Search for leptoquark pairs decaying to $\nu\nu$ + jets in $p\bar p$ collisions at $\sqrt s$ = 1.8 TeV

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Search for leptoquark pairs decaying to νν + jets in p¯

p

collisions at

√

s = 1.8 TeV

V.M. Abazov, B. Abbott, Abdelmalek Abdesselam, M. Abolins, V. Abramov,

B.S. Acharya, D.L. Adams, M. Adams, S.N. Ahmed, G.D. Alexeev, et al.

To cite this version:

V.M. Abazov, B. Abbott, Abdelmalek Abdesselam, M. Abolins, V. Abramov, et al.. Search for

leptoquark pairs decaying to νν + jets in p¯

p

collisions at

√

s = 1.8 TeV. Physical Review Letters,

arXiv:hep-ex/0111047 v1 14 Nov 2001

Search for Leptoquark Pairs Decaying to νν + jets in pp Collisions at

√

s=1.8 TeV

V.M. Abazov,23 _{B. Abbott,}57 _{A. Abdesselam,}11 _{M. Abolins,}50_{V. Abramov,}26_{B.S. Acharya,}17 _{D.L. Adams,}59 M. Adams,37_{S.N. Ahmed,}21 _{G.D. Alexeev,}23 _{A. Alton,}49_{G.A. Alves,}2_{N. Amos,}49 _{E.W. Anderson,}42 _{Y. Arnoud,}9

C. Avila,5_{M.M. Baarmand,}54_{V.V. Babintsev,}26 _{L. Babukhadia,}54_{T.C. Bacon,}28_{A. Baden,}46_{B. Baldin,}36 P.W. Balm,20_{S. Banerjee,}17 _{E. Barberis,}30 _{P. Baringer,}43_{J. Barreto,}2 _{J.F. Bartlett,}36_{U. Bassler,}12 _{D. Bauer,}28

A. Bean,43 _{F. Beaudette,}11 _{M. Begel,}53 _{A. Belyaev,}35_{S.B. Beri,}15 _{G. Bernardi,}12 _{I. Bertram,}27 _{A. Besson,}9 R. Beuselinck,28 _{V.A. Bezzubov,}26 _{P.C. Bhat,}36 _{V. Bhatnagar,}15_{M. Bhattacharjee,}54 _{G. Blazey,}38_{F. Blekman,}20

S. Blessing,35 _{A. Boehnlein,}36 _{N.I. Bojko,}26 _{F. Borcherding,}36 _{K. Bos,}20_{T. Bose,}52 _{A. Brandt,}59 _{R. Breedon,}31 G. Briskin,58_{R. Brock,}50_{G. Brooijmans,}36_{A. Bross,}36_{D. Buchholz,}39 _{M. Buehler,}37 _{V. Buescher,}14_{V.S. Burtovoi,}26

J.M. Butler,47 _{F. Canelli,}53_{W. Carvalho,}3_{D. Casey,}50_{Z. Casilum,}54 _{H. Castilla-Valdez,}19_{D. Chakraborty,}38 K.M. Chan,53_{S.V. Chekulaev,}26 _{D.K. Cho,}53_{S. Choi,}34_{S. Chopra,}55_{J.H. Christenson,}36 _{M. Chung,}37_{D. Claes,}51 A.R. Clark,30 _{L. Coney,}41 _{B. Connolly,}35_{W.E. Cooper,}36_{D. Coppage,}43_{S. Cr´ep´e-Renaudin,}9 _{M.A.C. Cummings,}38

