Search for Leptoquark Pairs Decaying to $\boldmath \nu\nu$ + jets in $\boldmath p\overline{p}~$ Collisions at $\boldmath \sqrt{s}$=1.8 TeV\\
%% names begin here 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\'{o}mez
$^{5}$ G.~G\'{o}mez
$^{46}$ P.I.~Goncharov
$^{26}$ J.L.~Gonz\'alez~Sol\'{\i}s
$^{19}$ 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
$^{39,\dag}$ 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\'{\i}n
$^{1}$ 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
$^{48}$ Q.~Xu
$^{49}$ R.~Yamada
$^{36}$ P.~Yamin
$^{55}$ T.~Yasuda
$^{36}$ Y.A.~Yatsunenko
$^{23}$ K.~Yip
$^{55}$ S.~Youssef
$^{35}$ J.~Yu
$^{36}$ Z.~Yu
$^{39}$ M.~Zanabria
$^{5}$ X.~Zhang
$^{57}$ H.~Zheng
$^{41}$ B.~Zhou
$^{49}$ Z.~Zhou
$^{42}$ M.~Zielinski
$^{53}$ D.~Zieminska
$^{40}$ A.~Zieminski
$^{40}$ V.~Zutshi
$^{55}$ E.G.~Zverev
$^{25}$ and~A.~Zylberstejn$^{13}$
\vskip 0.30cm \centerline{(D\O\ Collaboration)} \vskip 0.30cm
\input abstract.tex %We present the results of a search for leptoquark ($LQ$) pairs using (85.2 %$\pm$ 3.7) pb$^{-1}$ of $p{\bar p}$ collider data collected by the D\O\ %experiment at the Fermilab Tevatron from 1994-1996. We observe no evidence %for leptoquark production and set a limit on $\sigma (p{\bar p} \rightarrow %LQ\overline{LQ} \rightarrow \nu\nu~+~jets)$ as a function of the mass of the %leptoquark ($M_{LQ}$), assuming BR($LQ \rightarrow \nu q$)=100\%. At the %95\% confidence level, we exclude scalar leptoquarks for $M_{LQ} <$ 99 %GeV/c$^2$, and vector leptoquarks for $M_{LQ} <$ 178 GeV/c$^2$ assuming %couplings that result in the minimum production cross section.
abstract.tex
We present the results of a search for leptoquark ($LQ$) pairs in (85.2  
$\pm$ 3.7) pb$^{-1}$ of $p{\bar p}$ collider data collected by the D\O\ 
experiment at the Fermilab Tevatron.  We observe no evidence for leptoquark 
production and set a limit on $\sigma (p{\bar p} \rightarrow LQ\overline{LQ} 
\rightarrow \nu\nu$ + jets) as a function of the mass of the leptoquark 
($m_{LQ}$).  Assuming the decay LQ $\rightarrow \nu q$, we exclude scalar 
leptoquarks for $m_{LQ} <$ 98 GeV/$c^2$, and vector leptoquarks for $m_{LQ} 
<$ 200 GeV/$c^2$ and coupling which produces the minimum cross section, at a 
95\% confidence level. 

body.tex
The observed symmetry between the lepton ($l$) and quark ($q$) sectors 
suggests the existence of a force connecting the two that is mediated by
leptoquark ($LQ$) particles that couple directly to both leptons and 
quarks.  Such particles arise naturally as vector \cite{lq} or scalar bosons 
\cite{lqhiggs} in Grand Unified Theories \cite{lq}, as composite particles 
\cite{lqcomp}, as techniparticles \cite{lqtechni}, or as R-parity violating 
supersymmetric particles \cite{rpvio}.
\par 
Leptoquarks would carry both color and fractional electric charge.  They could 
be pair-produced at the Fermilab Tevatron through a virtual gluon ($g$) in the 
strong process $p{\bar p} \rightarrow g \rightarrow LQ\overline{LQ} + X$, 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 ($\sigma _{\rm min}$), 
Minimal Vector coupling (MV), and Yang-Mills coupling (YM) \cite{mc}.   
\par 
Limits from flavor-changing neutral currents imply that leptoquarks of low 
mass ${\cal O}$(TeV) couple only within a single generation \cite{generation}, 
and the decays of leptoquark pairs would therefore be expected to yield one of 
three possible final states:  $l^{\pm}l^{\mp} q\overline{q}$, 
$l^{\pm}\! \! \! \stackrel{(-)}{\nu} \! \! \! q\overline{q}$, and 
$\nu\overline{\nu} q\overline{q}$.  This analysis \cite{thesis} is based on 
the $\nu\overline{\nu} q\overline{q}$ final state, and is sensitive to 
leptoquarks of all three generations.  In a previous study of this final state 
\cite{d01a} with the assumed decay $LQ \rightarrow \nu q$, D\O\ set limits of 
$m_{LQ} >$ 79 GeV/$c^2$ for scalar leptoquarks, and $m_{LQ} >$ 144 GeV/$c^2$, 
159 GeV/$c^2$, and 206 GeV/$c^2$, for vector leptoquarks with couplings that 
correspond to $\sigma _{\rm min}$, MV, and YM couplings, respectively 
\cite{d01a,d01avector}.  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 $LQ \rightarrow \nu q$, and set mass limits of 123 (148) GeV/$c^2$ 
for second (third) generation scalar leptoquarks, and 171 (199) GeV/$c^2$ and 
222 (250) GeV/$c^2$ for second (third) generation vector leptoquarks with MV 
and YM couplings, respectively \cite{cdfg23}.  The OPAL collaboration has 
searched $\sqrt{s}$=183 GeV $e^+e^-$ collisions for vector and scalar 
leptoquarks with specific weak isospins and decay modes \cite{OPAL}.  For 
first and second generation scalar leptoquarks with weak isospin of 1 and the 
decay $LQ \rightarrow \nu q$, OPAL has set a mass limit of 84.8 
GeV/$c^2$.  For other values of weak isospin, the mass limit ranges from 71.6 
GeV/$c^2$ to 80.8 GeV/$c^2$.  Our new results extend the range of sensitivity 
of the vector leptoquark searches and the first generation scalar leptoquark 
searches.
\par 
The D\O\ detector \cite{detector} consists of three major subsystems:  an  
inner detector for tracking charged particles; an uranium/liquid-argon 
calorimeter for measuring electromagnetic and hadronic showers; and a muon  
spectrometer.  The inner detector consists of two outer drift chambers 
separately covering the regions $|\eta|<1$ and 1.2 $< |\eta| <$ 2.8, and an
inner drift chamber covering the region $|\eta|<$2.  The calorimeter consists
of three cryostats supplemented with scintillators between the cryostats.  The 
Main Ring beam pipe used to accelerate and inject protons and antiprotons into 
the Tevatron traverses the hadronic region of the calorimeter at 
$\phi$=100$^{\circ}$.  The jets measured with the calorimeter have a 
resolution of approximately $\delta E$=0.8$\sqrt{E}$ ($E$ in GeV).  We measure 
the missing transverse energy (\met) by summing the calorimeter energy 
vectorially in the plane transverse to the beam.  The projection of \met\ on a 
given axis has a resolution of $\delta$\mexy\ =1.08 GeV + 
0.019($\Sigma |E_{x,y}|$) ($E_{x,y}$ in GeV).  
\par 
The event sample for our search is collected with a trigger requiring a jet 
with $E_T >$ 25 GeV, a second jet with $E_T >$ 10 GeV, \met\ $>$ 25 GeV, and 
the azimuthal angle between any jet and \met ($\Delta\phi$(jet, \met)) greater
than 14.3$^{\circ}$.  The trigger does not collect data in the 0.4 seconds 
following 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 measurement in a particular region 
of the calorimeter, by energy measurement isolated to a single calorimeter 
cell, and by documented subsystem malfunctions.  The integrated luminosity for 
this sample corresponds to 85.2 $\pm$ 3.7 pb$^{-1}$.  
\par
We select events with well-understood trigger efficiency by requiring at least 
two jets with $E_T >$ 50 GeV, \met\ $>$ 40 GeV, $\Delta\phi$(jet, \met) $ >$ 
30$^{\circ}$, and $\Delta {\cal R} $(jet, jet) $>$ 1.5, where 
${\Delta \cal R}=\sqrt{(\Delta\eta)^2 + (\Delta\phi)^2}$, $\eta$ is the jet 
pseudorapidity, and $\phi$ is the jet azimuthal angle.  Jets are defined as 
the calorimeter energy within a ${\Delta \cal R}=0.5$ cone.  We reduce 
cosmic-ray backgrounds by rejecting events containing jets with little energy 
in the electromagnetic sections 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.
\par 
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 production.  We use Monte Carlo generators to simulate the kinematics 
and topologies of events with $W$ or $Z$ bosons or top quarks, and a 
{\sc geant}-based simulation \cite{geant} of the detector to predict the 
acceptance for these events.
\par
The $W$ and $Z$ backgrounds correspond to processes involving only neutrinos 
and jets ($Z$ + 2 jets $\rightarrow \nu\nu$ + 2 jets and $W$ + jet 
$\rightarrow \tau\nu +$ jet, with $\tau \rightarrow $ hadrons + $\nu$), 
processes with undetected charged leptons ($W$ + 2 jets $\rightarrow l^{\pm} 
\nu$ + 2 jets, $Z$ + 2 jets $\rightarrow \mu\mu$ + 2 jets, and $Z$ + jet 
$\rightarrow \tau\tau$ + jet, with one $\tau \rightarrow$ hadrons + $\nu$), 
and processes in which an electron is misidentified as a jet ($W$ + jet 
$\rightarrow e\nu$ + jet and $W$ + jet $\rightarrow \tau\nu$ + jet, with $\tau
\rightarrow e\nu\nu$).  We use the {\sc pythia} Monte Carlo generator 
\cite{pythia} to generate the $W/Z$ + jet processes, and the {\sc vecbos} 
Monte Carlo generator \cite{vecbos} to generate the $W/Z$ + 2 jets 
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.
\par 
To obtain the background from $t{\bar t}$, $t{\bar b}$, and ${\bar t}b$ 
production, where the top quark decays to an unobserved charged lepton, a 
neutrino, and a jet, we use our measured cross section for $t{\bar t}$ 
production \cite{ttbar}, and the calculated next-to-leading-order cross 
section for the single-top production processes \cite{singlet}.  We use 
the {\sc herwig} Monte Carlo \cite{herwig} program to generate $t{\bar t}$ 
events and the \footnotesize CompHEP \normalsize Monte Carlo \cite{comphep} 
program to generate $t{\bar b}$ and ${\bar t}b$ events. 
\par 
The multijet background arises primarily from a mismeasurement of the 
interaction vertex or of jet energy.  To reduce the number of events with 
mismeasured vertices, we use the central drift chamber (CDC) to associate 
tracks with the two highest $E_T$ jets, if those jets are in the fiducial 
volume of the CDC ($|\eta|\leq$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 chosen 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 $\Delta\phi$ 
between the \met\ vector and the direction of the jet with the second highest 
$E_T$ exceed 60$^{\circ}$.  Table \ref{tbl:initial_cuts} shows the number of 
events remaining in the data after each additional selection criterion.

