-0.4 -0.2 0.0 0.2 0.4

γ 40

h

-0.04 -0.02 0.00 0.02

0.04 DØ

(b)

FIG. 4. The 95% C.L. two-dimensional exclusion limits for