D. Cutts,58_{G.A. Davis,}53_{K. Davis,}29_{K. De,}59_{S.J. de Jong,}21_{K. Del Signore,}49_{M. Demarteau,}36 _{R. Demina,}44 P. Demine,9_{D. Denisov,}36_{S.P. Denisov,}26_{S. Desai,}54_{H.T. Diehl,}36_{M. Diesburg,}36 _{S. Doulas,}48 _{Y. Ducros,}13 L.V. Dudko,25_{S. Duensing,}21 _{L. Duflot,}11_{S.R. Dugad,}17_{A. Duperrin,}10_{A. Dyshkant,}38_{D. Edmunds,}50_{J. Ellison,}34 J.T. Eltzroth,59 _{V.D. Elvira,}36_{R. Engelmann,}54_{S. Eno,}46 _{G. Eppley,}61_{P. Ermolov,}25_{O.V. Eroshin,}26_{J. Estrada,}53

H. Evans,52 _{V.N. Evdokimov,}26_{T. Fahland,}33 _{S. Feher,}36 _{D. Fein,}29_{T. Ferbel,}53 _{F. Filthaut,}21 _{H.E. Fisk,}36 Y. Fisyak,55 _{E. Flattum,}36 _{F. Fleuret,}12 _{M. Fortner,}38_{H. Fox,}39_{K.C. Frame,}50 _{S. Fu,}52 _{S. Fuess,}36 _{E. Gallas,}36

A.N. Galyaev,26_{M. Gao,}52_{V. Gavrilov,}24_{R.J. Genik II,}27 _{K. Genser,}36 _{C.E. Gerber,}37 _{Y. Gershtein,}58 R. Gilmartin,35_{G. Ginther,}53 _{B. G´omez,}5 _{G. G´omez,}46_{P.I. Goncharov,}26_{J.L. Gonz´alez Sol´ıs,}19_{H. Gordon,}55 L.T. Goss,60_{K. Gounder,}36_{A. Goussiou,}28_{N. Graf,}55_{G. Graham,}46_{P.D. Grannis,}54_{J.A. Green,}42 _{H. Greenlee,}36

Z.D. Greenwood,45_{S. Grinstein,}1 _{L. Groer,}52 _{S. Gr¨unendahl,}36_{A. Gupta,}17_{S.N. Gurzhiev,}26_{G. Gutierrez,}36 P. Gutierrez,57_{N.J. Hadley,}46_{H. Haggerty,}36_{S. Hagopian,}35_{V. Hagopian,}35_{R.E. Hall,}32 _{P. Hanlet,}48 _{S. Hansen,}36

J.M. Hauptman,42 _{C. Hays,}52 _{C. Hebert,}43 _{D. Hedin,}38_{J.M. Heinmiller,}37 _{A.P. Heinson,}34_{U. Heintz,}47 M.D. Hildreth,41 _{R. Hirosky,}62 _{J.D. Hobbs,}54 _{B. Hoeneisen,}8 _{Y. Huang,}49_{I. Iashvili,}34_{R. Illingworth,}28_{A.S. Ito,}36 M. Jaffr´e,11_{S. Jain,}17_{R. Jesik,}28_{K. Johns,}29 _{M. Johnson,}36 _{A. Jonckheere,}36 _{H. J¨ostlein,}36_{A. Juste,}36 _{W. Kahl,}44

S. Kahn,55_{E. Kajfasz,}10 _{A.M. Kalinin,}23_{D. Karmanov,}25_{D. Karmgard,}41_{R. Kehoe,}50 _{A. Khanov,}44 A. Kharchilava,41_{S.K. Kim,}18_{B. Klima,}36_{B. Knuteson,}30 _{W. Ko,}31_{J.M. Kohli,}15_{A.V. Kostritskiy,}26_{J. Kotcher,}55

B. Kothari,52 _{A.V. Kotwal,}52_{A.V. Kozelov,}26_{E.A. Kozlovsky,}26 _{J. Krane,}42 _{M.R. Krishnaswamy,}17_{P. Krivkova,}6 S. Krzywdzinski,36 _{M. Kubantsev,}44_{S. Kuleshov,}24 _{Y. Kulik,}54_{S. Kunori,}46 _{A. Kupco,}7 _{V.E. Kuznetsov,}34 G. Landsberg,58_{W.M. Lee,}35 _{A. Leflat,}25_{C. Leggett,}30 _{F. Lehner,}36,∗ _{C. Leonidopoulos,}52_{J. Li,}59 _{Q.Z. Li,}36_{X. Li,}4