\begin{table}[ht]
\begin{center}
\begin{tabular}{cc}
Selection criterion                               & \# of Events \\
\hline
  2 Jets + \met\ Trigger                  & 503,557       \\
  No accelerator noise or detector malfunctions &   399,557       \\
  Leading jet $E_T \geq$ 50 GeV           & 236,339       \\
  Second jet $E_T \geq$ 50 GeV            & 86,826       \\
  \met\ $\geq$ 40 GeV                     &  8,996       \\
  $\Delta\phi$(jet,\met) $\geq$ 30$^o$    &  1,567       \\
  $\Delta {\cal R} $(jet, jet) $>$ 1.5    &  1,495  \\
  Jet EM Fraction cuts                    &  1,358  \\
  No isolated muons                       &  1,332  \\
  Leading or second jet $|\eta|\leq$1.0; all jets $|\eta|\leq$4.0&   1,071     \\
  $|$Jet vertex - Primary vertex$|<$ 15 cm  & 401 \\
  $\Delta\phi$(jet 2,\met) $\geq$ 60$^o$  &  231  \\
\end{tabular}
\end{center}
\vskip -0.1in
\caption{The set of criteria imposed on the 2 jets + \met\ data sample and 
the number of events that pass each additional selection criterion. }
\label{tbl:initial_cuts}
\end{table}

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 ($|\eta|\leq$1) 
jets, we require both vertices to fall within this range.  We normalize these 
events to the search sample using a multijet-dominated sample 
($\Delta\phi$(jet 2, \met)$< 60^{\circ}$).  The expected multijet background
is:
\large
\begin{center}
$N_{mj}=N_{\Delta\phi > 60^{\circ}}^{15 < z < 50}(\frac{N^{z  < 15}}{N^{15 < z  < 50}})_{\Delta\phi < 60^{\circ}}$
\end{center}  
\normalsize
\noindent 
We choose the upper bound of 50 cm to provide the best match between expected 
background and data for events with \met\ between 30 GeV and 40 GeV, a region
dominated by multijet events.  We predict 162.8 $\pm$ 23.7 multijet and 51.9 
%(Table \ref{tbl:qcd3040})
$\pm$ 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 \ref{tbl:datbd} shows the total 
expected background and the observed number of events for the final 2 jets + 
\met\ data sample. 
\par 
To model the characteristics of leptoquark production, we use scalar  
leptoquark events generated with the {\sc pythia} Monte Carlo program and 
vector leptoquark events generated with the 
\footnotesize CompHEP \normalsize Monte Carlo program.  The cross sections for 
scalar leptoquark production have been calculated to next-to-leading order 
\cite{kraemer}, while those for vector leptoquark production have been 
calculated to leading order \cite{vlqxsec}.  The calculations use a QCD
renormalization and factorization scale of $\mu$=$m_{LQ}$, with theoretical 
uncertainties estimated by changing the scale to $\mu$=$m_{LQ}$/2 and 
$\mu$=2$m_{LQ}$.  For scalar leptoquarks we use the smaller predicted cross 
section ($\mu$=2$m_{LQ}$) for determining the mass limits on $LQ$'s.
\par 
Failure to observe any hypothetical signal at 95\% confidence level (C.L.)
corresponds approximately to a downward fluctuation of that signal by two 
standard deviations.  Hence, to increase the sensitivity of our search for the
production of leptoquarks that decay to $\nu q$, we search for leptoquarks 
that would produce excesses of approximately two standard deviations.  We 
separately optimize our selection criteria for the production of 100 GeV/$c^2$ 
scalar leptoquarks and for 200 GeV/$c^2$ vector leptoquarks with Minimal 
Vector coupling.  Other choices of leptoquark masses do not significantly 
affect our results.  We use the {\sc jetnet} \cite{jetnet} neural network 
program to isolate regions of significant leptoquark production, with \met\ 
and $\Delta\phi$(jet, jet) as inputs for scalar leptoquarks, and \met\ and the 
$E_T$ of the jet with the second highest $E_T$ as inputs for vector 
leptoquarks.  The values of the neural network output variables and the 
thresholds for these masses are shown in Fig. \ref{fig:nnout}.  The thresholds 
are chosen to maximize the quantity:
\large
\begin{center}
$\frac{N_{lq}}{\sqrt{N_{lq}+N_{\rm back}+(\Delta N_{lq})^2+(\Delta N_{\rm back})^2}}$,
\end{center}  
\normalsize
\noindent 
where $N_{lq}$ and $N_{\rm back}$ are the number of signal and background
events, respectively, and $\Delta N_{lq}$ and $\Delta N_{\rm back}$ are their
associated uncertainties.  This quantity reflects the inverse of the 
fractional uncertainty on signal.  After applying these thresholds, we expect
56.0$^{+8.1} \! \! \! \! \! \! \! \! \! \! \! _{-8.2}$ events and observe
58 events for the scalar leptoquark optimization, and expect
13.3$^{+2.8} \! \! \! \! \! \! \! \! \! \! \!  _{-2.6}$ events and observe
10 events for the vector leptoquark optimization.
%Table \ref{tbl:nn} shows the number of 
%events observed, the number expected from background, and the 95\% C.L. 
%upper limits on the cross sections for $LQ$ production.