J.G.R. Lima,3_{D. Lincoln,}36_{S.L. Linn,}35_{J. Linnemann,}50_{R. Lipton,}36_{A. Lucotte,}9 _{L. Lueking,}36 _{C. Lundstedt,}51 C. Luo,40_{A.K.A. Maciel,}38 _{R.J. Madaras,}30_{V.L. Malyshev,}23 _{V. Manankov,}25_{H.S. Mao,}4_{T. Marshall,}40 M.I. Martin,38 _{K.M. Mauritz,}42 _{A.A. Mayorov,}40_{R. McCarthy,}54 _{T. McMahon,}56_{H.L. Melanson,}36 _{M. Merkin,}25

K.W. Merritt,36_{C. Miao,}58_{H. Miettinen,}61 _{D. Mihalcea,}38_{C.S. Mishra,}36 _{N. Mokhov,}36_{N.K. Mondal,}17 H.E. Montgomery,36_{R.W. Moore,}50 _{M. Mostafa,}1 _{H. da Motta,}2_{E. Nagy,}10_{F. Nang,}29_{M. Narain,}47 V.S. Narasimham,17_{N.A. Naumann,}21_{H.A. Neal,}49 _{J.P. Negret,}5 _{S. Negroni,}10_{T. Nunnemann,}36 _{D. O’Neil,}50

V. Oguri,3_{B. Olivier,}12_{N. Oshima,}36 _{P. Padley,}61_{L.J. Pan,}39 _{K. Papageorgiou,}37_{A. Para,}36_{N. Parashar,}48 R. Partridge,58_{N. Parua,}54_{M. Paterno,}53 _{A. Patwa,}54_{B. Pawlik,}22_{J. Perkins,}59_{O. Peters,}20 _{P. P´etroff,}11 R. Piegaia,1_{B.G. Pope,}50 _{E. Popkov,}47 _{H.B. Prosper,}35 _{S. Protopopescu,}55 _{M.B. Przybycien,}39,† _{J. Qian,}49

R. Raja,36_{S. Rajagopalan,}55_{E. Ramberg,}36_{P.A. Rapidis,}36 _{N.W. Reay,}44 _{S. Reucroft,}48 _{M. Ridel,}11 M. Rijssenbeek,54 _{F. Rizatdinova,}44_{T. Rockwell,}50 _{M. Roco,}36_{C. Royon,}13_{P. Rubinov,}36_{R. Ruchti,}41 J. Rutherfoord,29_{B.M. Sabirov,}23 _{G. Sajot,}9_{A. Santoro,}2_{L. Sawyer,}45 _{R.D. Schamberger,}54_{H. Schellman,}39 A. Schwartzman,1 _{N. Sen,}61_{E. Shabalina,}37_{R.K. Shivpuri,}16_{D. Shpakov,}48_{M. Shupe,}29 _{R.A. Sidwell,}44_{V. Simak,}7

H. Singh,34_{J.B. Singh,}15_{V. Sirotenko,}36_{P. Slattery,}53 _{E. Smith,}57_{R.P. Smith,}36_{R. Snihur,}39_{G.R. Snow,}51 J. Snow,56_{S. Snyder,}55 _{J. Solomon,}37_{Y. Song,}59_{V. Sor´ın,}1_{M. Sosebee,}59 _{N. Sotnikova,}25_{K. Soustruznik,}6

M. Souza,2 _{N.R. Stanton,}44_{G. Steinbr¨uck,}52 _{R.W. Stephens,}59 _{F. Stichelbaut,}55 _{D. Stoker,}33 _{V. Stolin,}24 A. Stone,45_{D.A. Stoyanova,}26_{M.A. Strang,}59_{M. Strauss,}57 _{M. Strovink,}30_{L. Stutte,}36 _{A. Sznajder,}3 _{M. Talby,}10

W. Taylor,54_{S. Tentindo-Repond,}35_{S.M. Tripathi,}31_{T.G. Trippe,}30 _{A.S. Turcot,}55 _{P.M. Tuts,}52_{V. Vaniev,}26 R. Van Kooten,40 _{N. Varelas,}37 _{L.S. Vertogradov,}23_{F. Villeneuve-Seguier,}10 _{A.A. Volkov,}26_{A.P. Vorobiev,}26

H.D. Wahl,35_{H. Wang,}39_{Z.-M. Wang,}54_{J. Warchol,}41 _{G. Watts,}63_{M. Wayne,}41_{H. Weerts,}50 _{A. White,}59 J.T. White,60_{D. Whiteson,}30 _{D.A. Wijngaarden,}21_{S. Willis,}38_{S.J. Wimpenny,}34_{J. Womersley,}36_{D.R. Wood,}48

V. Zutshi,55 _{E.G. Zverev,}25 _{and A. Zylberstejn}13 (DØ Collaboration)

1_{Universidad de Buenos Aires, Buenos Aires, Argentina} 2_{LAFEX, Centro Brasileiro de Pesquisas F´ısicas, Rio de Janeiro, Brazil}

3_{Universidade do Estado do Rio de Janeiro, Rio de Janeiro, Brazil} 4_{Institute of High Energy Physics, Beijing, People’s Republic of China}

5_{Universidad de los Andes, Bogot´a, Colombia}

6_{Charles University, Center for Particle Physics, Prague, Czech Republic}

7_{Institute of Physics, Academy of Sciences, Center for Particle Physics, Prague, Czech Republic} 8_{Universidad San Francisco de Quito, Quito, Ecuador}

9_{Institut des Sciences Nucl´eaires, IN2P3-CNRS, Universite de Grenoble 1, Grenoble, France} 10_{CPPM, IN2P3-CNRS, Universit´e de la M´editerran´ee, Marseille, France}

11_{Laboratoire de l’Acc´el´erateur Lin´eaire, IN2P3-CNRS, Orsay, France} 12_{LPNHE, Universit´es Paris VI and VII, IN2P3-CNRS, Paris, France} 13_{DAPNIA/Service de Physique des Particules, CEA, Saclay, France}

14_{Universit¨at Mainz, Institut f¨ur Physik, Mainz, Germany} 15_{Panjab University, Chandigarh, India}

16_{Delhi University, Delhi, India}

17_{Tata Institute of Fundamental Research, Mumbai, India} 18_{Seoul National University, Seoul, Korea}

19_{CINVESTAV, Mexico City, Mexico}

20_{FOM-Institute NIKHEF and University of Amsterdam/NIKHEF, Amsterdam, The Netherlands}

21_{University of Nijmegen/NIKHEF, Nijmegen, The Netherlands}

22_{Institute of Nuclear Physics, Krak´ow, Poland} 23_{Joint Institute for Nuclear Research, Dubna, Russia} 24_{Institute for Theoretical and Experimental Physics, Moscow, Russia}

25_{Moscow State University, Moscow, Russia} 26_{Institute for High Energy Physics, Protvino, Russia}

27_{Lancaster University, Lancaster, United Kingdom} 28_{Imperial College, London, United Kingdom} 29_{University of Arizona, Tucson, Arizona 85721}

30_{Lawrence Berkeley National Laboratory and University of California, Berkeley, California 94720} 31_{University of California, Davis, California 95616}

32_{California State University, Fresno, California 93740} 33_{University of California, Irvine, California 92697} 34_{University of California, Riverside, California 92521} 35_{Florida State University, Tallahassee, Florida 32306} 36_{Fermi National Accelerator Laboratory, Batavia, Illinois 60510}

37_{University of Illinois at Chicago, Chicago, Illinois 60607} 38_{Northern Illinois University, DeKalb, Illinois 60115}