%\begin{table}[ht] 
%\begin{center} 
%\begin{tabular}{|cc|} 
%Type of Events 						& \# of Events \\ 
%\hline
% Multijet 					& 162.8 $\pm$ 23.7\\ 
%%\hline 
%W, Z, and top 					& 51.9 $\pm$ 7.0 \\ 
%%\hline 
%$~~~~~~~~~~~~~~$ Total background $~~~~~~~~~~~~~~$& 214.7 $\pm$ 24.7 \\ 
% Data & 224 \\ 
%\end{tabular} 
%\end{center} 
%%\vskip -0.1in 
%\caption{The expected and observed numbers of events for \met\ between 30 GeV 
%and 40 GeV.  This region is dominated by multijet events.} 
%\label{tbl:qcd3040} 
%\end{table} 

\begin{table}[hptb] 
\begin{center} 
\begin{tabular}{cc} 
Type of Events & \# of Events \\ 
\hline
 Multijet & 58.8 $\pm$ 14.1 $\pm$ 12.9\\ 
%\hline 
($W \rightarrow e\nu$) + jet & 51.9 $\pm$ 7.0 $^{+13.7} \! \! \! \! \! \! \! \! \! \! \! \! \! _{-8.9}$ \\ 
%\hline 
($W \rightarrow \tau\nu$) + jet & 46.3 $\pm$ 5.0 $^{+8.9} \! \! \! \! \! \! \! \! \! \! _{-7.7}$ \\ 
%\hline 
($Z \rightarrow \nu\nu$) + 2 jets & 36.1 $\pm$ 7.7 $^{+9.0} \! \! \! \! \! \! \! \! \! \! \! _{-5.5}$ \\ 
%\hline 
($W \rightarrow \mu\nu$) + 2 jets & 18.7 $\pm$ 3.5 $^{+4.2} \! \! \! \! \! \! \! \! \! \! _{-3.7}$ \\ 
%\hline 
$t{\bar t} \rightarrow$ $l^{\pm}\nu$ + 4 jets & 10.6 $\pm$ 2.0 $\pm$ 2.3 \\ 
%\hline 
($W \rightarrow e\nu$) + 2 jets & 8.3 $\pm$ 2.5 $^{+2.0} \! \! \! \! \! \! \! \! \! \! _{-2.5}$ \\ 
%\hline 
($W \rightarrow \tau\nu$) + 2 jets & 5.6 $\pm$ 1.7 $^{+1.4} \! \! \! \! \! \! \! \! \! \! _{-0.8}$ \\ 
%\hline 
$tb \rightarrow$ $l^{\pm}\nu$ + 2 jets & 2.0 $\pm$ 0.3 $\pm$ 0.2 \\  
%\hline 
($Z \rightarrow \tau\tau$) + jet & 2.0 $\pm$ 0.4 $^{+0.6} \! \! \! \! \! \! \! \! \! \! _{-0.3}$ \\ 
%\hline 
($Z \rightarrow \mu\mu$) + 2 jets & 1.7 $\pm$ 0.4 $^{+0.4} \! \! \! \! \! \! \! \! \! \! _{-0.3}$ \\ 
%\hline 
$~~~~~~~~~~~~~$ Total background $~~~~~~~~~~~~~$& 242.0 $\pm$ 18.9 $^{+23.3} \! \! \! \! \! \! \! \! \! \! \! \! \! _{-19.0}$ \\ 
 Data & 231 \\ 
\end{tabular} 
\end{center} 
%\vskip -0.1in 
\caption{The expected and observed numbers of events in the final 2 jets + 
\met\ sample.} 
\label{tbl:datbd} 
\end{table} 

\begin{figure}[!htbp]  
\begin{minipage}[htb]{4.3cm} 
\epsfysize = 4.3cm  
\centerline{\epsffile{slqnnout.eps}} 
\end{minipage} 
\hskip -0.1cm
\begin{minipage}[htb]{4.3cm} 
\epsfysize = 4.3cm  
\centerline{\epsffile{vlqnnout.eps}} 
\end{minipage} 
\caption{The neural network output for data (points), for background (solid
histogram), and for leptoquarks (dashed histogram).  The optimization is for 
100 GeV/$c^2$ scalar leptoquarks (left) and 200 GeV/$c^2$ vector leptoquarks 
with Minimal Vector coupling (right).  We remove events to the left of the 
arrows.} 
\label{fig:nnout} 
\end{figure} 
 
%\begin{table}[hptb] 
%\begin{center} 
%\begin{tabular}{|cccc|} 
%Leptoquark & $N_{\rm data}$       & $N_{\rm back}$      & $\sigma ^{95\%}$ (pb)\\ 
%\hline
% Scalar & 58 & 56.0 $^{+8.1} \! \! \! \! \! \! \! \! \! \! \! _{-8.2}$ & 11.1 \\ 
% Vector & 10 & 13.3 $^{+2.8} \! \! \! \! \! \! \! \! \! \! \!  _{-2.6}$ & 0.60 \\ 
%\end{tabular} 
%\end{center} 
%%\vskip -0.1in 
%\caption{The data, the expected background, and the 95\% confidence level  
%limits on the cross sections for $LQ$ production.} 
%\label{tbl:nn} 
%\end{table} 

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. \ref{fig:lqlim}) as a 
function of leptoquark mass.  We calculate the limit using a Bayesian method 
\cite{bayes} with a flat prior for the signal and Gaussian priors for 
background and acceptance uncertainties.  The equivalent limits on mass are 98 
GeV/$c^2$ for scalar leptoquarks, and 200 GeV/$c^2$, 238 GeV/$c^2$, and 
298 GeV/$c^2$ for vector leptoquarks with couplings corresponding to the 
minimum cross section $\sigma _{\rm min}$, Minimal Vector coupling, and 
Yang-Mills coupling, respectively.  We summarize the D\O\ mass limits as a 
function of the branching fraction ${\cal B}$($LQ \rightarrow l^{\pm}q$) for 
first \cite{d01a} and second \cite{d0g2} generation leptoquarks in Fig. 
\ref{fig:brvmass}.  These limits combine searches using the final states 
$l^{\pm}l^{\mp} q\overline{q}$, $l^{\pm}\! \! \! \stackrel{(-)}{\nu} \! \! \! 
q\overline{q}$, and $\nu\overline{\nu} q\overline{q}$.  We note that the gap 
at small values of ${\cal B}$($LQ \rightarrow l^{\pm}q$) in previous analyses 
has been filled as a result of this investigation.

\begin{figure}[htb] 
\begin{center}
\begin{minipage}[htb]{5.5cm} 
\epsfysize = 5.5cm  
\centerline{\epsffile{slqlim.eps}} 
\end{minipage} 
\begin{minipage}[htb]{5.5cm} 
\epsfysize = 5.5cm  
\centerline{\epsffile{vlqlim.eps}} 
\end{minipage} 
\caption{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 
$\nu q$.  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 generations. }
\label{fig:lqlim} 
\end{center}
\end{figure} 
\begin{figure}[htb] 
\begin{center}
\begin{minipage}[htb]{7.0cm} 
\epsfysize = 7.0cm  
\centerline{\epsffile{g1.eps}} 
\end{minipage} 
\begin{minipage}[htb]{7.0cm} 
\epsfysize = 7.0cm  
\centerline{\epsffile{g2.eps}} 
\end{minipage} 
\caption{The region of ${\cal B}$($LQ \rightarrow l^{\pm}q$) vs. mass for 
first-generation (top) and second-generation (bottom) leptoquarks excluded by 
D\O.} 
\label{fig:brvmass} 
\end{center}
\end{figure} 

prl_v7.tex
% LATEX (RevTex) template for submission of papers to ICHEP98, 
\documentstyle[prl,epsf,aps,floats]{revtex}  % use for PRL mode
%\documentstyle[doublespace,prl,epsf,aps,floats]{revtex}  % use for PRL mode
%\def\baselinestretch{2}
%use this for prd format
%\documentstyle[prd,epsfig,aps,amsmath,floats,tighten]{revtex}
%use this for preprint
%\documentstyle[preprint,eqsecnum,epsfig,aps,floats,tighten]{revtex} % for preprint mode
% Following 4 commands allow a Figure to occupy most of a column without being
% placed on a separate page which includes no text.
%
%\usepackage[]{graphics}
%\usepackage[dvips]{color}
%\usepackage{subfigure}
%\usepackage{epsfig}
%\input epsf
\input psfig
%\input d0_style.tex
%
%
%\input eps_graphics
\renewcommand{\topfraction}{0.95}
\renewcommand{\bottomfraction}{0.95}
\renewcommand{\floatpagefraction}{0.95}
\renewcommand{\textfraction}{0.05}