39_{Northwestern University, Evanston, Illinois 60208} 40_{Indiana University, Bloomington, Indiana 47405} 41_{University of Notre Dame, Notre Dame, Indiana 46556}

42_{Iowa State University, Ames, Iowa 50011} 43_{University of Kansas, Lawrence, Kansas 66045} 44_{Kansas State University, Manhattan, Kansas 66506} 45_{Louisiana Tech University, Ruston, Louisiana 71272} 46_{University of Maryland, College Park, Maryland 20742}

47_{Boston University, Boston, Massachusetts 02215} 48_{Northeastern University, Boston, Massachusetts 02115}

49_{University of Michigan, Ann Arbor, Michigan 48109} 50_{Michigan State University, East Lansing, Michigan 48824}

51_{University of Nebraska, Lincoln, Nebraska 68588} 52_{Columbia University, New York, New York 10027} 53_{University of Rochester, Rochester, New York 14627} 54_{State University of New York, Stony Brook, New York 11794}

55_{Brookhaven National Laboratory, Upton, New York 11973} 56_{Langston University, Langston, Oklahoma 73050}

58_{Brown University, Providence, Rhode Island 02912} 59_{University of Texas, Arlington, Texas 76019} 60_{Texas A&M University, College Station, Texas 77843}

61_{Rice University, Houston, Texas 77005} 62_{University of Virginia, Charlottesville, Virginia 22901} 63_{University of Washington, Seattle, Washington 98195}

We present the results of a search for leptoquark (LQ) pairs in (85.2_{± 3.7) pb}−1_{of p¯p collider data}

collected by the DØ experiment at the Fermilab Tevatron. We observe no evidence for leptoquark production and set a limit on σ(p¯p_{→ LQLQ → νν + jets) as a function of the mass of the leptoquark} (mLQ). Assuming the decay LQ→ νq, we exclude scalar leptoquarks for mLQ < 98 GeV/c2, and

vector leptoquarks for mLQ< 200 GeV/c2and coupling which produces the minimum cross section,

The observed symmetry between the lepton (l) and quark (q) sectors suggests the existence of a force con-necting the two that is mediated by leptoquark (LQ) particles that couple directly to both leptons and quarks. Such particles arise naturally as vector [1] or scalar bosons [2] in Grand Unified Theories [1], as composite particles [3], as techniparticles [4], or as R-parity violat-ing supersymmetric particles [5].

Leptoquarks would carry both color and fractional electric charge. They could be pair-produced at the Fer-milab Tevatron through a virtual gluon (g) in the strong process p¯p_{→ g → LQLQ + X, with a production cross} section that, for scalar leptoquarks, is independent of the LQ_{− q − l coupling. For vector leptoquarks, we consider} the specific cases of the coupling resulting in the minimal cross section (σmin), Minimal Vector coupling (MV), and Yang-Mills coupling (YM) [6].

Limits from flavor-changing neutral currents imply that leptoquarks of low massO(TeV) couple only within a single generation [7], and the decays of leptoquark pairs would therefore be expected to yield one of three possible final states: l±_l∓_{qq, l}±(−)_{ν qq, and ννqq. This analysis [8]} is based on the ννqq final state, and is sensitive to lep-toquarks of all three generations. In a previous study of this final state [9] with the assumed decay LQ_{→ νq, DØ} set limits of mLQ > 79 GeV/c2 _{for scalar leptoquarks,} and mLQ> 144 GeV/c2_{, 159 GeV/c}2_{, and 206 GeV/c}2_, for vector leptoquarks with couplings that correspond to σmin, MV, and YM couplings, respectively [9,10]. The present analysis is based on a factor of ten increase in data over the previous analysis. The CDF collaboration has conducted a search for second and third generation leptoquarks, also assuming the decay LQ→ νq, and set mass limits of 123 (148) GeV/c2 _{for second (third)} gen-eration scalar leptoquarks, and 171 (199) GeV/c2 _and 222 (250) GeV/c2 _{for second (third) generation vector} leptoquarks with MV and YM couplings, respectively [11]. The OPAL collaboration has searched√s=183 GeV e+_e− _{collisions for vector and scalar leptoquarks with} specific weak isospins and decay modes [12]. For first and second generation scalar leptoquarks with weak isospin of 1 and the decay LQ_{→ νq, OPAL has set a mass limit of} 84.8 GeV/c2_{. For other values of weak isospin, the mass} limit ranges from 71.6 GeV/c2_{to 80.8 GeV/c}2_{. Our new} results extend the range of sensitivity of the vector lepto-quark searches and the first generation scalar leptolepto-quark searches.