% A useful Journal macro
\def\Journal#1#2#3#4{{#1} {\bf #2}, #3 (#4)}

% Journal names used here
\def\NIM{Nucl. Instrum. Methods A}
\def\NP{Nucl. Phys. B}
\def\PRL{Phys. Rev. Lett.}
\def\PRD{Phys. Rev. D}
\def\RMP{Rev. Mod. Phys.} 
%
% D0 Style Definitions
% Definitions of commonly used symbols
%
% particles
\def\pbar{$\overline{p}~$}               %pbar
\def\tbar{$\overline{t}~$}               %tbar
\def\bbar{$\overline{b}~$}               %bbar
\def\pizero{$\pi^0~$}                    %pizero
\def\jpsi{$J/\psi~$}                     %J/psi
\def\ppbar{$p\overline{p}~$}             %ppbar
\def\pbarp{$\overline{p}p~$}             %pbarp
\def\qqbar{$q\overline{q}~$}             %qqbar
\def\ttbar{$t\overline{t}~$}             %ttbar
\def\bbbar{$b\overline{b}~$}             %bbbar
\def\epm{$e^+e^-~$}                      %e+e-
\def\wino{$\widetilde W~$}               %Wino
\def\zino{$\widetilde Z~$}               %Zino
\def\squark{$\widetilde q~$}             %squark
\def\gluino{$\widetilde g~$}             %gluino
%
% kinematics, physics quantities, etc.
\def\pt{$p_T$}                          %pT
\def\et{$E_T$}                          %ET
\def\met{\mbox{${\hbox{$E$\kern-0.6em\lower-.1ex\hbox{/}}}_T$}} %missing ET
\def\mex{\mbox{${\hbox{$E$\kern-0.6em\lower-.1ex\hbox{/}}}_x$}} %missing Ex
\def\mey{\mbox{${\hbox{$E$\kern-0.6em\lower-.1ex\hbox{/}}}_y$}} %missing Ey
\def\mexy{\mbox{${\hbox{$E$\kern-0.6em\lower-.1ex\hbox{/}}}_{x,y}$}} %missing Exy
\def\ptw{$p_T^W$} 
\def\htran{$H_T~$}                       %HT
\def\aplan{$\cal{A}~$}                   %aplanarity
\def\iso{$\cal{I}~$}                     %isolation variable
\def\remu{${\cal{R}}_{e\mu}~$}           %distance between e and mu in eta-phi
\def\rmu{$\Delta\cal{R}_{\mu}~$}         %distance between mu and jet cone axis
\def\sigtot{$\sigma_{\rm tot}~$}         %sigma total
\def\sigtop{$\sigma_{t \overline{t}}~$}  %sigma_ttbar
\def\njet{$N_{\rm jet}~$}                %N_jet
\def\sinthw{sin$^2 \theta_W~$}           %sin^2 th_W
\def\alphas{$\alpha_{\scriptscriptstyle S}~$}                %alpha_s
\def\alphaem{$\alpha_{\scriptscriptstyle{\rm EM}}~$}         %alpha_EM
%
% units
\def\ipb{pb$^{-1}~$}                     %inverse picobarns
\def\gevcc{GeV/c$^2~$}                   %GeV/c^2
\def\gevc{GeV/c}                         %GeV/c
\def\gev{GeV}                            %GeV
\def\deg{$^\circ~$}                      %degree sign
%
% luminosity stuff
\def\lum{$\cal{L}~$}                     %luminosity
\def\lumint{$\int\! {\cal{L}} dt~$}      %integrated luminosity
\def\lumunits{cm$^{-2}$s$^{-1}~$}        %luminosity units
%
% masses
\def\mt{$m_t~$}                          %m_top
\def\mb{$m_b~$}                          %m_bottom
\def\mw{$M_W~$}                          %M_W
\def\mz{$M_Z~$}                          %M_Z
%
% misc.
\def\D0{D\O}                            %D0
\def\etal{{\sl et al.}}                 %et al. - no preceeding comma
\def\vs{{\sl vs.}}                      %vs.
%
\newcommand{\W}{{\it W}}
\newcommand{\Z}{{\it Z}}
\newcommand{\photon}{direct-$\gamma$}
\newcommand{\Photon}{Direct-$\gamma$}
\newcommand{\PHOTON}{DIRECT-$\gamma$}
\newcommand{\Et}{$E_{\rm T}$}
\newcommand{\Dzero}{D\O\ }
\newcommand{\ie}{{\rm i.e.}}
\newcommand{\eg}{{\rm e.g.}}
\newcommand{\pte}{$p_{\rm T}(e)$}
\newcommand{\ptn}{$p_{\rm T}(\nu)$}
\newcommand{\MW}{$M_W$}
\newcommand{\Wev}{$W\rightarrow e\nu$}
\newcommand{\Wj}{$W+\ge 1$ jet}
\newcommand{\Etmin}{$E_{\rm T}^{min}$}
%%
\def\Zee{\mbox{$Z \rightarrow e e \ $}}
%%
%% PG macros
\newcommand{\nunit}[2]{#1\,\mbox{#2}}
\begin{document}
\lefthyphenmin=2
\righthyphenmin=3

%new 060600
%\setlength{\baselineskip}{24pt}
%\preprint{FERMILAB-Pub-97/109-E}
%
% ======> Title of the paper goes here <====================
%
\title{
Search for Leptoquark Pairs Decaying to $\boldmath \nu\nu$ + jets in 
$\boldmath p\overline{p}~$ Collisions at $\boldmath \sqrt{s}$=1.8 TeV\\
}

% LIST_OF_AUTHORS_R1.TEX                 8/31/01            
%
\author{                                                                      
%% names begin here                                                           
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\'{o}mez,$^{5}$                                                           
G.~G\'{o}mez,$^{46}$                                                          
P.I.~Goncharov,$^{26}$                                                        
J.L.~Gonz\'alez~Sol\'{\i}s,$^{19}$                                            
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,$^{39,\dag}$                                                  
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\'{\i}n,$^{1}$                                                          
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,$^{48}$                                                             
Q.~Xu,$^{49}$                                                                 
R.~Yamada,$^{36}$                                                             
P.~Yamin,$^{55}$                                                              
T.~Yasuda,$^{36}$                                                             
Y.A.~Yatsunenko,$^{23}$                                                       
K.~Yip,$^{55}$                                                                
S.~Youssef,$^{35}$                                                            
J.~Yu,$^{36}$                                                                 
Z.~Yu,$^{39}$                                                                 
M.~Zanabria,$^{5}$                                                            
X.~Zhang,$^{57}$                                                              
H.~Zheng,$^{41}$                                                              
B.~Zhou,$^{49}$                                                               
Z.~Zhou,$^{42}$                                                               
M.~Zielinski,$^{53}$                                                          
D.~Zieminska,$^{40}$                                                          
A.~Zieminski,$^{40}$                                                          
V.~Zutshi,$^{55}$                                                             
E.G.~Zverev,$^{25}$                                                           
and~A.~Zylberstejn$^{13}$                                                     
\\                                                                            
\vskip 0.30cm                                                                 
\centerline{(D\O\ Collaboration)}                                             
\vskip 0.30cm                                                                 
}                                                                             
\address{                                                                     
\centerline{$^{1}$Universidad de Buenos Aires, Buenos Aires, Argentina}       
\centerline{$^{2}$LAFEX, Centro Brasileiro de Pesquisas F{\'\i}sicas,         
                  Rio de Janeiro, Brazil}                                     
\centerline{$^{3}$Universidade do Estado do Rio de Janeiro,                   
                  Rio de Janeiro, Brazil}                                     
\centerline{$^{4}$Institute of High Energy Physics, Beijing,                  
                  People's Republic of China}                                 
\centerline{$^{5}$Universidad de los Andes, Bogot\'{a}, Colombia}             
\centerline{$^{6}$Charles University, Center for Particle Physics,            
                  Prague, Czech Republic}                                     
\centerline{$^{7}$Institute of Physics, Academy of Sciences, Center           
                  for Particle Physics, Prague, Czech Republic}               
\centerline{$^{8}$Universidad San Francisco de Quito, Quito, Ecuador}         
\centerline{$^{9}$Institut des Sciences Nucl\'eaires, IN2P3-CNRS,             
                  Universite de Grenoble 1, Grenoble, France}                 
\centerline{$^{10}$CPPM, IN2P3-CNRS, Universit\'e de la M\'editerran\'ee,     
                  Marseille, France}                                          
\centerline{$^{11}$Laboratoire de l'Acc\'el\'erateur Lin\'eaire,              
                  IN2P3-CNRS, Orsay, France}                                  
\centerline{$^{12}$LPNHE, Universit\'es Paris VI and VII, IN2P3-CNRS,         
                  Paris, France}                                              
\centerline{$^{13}$DAPNIA/Service de Physique des Particules, CEA, Saclay,    
                  France}                                                     
\centerline{$^{14}$Universit{\"a}t Mainz, Institut f{\"u}r Physik,            
                  Mainz, Germany}                                             
\centerline{$^{15}$Panjab University, Chandigarh, India}                      
\centerline{$^{16}$Delhi University, Delhi, India}                            
\centerline{$^{17}$Tata Institute of Fundamental Research, Mumbai, India}     
\centerline{$^{18}$Seoul National University, Seoul, Korea}                   
\centerline{$^{19}$CINVESTAV, Mexico City, Mexico}                            
\centerline{$^{20}$FOM-Institute NIKHEF and University of                     
                  Amsterdam/NIKHEF, Amsterdam, The Netherlands}               
\centerline{$^{21}$University of Nijmegen/NIKHEF, Nijmegen, The               
                  Netherlands}                                                
\centerline{$^{22}$Institute of Nuclear Physics, Krak\'ow, Poland}            
\centerline{$^{23}$Joint Institute for Nuclear Research, Dubna, Russia}       
\centerline{$^{24}$Institute for Theoretical and Experimental Physics,        
                   Moscow, Russia}                                            
\centerline{$^{25}$Moscow State University, Moscow, Russia}                   
\centerline{$^{26}$Institute for High Energy Physics, Protvino, Russia}       
\centerline{$^{27}$Lancaster University, Lancaster, United Kingdom}           
\centerline{$^{28}$Imperial College, London, United Kingdom}                  
\centerline{$^{29}$University of Arizona, Tucson, Arizona 85721}              
\centerline{$^{30}$Lawrence Berkeley National Laboratory and University of    
                  California, Berkeley, California 94720}                     
\centerline{$^{31}$University of California, Davis, California 95616}         
\centerline{$^{32}$California State University, Fresno, California 93740}     
\centerline{$^{33}$University of California, Irvine, California 92697}        
\centerline{$^{34}$University of California, Riverside, California 92521}     
\centerline{$^{35}$Florida State University, Tallahassee, Florida 32306}      
\centerline{$^{36}$Fermi National Accelerator Laboratory, Batavia,            
                   Illinois 60510}                                            
\centerline{$^{37}$University of Illinois at Chicago, Chicago,                
                   Illinois 60607}                                            
\centerline{$^{38}$Northern Illinois University, DeKalb, Illinois 60115}      
\centerline{$^{39}$Northwestern University, Evanston, Illinois 60208}         
\centerline{$^{40}$Indiana University, Bloomington, Indiana 47405}            
\centerline{$^{41}$University of Notre Dame, Notre Dame, Indiana 46556}       
\centerline{$^{42}$Iowa State University, Ames, Iowa 50011}                   
\centerline{$^{43}$University of Kansas, Lawrence, Kansas 66045}              
\centerline{$^{44}$Kansas State University, Manhattan, Kansas 66506}          
\centerline{$^{45}$Louisiana Tech University, Ruston, Louisiana 71272}        
\centerline{$^{46}$University of Maryland, College Park, Maryland 20742}      
\centerline{$^{47}$Boston University, Boston, Massachusetts 02215}            
\centerline{$^{48}$Northeastern University, Boston, Massachusetts 02115}      
\centerline{$^{49}$University of Michigan, Ann Arbor, Michigan 48109}         
\centerline{$^{50}$Michigan State University, East Lansing, Michigan 48824}   
\centerline{$^{51}$University of Nebraska, Lincoln, Nebraska 68588}           
\centerline{$^{52}$Columbia University, New York, New York 10027}             
\centerline{$^{53}$University of Rochester, Rochester, New York 14627}        
\centerline{$^{54}$State University of New York, Stony Brook,                 
                   New York 11794}                                            
\centerline{$^{55}$Brookhaven National Laboratory, Upton, New York 11973}     
\centerline{$^{56}$Langston University, Langston, Oklahoma 73050}             
\centerline{$^{57}$University of Oklahoma, Norman, Oklahoma 73019}            
\centerline{$^{58}$Brown University, Providence, Rhode Island 02912}          
\centerline{$^{59}$University of Texas, Arlington, Texas 76019}               
\centerline{$^{60}$Texas A\&M University, College Station, Texas 77843}       
\centerline{$^{61}$Rice University, Houston, Texas 77005}                     
\centerline{$^{62}$University of Virginia, Charlottesville, Virginia 22901}   
\centerline{$^{63}$University of Washington, Seattle, Washington 98195}       
}                                                                             
%end                                                                          

\maketitle
%
% ==============> Text of the abstract goes here <=====================
% 
\begin{abstract}
\input abstract.tex
%We present the results of a search for leptoquark ($LQ$) pairs using (85.2 
%$\pm$ 3.7) pb$^{-1}$ of $p{\bar p}$ collider data collected by the D\O\ 
%experiment at the Fermilab Tevatron from 1994-1996.  We observe no evidence 
%for leptoquark production and set a limit on $\sigma (p{\bar p} \rightarrow 
%LQ\overline{LQ} \rightarrow \nu\nu~+~jets)$ as a function of the mass of the 
%leptoquark ($M_{LQ}$), assuming BR($LQ \rightarrow \nu q$)=100\%.  At the 
%95\% confidence level, we exclude scalar leptoquarks for $M_{LQ} <$ 99 
%GeV/c$^2$, and vector leptoquarks for $M_{LQ} <$ 178 GeV/c$^2$ assuming 
%couplings that result in the minimum production cross section.
\end{abstract}
%\pacs{PACS numbers 14.65.Ha, 13.85.Qk, 13.85.Ni}