The DØ detector [13] consists of three major subsys-tems: an inner detector for tracking charged particles; an uranium/liquid-argon calorimeter for measuring elec-tromagnetic and hadronic showers; and a muon spec-trometer. The inner detector consists of two outer drift chambers separately covering the regions_{|η| < 1 and 1.2} <_{|η| < 2.8, and an inner drift chamber covering the} re-gion_{|η| <2. The calorimeter consists of three cryostats}

supplemented with scintillators between the cryostats. The Main Ring beam pipe used to accelerate and in-ject protons and antiprotons into the Tevatron traverses the hadronic region of the calorimeter at φ=100◦. The jets measured with the calorimeter have a resolution of approximately δE=0.8√E (E in GeV). We measure the missing transverse energy (E/_T) by summing the calorime-ter energy vectorially in the plane transverse to the beam. The projection of E/_T on a given axis has a resolution of δE/x,y =1.08 GeV + 0.019(Σ|Ex,y|) (Ex,y in GeV).

The event sample for our search is collected with a trig-ger requiring a jet with ET > 25 GeV, a second jet with ET > 10 GeV, E/_T > 25 GeV, and the azimuthal angle between any jet and E/_T(∆φ(jet, E/_T)) greater than 14.3◦_. The trigger does not collect data in the 0.4 seconds fol-lowing Main Ring injection, or within 800 nanoseconds of protons or antiprotons passing through the detector. We remove data affected by accelerator noise or detector malfunctions. The former are identified by significant energy measurement in the region surrounding the Main Ring. The latter are identified by recurring energy mea-surement in a particular region of the calorimeter, by energy measurement isolated to a single calorimeter cell, and by documented subsystem malfunctions. The inte-grated luminosity for this sample corresponds to 85.2_± 3.7 pb−1_.

We select events with well-understood trigger efficiency by requiring at least two jets with ET > 50 GeV, E/_T > 40 GeV, ∆φ(jet, E/_T) > 30◦, and ∆R(jet, jet) > 1.5, where ∆R =p(∆η)2_{+ (∆φ)}2_{, η is the jet} pseudorapid-ity, and φ is the jet azimuthal angle. Jets are defined as the calorimeter energy within a ∆R = 0.5 cone. We reduce cosmic-ray backgrounds by rejecting events con-taining jets with little energy in the electromagnetic sec-tions of the calorimetry. Backgrounds arising from W or Z boson production are reduced by rejecting events with isolated muons or jets with a large fraction of their energy measured in the electromagnetic calorimeter.

The remaining backgrounds in the sample consist of events with jets produced in association with a W or a Z boson, and events from top quark and multijet pro-duction. We use Monte Carlo generators to simulate the kinematics and topologies of events with W or Z bosons or top quarks, and a geant-based simulation [14] of the detector to predict the acceptance for these events.

processes. We scale the generator cross sections to match the corresponding W/Z + jet(s) cross sections measured using decays into electrons. These cross sections were remeasured specifically for this analysis.

To obtain the background from t¯t, t¯b, and ¯tb pro-duction, where the top quark decays to an unobserved charged lepton, a neutrino, and a jet, we use our mea-sured cross section for t¯t production [17], and the cal-culated next-to-leading-order cross section for the single-top production processes [18]. We use the herwig Monte Carlo [19] program to generate t¯t events and the Com-pHEP Monte Carlo [20] program to generate t¯b and ¯tb events.