\newpage
\vfill\eject
\twocolumn
\input body.tex
%The observed symmetry between leptons ($l$) and quarks ($q$) suggests the   
%existence of a force connecting the two sectors.  The mediator of the force  
%would couple directly to both leptons and quarks, and is therefore known as a  
%leptoquark ($LQ$).  Leptoquarks can be the vector bosons of a Grand Unified  
%Theory \cite{lq}, Higgs particles \cite{lqhiggs}, composite particles  
%\cite{lqcomp}, techniparticles \cite{lqtechni}, R-parity violating 
%supersymmetric particles \cite{rpvio}, or the particles of a hitherto 
%unproposed theory. 
%\par
%Leptoquarks carry both color and fractional electric charge, defining their 
%production mechanism.  The Fermilab Tevatron can produce pairs of leptoquarks 
%through the strong process $p{\bar p} \rightarrow g \rightarrow 
%LQ\overline{LQ} +  X$.  The production cross section is independent of the  
%coupling for scalar leptoquarks, but not for vector leptoquarks.  For these  
%particles, we study the specific cases of Yang-Mills coupling (YM), Minimal  
%Coupling (MC), and the coupling resulting in the minimal cross section  
%($\sigma _{min}$) \cite{mc}.  
%\par
%While intergenerational decay is theoretically possible, limits from 
%flavor-changing neutral currents require low mass (${\cal O}$(Tev)) 
%leptoquarks to couple only within a single generation 
%\cite{generation}.  Decays of leptoquark pairs result in one of three possible 
%final states:  $l^{\pm}l^{\mp} qq$; $l^{\pm}\nu qq$; and $\nu\nu qq$.  This 
%analysis \cite{thesis} studies the $\nu\nu qq$ final state, and assumes 
%BR($LQ \rightarrow \nu q$)=100\%.  Using this branching ratio, D\O\ has used 
%data taken during the 1992-1993 collider run to set limits of $M_{LQ} >$ 79 
%GeV/c$^2$ for scalar leptoquarks, and $M_{LQ} >$ 145 GeV/c$^2$, 160 GeV/c$^2$, 
%and 205 GeV/c$^2$, for $\sigma _{min}$, MC, and YM vector leptoquarks, 
%respectively \cite{d01a}.  The CDF collaboration has conducted a search for 
%second and third generation leptoquarks with BR($LQ \rightarrow \nu q$)=100\% 
%and set mass limits of 123 (148) GeV/c$^2$ for second (third) generation 
%scalar leptoquarks and 171 (199) GeV/c$^2$ and 222 (250) GeV/c$^2$ for second 
%(third) generation vector leptoquarks with MC and YM couplings, respectively 
%\cite{cdfg23}.  
%\par
%The D\O\ detector \cite{detector} consists of three major subsystems:  An 
%inner detector for tracking charged particles; a uranium-liquid argon 
%calorimeter for measuring electromagnetic and hadronic showers, and a muon 
%spectrometer.  The jets measured with the calorimeter have an energy 
%resolution of approximately $\sigma(E)$=0.8$\sqrt{E}$ ($E$ in GeV).  We 
%measure \met\ by summing the calorimeter energy in the direction transverse to 
%the beam.  The measurement has a resolution of $\sigma$=1.08 GeV + 
%0.019($\Sigma |E_T|$) ($E_T$ in GeV).
%\par
%We obtain the event sample from a trigger requiring 2 high $p_T$ jets, \met, 
%and a minimum azimuthal angle between any jet and \met.  The integrated 
%luminosity after removing events corrupted by accelerator and detector 
%malfunctions is 85.2 $\pm$ 3.7 pb$^{-1}$.  We select events with a high trigger
%efficiency:  2 jets with $E_T >$ 50 GeV; \met $>$ 40 GeV; $\Delta\phi 
%(jet,~$\met$) >$ 30$^{\circ}$; and $\Delta {\cal R} (jet,~jet) >$ 1.5, where 
%$\Delta {\cal R}=\sqrt{(\Delta\eta)^2 + (\Delta\phi)^2}$, $\eta$ is the jet
%pseudo-rapidity, and $\phi$ is the jet azimuthal angle.  We reduce backgrounds 
%originating from $W$ or $Z$ production by rejecting events with isolated muons 
%or highly electromagnetic jets.  We reduce cosmic ray backgrounds by rejecting 
%events with jets containing little electromagnetic activity.
%\par
%The backgrounds to the sample consist of jets produced in association with a 
%$W$ or a $Z$; top quark production; and multijet production.  The $W$ and $Z$
%backgrounds consist of processes with only neutrinos and hadronic jets ($W 
%\rightarrow \tau _h\nu +jet,~Z \rightarrow \nu\nu ~+~2~jets$), processes with 
%unobserved charged leptons ($W \rightarrow l^{\pm} \nu ~+~2~jets,~Z 
%\rightarrow \mu\mu ~+~2~jets, ~Z \rightarrow \tau _h \tau _l ~+~jet$), and 
%processes with an electron appearing as a jet ($W \rightarrow e\nu ~+~jet, 
%~W \rightarrow \tau _e \nu ~+~jet$).  We use the \small PYTHIA \normalsize 
%Monte Carlo generator \cite{pythia} to predict the acceptances of the $W/Z$ + 
%jet processes, and the \small VECBOS \normalsize Monte Carlo generator 
%\cite{vecbos} to predict the acceptances of the $W/Z$ + 2 jets processes.  We 
%normalize the generator cross sections to the cross sections measured using 
%the $W$ and $Z$ electronic decays.  
%\par
%The top background consists of $t{\bar t}$, $t{\bar b}$, and ${\bar t}b$ 
%production, where the top quark decays to an unobserved charged lepton, a
%neutrino, and a jet.  We use the D\O\ measured cross section for $t{\bar t}$
%production \cite{ttbar} and the next-to-leading order theoretical cross 
%section for the other processes \cite{singlet}.  We use the 
%\small HERWIG \normalsize generator \cite{herwig} to predict the acceptance of 
%the $t{\bar t}$ process, and \small CompHEP \normalsize \cite{comphep} to 
%predict the acceptances of the $t{\bar b}$ and ${\bar t}b$ processes.
%\par
%The multijet background arises primarily from 2 sources:  Vertex measurement
%error; and jet energy loss.  To reduce the number of events with vertex 
%measurement error, we use the central detector chamber (CDC) to associate 
%charged tracks with each central high $p_T$ jet.  We use the tracks to 
%determine the jet vertex position and require this position to be no further 
%than 15 cm from the event vertex position.  The event vertex position is 
%determined using all of the tracks in the event.  We reduce the number of 
%events with significant jet energy loss by requiring that the angle between 
%the \met\ and the jet with the second highest measured $p_T$ be greater than 
%60$^{\circ}$.
%\par
%To predict the multijet background remaining in our sample, we use the sample 
%of events with the jet vertex position deviating by 15 cm to 50 cm from the 
%event vertex position.  We normalize this sample to the event sample using 
%events with $\Delta\phi (jet~2,~$\met$) < 60 ^{\circ}$.  We choose the 50 cm 
%value to provide the best agreement between the background prediction and the 
%data for the \met\ region of 30 GeV to 40 GeV, which is dominated by multijet 
%events (Table \ref{tbl:qcd3040}).  Changing this value to 100 cm increases the 
%multijet prediction by 22\% in this region; we take this to be the systematic 
%error of the method.  Table \ref{tbl:datbd} shows the total expected background
%and the observed number of events for the 2 jets + \met\ data sample.
%\par
%To model the characteristics of leptoquark production, we use scalar 
%leptoquark events generated with \small PYTHIA \normalsize and vector 
%leptoquark events generated with \small CompHEP\normalsize .  The cross 
%sections for scalar leptoquark production have been calculated at 
%next-to-leading order \cite{kraemer}, while those for vector leptoquark 
%production have been calculated at leading order \cite{vlqxsec}.  The 
%calculations use a mass scale of $\mu$=M$_{LQ}$, with theoretical 
%uncertainties estimated by changing the scale to $\mu$=M$_{LQ}$/2 and 
%$\mu$=2M$_{LQ}$.  We use the minimum cross section ($\mu$=2M$_{LQ}$) in our 
%optimization and in determining our mass limits.  
%\par
%We optimize for the production of 100 GeV/c$^2$ scalar leptoquarks and 200 
%GeV/c$^2$ vector leptoquarks, since leptoquarks with either of these masses 
%give a $\sim$2$\sigma$ excess if they exist.  We use the 
%\small JETNET \normalsize \cite{jetnet} neural network, with the \met\ and 
%$\Delta\phi (jet~1,~jet~2)$ distributions as inputs for scalar leptoquarks, 
%and the \met\ and second jet $p_T$ distributions as inputs for vector 
%leptoquarks.  We show the neural network outputs and the chosen cuts for both 
%of these masses in Fig. \ref{fig:nnout}.  The cuts are chosen to maximize the 
%number of $\sigma$ excess we would observe if leptoquark events were in our 
%sample:

%\begin{center} 
%$n_{\sigma}=\frac{N_{lq}}{\sqrt{N_{lq}+N_{background}+\Delta N_{lq}^2+\Delta N_{background}^2}}$ 
%\end{center} 

%\noindent
%We show the numbers of events after these cuts in Table \ref{tbl:nn}.