The multijet background arises primarily from a mis-measurement of the interaction vertex or of jet energy. To reduce the number of events with mismeasured ver-tices, we use the central drift chamber (CDC) to asso-ciate tracks with the two highest ET jets, if those jets are in the fiducial volume of the CDC (|η| ≤1). These tracks are used to determine the point-of-origin of each jet, which is required to be no further than 15 cm from the reconstructed event vertex (the latter is determined from all tracks in the event). The 15 cm value is cho-sen to maximize the inverse of the fractional uncertainty on signal (see below). We reduce the number of events with poorly measured jet energies by requiring that the azimuth ∆φ between the E/T vector and the direction of the jet with the second highest ET exceed 60◦. Table I shows the number of events remaining in the data after each additional selection criterion.

Selection criterion # of Events

2 Jets + E/_T Trigger 503,557

No accelerator noise or detector malfunctions 399,557

Leading jet ET ≥ 50 GeV 236,339

Second jet ET ≥ 50 GeV 86,826

E/_{T ≥ 40 GeV} 8,996

∆φ(jet,E/_T)≥ 30o _1,567

∆_{R(jet, jet) > 1.5} 1,495

Jet EM Fraction cuts 1,358

No isolated muons 1,332

Leading or second jet|η| ≤1.0; all jets |η| ≤4.0 1,071

|Jet vertex - Primary vertex| < 15 cm 401

∆φ(jet 2,E/_T)_{≥ 60}o ₂₃₁

TABLE I. The set of criteria imposed on the 2 jets + E/_T data sample and the number of events that pass each addi-tional selection criterion.

To estimate the remaining multijet background in our search sample, we count events in which jet-based vertex positions deviate by 15 cm to 50 cm from the position of the event vertex. In events with two central (_{|η| ≤1)} jets, we require both vertices to fall within this range. We normalize these events to the search sample using a

multijet-dominated sample (∆φ(jet 2, E/_T)< 60◦). The expected multijet background is:

N

= N

(

N15<z<50

)

We choose the upper bound of 50 cm to provide the best match between expected background and data for events with E/_T between 30 GeV and 40 GeV, a region domi-nated by multijet events. We predict 162.8_{± 23.7} multi-jet and 51.9_{± 7.0 W , Z, and top events in this sample.} We observe 224 events in the data. Changing the vertex threshold to 100 cm increases the multijet background prediction by 22% in this region, which we take as an estimate of the systematic error of the method. Table II shows the total expected background and the observed number of events for the final 2 jets + E/_T data sample.

To model the characteristics of leptoquark produc-tion, we use scalar leptoquark events generated with the pythia Monte Carlo program and vector leptoquark events generated with the CompHEP Monte Carlo

pro-gram. The cross sections for scalar leptoquark produc-tion have been calculated to next-to-leading order [21], while those for vector leptoquark production have been calculated to leading order [22]. The calculations use a QCD renormalization and factorization scale of µ=mLQ, with theoretical uncertainties estimated by changing the scale to µ=mLQ/2 and µ=2mLQ. For scalar leptoquarks we use the smaller predicted cross section (µ=2mLQ) for determining the mass limits on LQ’s.

Failure to observe any hypothetical signal at 95% confi-dence level (C.L.) corresponds approximately to a down-ward fluctuation of that signal by two standard devia-tions. Hence, to increase the sensitivity of our search for the production of leptoquarks that decay to νq, we search for leptoquarks that would produce excesses of approxi-mately two standard deviations. We separately optimize our selection criteria for the production of 100 GeV/c2 scalar leptoquarks and for 200 GeV/c2_{vector leptoquarks} with Minimal Vector coupling. Other choices of lepto-quark masses do not significantly affect our results. We use the jetnet [23] neural network program to isolate re-gions of significant leptoquark production, with E/_T and ∆φ(jet, jet) as inputs for scalar leptoquarks, and E/_T and the ET of the jet with the second highest ET as inputs for vector leptoquarks. The values of the neural network output variables and the thresholds for these masses are shown in Fig. 1. The thresholds are chosen to maximize the quantity:

Nlq

√

Nlq+Nback+(∆Nlq)2+(∆Nback)2

,

their associated uncertainties. This quantity reflects the inverse of the fractional uncertainty on signal. After ap-plying these thresholds, we expect 56.0+8.1

−8.2 events and observe 58 events for the scalar leptoquark optimization, and expect 13.3+2.8

−2.6events and observe 10 events for the vector leptoquark optimization.