%\begin{table}[hptb]
%\begin{center}
%\begin{tabular}{|cc|}
%Event Sample & Number of Events \\
%\hline
%\hline Multijet & 162.8 $\pm$ 23.7\\
%\hline W, Z, and top & 51.9 $\pm$ 7.0 \\
%\hline
%\hline Total background & 214.7 $\pm$ 24.7 \\
%\hline
%\hline Data & 224 \\
%\end{tabular}
%\end{center}
%\vskip -0.1in
%\caption{The expected and observed numbers of events in the multijet 
%dominated sample of \met\ between 30 GeV and 40 GeV.}
%\label{tbl:qcd3040}
%\end{table}
%\begin{table}[hptb]
%\begin{center}
%\begin{tabular}{|cc|}
%Background & Number of Events \\
%\hline
%\hline Multijet & 58.8 $\pm$ 14.1 $\pm$ 12.9\\
%\hline ($W \rightarrow e\nu )~+~jet$ & 51.9 $\pm$ 7.0 $^{+13.7} \! \! \! \! \! \! \! \! \! \! \! \! \! _{-8.9}$ \\
%\hline ($W \rightarrow \tau\nu )~+~jet$ & 46.3 $\pm$ 5.0 $^{+8.9} \! \! \! \! \! \! \! \! \! \! _{-7.7}$ \\
%\hline ($Z \rightarrow \nu\nu )~+~2~jets$ & 36.1 $\pm$ 7.7 $^{+9.0} \! \! \! \! \! \! \! \! \! \! \! _{-5.5}$ \\
%\hline ($W \rightarrow \mu\nu )~+~2~jets$ & 18.7 $\pm$ 3.5 $^{+4.2} \! \! \! \! \! \! \! \! \! \! _{-3.7}$ \\
%\hline $t{\bar t} \rightarrow$ $l^{\pm}\nu ~+~4~jets$ & 10.6 $\pm$ 2.0 $\pm$ 2.3 \\
%\hline ($W \rightarrow e\nu )~+~2~jets$ & 8.3 $\pm$ 2.5 $^{+2.0} \! \! \! \! \! \! \! \! \! \! _{-2.5}$ \\
%\hline ($W \rightarrow \tau\nu )~+~2~jets$ & 5.6 $\pm$ 1.7 $^{+1.4} \! \! \! \! \! \! \! \! \! \! _{-0.8}$ \\
%\hline $tb \rightarrow$ $l^{\pm}\nu ~+~2~jets$ & 2.0 $\pm$ 0.3 $\pm$ 0.2 \\ 
%\hline ($Z \rightarrow \tau\tau )~+~jet$ & 2.0 $\pm$ 0.4 $^{+0.6} \! \! \! \! \! \! \! \! \! \! _{-0.3}$ \\
%\hline ($Z \rightarrow \mu\mu )~+~2~jets$ & 1.7 $\pm$ 0.4 $^{+0.4} \! \! \! \! \! \! \! \! \! \! _{-0.3}$ \\
%\hline
%\hline Total background & 242.0 $\pm$ 18.9 $^{+23.3} \! \! \! \! \! \! \! \! \! \! \! \! \! _{-19.0}$ \\
%\hline
%\hline Data & 231 \\
%\end{tabular}
%\end{center}
%\vskip -0.1in
%\caption{The expected and observed numbers of events in the 2 jets + \met\
%sample.}
%\label{tbl:datbd}
%\end{table}
%\begin{figure}[!htbp] 
%\begin{minipage}[htb]{9.5cm}
%\epsfysize = 9.5cm 
%\epsffile{slqnnout.eps}
%\end{minipage}
%\vskip -0.5cm
%\begin{minipage}[htb]{9.5cm}
%\epsfysize = 9.5cm 
%\epsffile{vlqnnout.eps}
%\end{minipage}
%\caption{The neural network output for the data, for background (solid), and 
%for leptoquarks (dashed).  We show the optimization for 100 GeV/c$^2$ scalar 
%leptoquarks (top) and 200 GeV/c$^2$ vector leptoquarks (bottom).  We mark the 
%chosen cuts with arrows.}
%\label{fig:nnout}
%\end{figure}
%\begin{table}[hptb]
%\begin{center}
%\begin{tabular}{|c|c|c|c|c|}
%Leptoquark & $N_{data}$       & $N_{background}$      & $n_{\sigma}$ & $\sigma ^{95\%}$ (pb)\\
%\hline 100 GeV/c$^2$ Scalar & 58 & 56.0 $^{+8.1} \! \! \! \! \! \! \! \! \! \! \! _{-8.2}$ & +2.1 & 10.8 \\
%\hline 200 GeV/c$^2$ Vector (MC) & 10 & 13.3 $^{+2.8} \! \! \! \! \! \! \! \! \!  _{-2.6}$ & +2.6 & 0.60 \\
%\end{tabular}
%\end{center}
%\caption{The data, the expected background, the number of $\sigma$ excess that
%would be observed in the presence of a signal, and the 95\% confidence level 
%cross section limit.}
%\label{tbl:nn}
%\end{table}
%After optimization, the observed number of events is consistent with the 
%expected background.  We show the corresponding 95\% confidence level cross 
%section limit as a function of leptoquark mass in Fig. \ref{fig:lqlim}.  We  
%calculate the limit using a Bayesian method with a flat prior for the signal 
%and Gaussian priors for background and acceptance uncertainties.  The 
%corresponding mass limits are 99 GeV/c$^2$ for scalar leptoquarks, and 178 
%GeV/c$^2$, 222 GeV/c$^2$, and 282 GeV/c$^2$ for $\sigma _{min}$, MC, and YM 
%vector leptoquarks, respectively.  We show the combined D\O\ mass limits as a 
%function of BR($LQ \rightarrow l^{\pm}q$) for first generation scalar 
%leptoquarks in Fig. \ref{fig:slqbr} and for second generation MC and YM vector
%leptoquarks in Fig. \ref{fig:vlqbr}.
%\begin{figure}[htb]
%\begin{minipage}[htb]{9.5cm}
%\epsfysize = 9.5cm 
%\epsffile{slqlim.eps}
%\end{minipage}
%\vskip -0.5cm
%\begin{minipage}[htb]{9.5cm}
%\epsfysize = 9.5cm 
%\epsffile{vlqlim.eps}
%\end{minipage}
%\caption{The 95\% confidence level cross section limits as a function of 
%leptoquark mass.  We show the mass limits for scalar (top) and vector (bottom)
%leptoquarks.}
%\label{fig:lqlim}
%\end{figure}
%\begin{figure}[htb]
%\epsfxsize = 9.5cm
%\centerline{\epsffile{mbeta_prl.eps}}
%\vskip -0.1in
%\caption{The D\O\ excluded mass vs. BR($LQ \rightarrow eq$) region for first 
%generation scalar leptoquarks.}
%\label{fig:slqbr}
%\end{figure}
%\begin{figure}[htb]
%\begin{minipage}[htb]{9.5cm}
%\epsfysize = 9.5cm 
%\epsffile{mvexcluded.eps}
%\end{minipage}
%\vskip -0.5cm
%\begin{minipage}[htb]{9.5cm}
%\epsfysize = 9.5cm    
%\epsffile{ymexcluded.eps}
%\end{minipage}
%\caption{The D\O\ excluded mass vs. BR($LQ \rightarrow \mu q$) region for second 
%generation MC (top) and YM (bottom) vector leptoquarks.}
%\label{fig:vlqbr}
%\end{figure}
% acknowledgement_paragraph_r1.tex
%
We thank the staffs at Fermilab and collaborating institutions, 
and acknowledge support from the 
Department of Energy and National Science Foundation (USA),  
Commissariat  \` a L'Energie Atomique and 
CNRS/Institut National de Physique Nucl\'eaire et 
de Physique des Particules (France), 
Ministry for Science and Technology and Ministry for Atomic 
   Energy (Russia),
CAPES and CNPq (Brazil),
Departments of Atomic Energy and Science and Education (India),
Colciencias (Colombia),
CONACyT (Mexico),
Ministry of Education and KOSEF (Korea),
CONICET and UBACyT (Argentina),
The Foundation for Fundamental Research on Matter (The Netherlands),
PPARC (United Kingdom),
Ministry of Education (Czech Republic),
and the A.P.~Sloan Foundation.
%
\input references.tex