Type of Events # of Events

Multijet 58.8_{± 14.1 ± 12.9} (W → eν) + jet 51.9± 7.0+13.7 −8.9 (W _{→ τν) + jet} 46.3_{± 5.0}+8.9 −7.7 (Z_{→ νν) + 2 jets} 36.1_{± 7.7}+9.0 −5.5 (W → µν) + 2 jets 18.7± 3.5+4.2 −3.7 t¯t_{→ l}±_{ν + 4 jets 10.6± 2.0 ± 2.3} (W → eν) + 2 jets 8.3± 2.5+2.0 −2.5 (W _{→ τν) + 2 jets} 5.6_{± 1.7}+1.4 −0.8 tb→ l±_{ν + 2 jets 2.0± 0.3 ± 0.2} (Z_{→ ττ) + jet} 2.0_{± 0.4}+0.6 −0.3 (Z_{→ µµ) + 2 jets} 1.7_{± 0.4}+0.4 −0.3 Total background 242.0_{± 18.9}+23.3 −19.0 Data 231

TABLE II. The expected and observed numbers of events in the final 2 jets + E/_T sample.

FIG. 1. The neural network output for data (points), for background (solid histogram), and for leptoquarks (dashed histogram). The optimization is for 100 GeV/c2_scalar

lepto-quarks (left) and 200 GeV/c2 _{vector leptoquarks with}

Mini-mal Vector coupling (right). We remove events to the left of the arrows.

After applying the optimal thresholds, we find that the observed number of events is consistent with the expected background, and that, consequently, we have no evidence for leptoquark production. This null result yields the 95% C.L. upper limit on cross section (Fig. 2) as a func-tion of leptoquark mass. We calculate the limit using a Bayesian method [24] with a flat prior for the signal and Gaussian priors for background and acceptance un-certainties. The equivalent limits on mass are 98 GeV/c2 for scalar leptoquarks, and 200 GeV/c2_{, 238 GeV/c}2_{, and} 298 GeV/c2_{for vector leptoquarks with couplings} corre-sponding to the minimum cross section σmin, Minimal Vector coupling, and Yang-Mills coupling, respectively.

We summarize the DØ mass limits as a function of the branching fraction B(LQ → l±_{q) for first [9] and} sec-ond [25] generation leptoquarks in Fig. 3. These limits combine searches using the final states l±l∓qq, l±(−)ν qq, and ννqq. We note that the gap at small values of B(LQ → l±_{q) in previous analyses has been filled as a} result of this investigation.

DØ (1992-93)

DØ (1994-96)

DØ (1994-96)

FIG. 2. Limits on cross section at 95% confidence level, as a function of leptoquark mass, for scalar (top) and vector (bottom) leptoquarks, and different theoretical predictions. We assume the LQ decays exclusively to νq. The theoretical predictions correspond to the production of leptoquarks of a single generation, while the experimental limit corresponds to the sum of contributions from leptoquarks of all three gener-ations.

Nether-FIG. 3. The region of B(LQ → l±_{q) vs. mass for}

first-generation (top) and second-generation (bottom) lepto-quarks excluded by DØ.

lands), PPARC (United Kingdom), Ministry of Educa-tion (Czech Republic), and the A.P. Sloan FoundaEduca-tion.

∗ _{Visitor from University of Zurich, Zurich, Switzerland.}

† _{Visitor from Institute of Nuclear Physics, Krakow,}

Poland.

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