%\begin{references}
%\input{list_of_visitor_addresses.tex}
%\bibitem{lq}
%H. Georgi and S. Glashow, Phys. Rev.
%Lett. \bf 32\rm , 438 (1974); J.C. Pati and 
%A. Salam, Phys. Rev. D \bf 10\rm , 275 (1974).
%
%\bibitem{lqhiggs}
%P.H. Frampton, Mod. Phys. Lett. \bf A7\rm, 
%\rm 559 (1992).  
%
%\bibitem{lqcomp}
%J. L. Hewett and T. G. Rizzo, Phys. Rep. \bf 183\rm, 
%193 (1989); E. Accomando \em et al.\rm , Phys. Rep. 
%\bf 299\rm, 1 (1998). 
%
%\bibitem{lqtechni} 
%E. Eichten and K. Lane, \em hep-ph/9609297 \rm (1996); 
%\em hep-ph/9609298 \rm (1996). 
%
%\bibitem{rpvio}
%D. Choudhury and S. Raychaudhuri, Phys. Lett. B\bf 401\rm ,
%367 (1997); G. Altarelli \em et al.\rm , Nucl. Phys. B\bf 506\rm, 
%3 (1997).
%
%\bibitem{mc} 
%\rm J. Bl$\rm\ddot{u}$mlein, E. Boos, and A. Kryukov, 
%Z. Phys. C \bf 76\rm, 137 (1997). 
% 
%\bibitem{generation}
%H.-U. Bengtsson {\it et al.},
%Phys. Rev. Lett. \bf 55\rm , 2762 (1985).
% 
%\bibitem{thesis} 
%C. Hays, Ph.D. thesis, Columbia University, 
%2001 (unpublished). 
%
%\bibitem{d01a}
%D. Acosta and S. Blessing, Ann. Rev.
%Nucl. Part. Sci. \bf 49\rm , 389 (1999).
%
%\bibitem{cdfg23}
%T. Affolder {\it el al.}, Phys. 
%Rev. Lett. \bf 85\rm , 2056 (2000).
%
%\bibitem{detector}
%D\O\ Collaboration, B. Abbott
%{\it et al.}, Nucl. Instrum. Methods Phys. Res. 
%A \bf 338\rm , 185 (1994).
%
%\bibitem{pythia}
%T. Sj$\rm\ddot{o}$strand, Comp. Phys. Comm. 
%\bf 82\rm , 74 (1994); v. 6.127.
%
%\bibitem{vecbos}
%F.A. Berends {\it et al.}, Nucl.
%Phys. \bf B357\rm , 32 (1991).
%
%\bibitem{ttbar}
%D\O\ Collaboration, B. Abbott {\it et al.},
%Phys. Rev. D \bf 60\rm , 012001 (1999).
%
%\bibitem{singlet}
%M.C. Smith and S. Willenbrock, Phys. Rev.
%D\bf 54\rm , 6696 (1996); T. Stelzer, Z. Sullivan, and 
%S. Willenbrock, Phys. Rev. D \bf 56\rm , 5919 (1997); \bf 58\rm ,
%094021 (1998).
%
%\bibitem{herwig}
%G.~Marchesini {\it et al.}, Comput. Phys. Commun. {\bf 67}, 465 
%(1992).
%
%\bibitem{comphep}
%A. Pukhov {\it et al.}, hep-ph/9908288; v. 3.0.
%
%\bibitem{jetnet}
%C. Peterson, T. R$\rm\ddot{o}$gnvaldsson, and L. 
%L$\rm\ddot{o}$nnblad, CERN-TH.7135/94 (1993); v. 3.4.
%
%\bibitem{kraemer} 
%\rm M. Kr$\rm\ddot{a}$mer {\it et al.}, 
%Phys. Rev. Lett. \bf 79\rm, 
%\rm 341 (1997). 
%
%\bibitem{vlqxsec} 
%\rm J. Bl$\rm\ddot{u}$mlein, E. Boos, and A. Kryukov, 
%hep-ph/9811271. 
% 
%\end{references}
%------------------------------------------------------------------------------

\end{document}

references.tex
\begin{references} 
% list_of_visitor_addresses_r1.tex                            8/31/01
%
%\bibitem[\dag]{name}
\bibitem[*]{lehner}
Visitor from University of Zurich, Zurich, Switzerland.
\bibitem[\dag]{przybycien}
Visitor from Institute of Nuclear Physics, Krakow, Poland.
%
\vskip 0.25cm
\bibitem{lq}  
H. Georgi and S. Glashow, Phys. Rev.  
Lett. \bf 32\rm , 438 (1974); J.C. Pati and   
A. Salam, Phys. Rev. D \bf 10\rm , 275 (1974).  
%  
\bibitem{lqhiggs}  
P.H. Frampton, Mod. Phys. Lett. A \bf 7\rm,   
\rm 559 (1992).    
%  
\bibitem{lqcomp}  
J. L. Hewett and T. G. Rizzo, Phys. Rep. \bf 183\rm,   
193 (1989); E. Accomando \em et al.\rm , Phys. Rep.   
\bf 299\rm, 1 (1998).   
%  
\bibitem{lqtechni}   
E. Eichten and K. Lane, hep-ph/9609297; hep-ph/9609298, Proceedings of 1996
Snowmass Summer Study.   
% 
\bibitem{rpvio}  
D. Choudhury and S. Raychaudhuri, Phys. Lett. B \bf 401\rm ,  
367 (1997); G. Altarelli \em et al.\rm , Nucl. Phys. B \bf 506\rm,   
3 (1997).  
% 
\bibitem{mc}   
\rm J. Bl$\rm\ddot{u}$mlein, E. Boos, and A. Kryukov,   
Z. Phys. C \bf 76\rm, 137 (1997).   
%  
\bibitem{generation}  
H.-U. Bengtsson, W.-S. Hou, A. Soni, and D. H. Stork,  
Phys. Rev. Lett. \bf 55\rm , 2762 (1985).  
%   
\bibitem{thesis} 
C. Hays, Ph.D. thesis, Columbia University, 
2001 (unpublished). 
%   
\bibitem{d01a}  
B. Abbott \em et al.\rm (D\O\ Collaboration), Phys. Rev. Lett. \bf 80\rm , 
2051 (1998).
%  
\bibitem{d01avector}  
V. M. Abazov \em et al.\rm (D\O\ Collaboration), hep-ex/0105072.
%  
\bibitem{cdfg23}  
T. Affolder {\it et al.}, Phys.   
Rev. Lett. \bf 85\rm , 2056 (2000).  
%  
\bibitem{OPAL}  
G. Abbiendi {\it et al.}, Eur. Phys. J. C \bf 13\rm , 15 (2000). 
%  
\bibitem{detector}  
B. Abbott {\it et al.} (D\O\ Collaboration), Nucl. Instrum. Methods Phys. Res. 
A \bf 338\rm , 185 (1994).  
%  
\bibitem{geant}  
R. Brun and F. Carminati, CERN Program Library Long Writeup, W5013, 1993 
(unpublished).  We used version 3.15.
%  
\bibitem{pythia}  
T. Sj$\rm\ddot{o}$strand, Comput. Phys. Commun. \bf 82\rm , 74 (1994).  We used
version 6.127.  
%  
\bibitem{vecbos}  
F.A. Berends {\it et al.}, Nucl. Phys. B\bf 357\rm , 32 (1991).  
%  
\bibitem{ttbar}  
B. Abbott {\it et al.} (D\O\ Collaboration), Phys. Rev. D \bf 60\rm , 012001 
(1999).  
%  
\bibitem{singlet}  
M.C. Smith and S. Willenbrock, Phys. Rev. D \bf 54\rm , 6696 (1996); T. 
Stelzer, Z. Sullivan, and  S. Willenbrock, Phys. Rev. D \bf 56\rm , 5919 
(1997); {\sl ibid.} \bf 58\rm , 094021 (1998); V. M. Abazov {\it et al.}, 
Phys. Lett. B \bf 517\rm , 282 (2001).
%  
\bibitem{herwig}  
G.~Marchesini {\it et al.}, Comput. Phys. Commun. {\bf 67}, 465   
(1992).  
%  
\bibitem{comphep}  
A. Pukhov {\it et al.}, hep-ph/9908288.  We used version 3.0.  
%  
\bibitem{kraemer}   
\rm M. Kr$\rm\ddot{a}$mer {\it et al.},   Phys. Rev. Lett. \bf 79\rm,   
\rm 341 (1997).   
%  
\bibitem{vlqxsec}   
\rm J. Bl$\rm\ddot{u}$mlein, E. Boos, and A. Kryukov,   
hep-ph/9811271.   
%   
\bibitem{jetnet}  
C. Peterson, T. R$\rm\ddot{o}$gnvaldsson, and L.   
L$\rm\ddot{o}$nnblad, Comput. Phys. Commun. {\bf 81}, 185 (1994).  We used 
version 3.4.  
%  
\bibitem{bayes}  
I. Bertram {\it et al.}, Fermilab-TM-2104 (unpublished).
%  
\bibitem{d0g2}   
B. Abbott {\it et al.} (D\O\ Collaboration), Phys. Rev. Lett. {\bf 84}, 2088 
(2000).
%
\end{references} 

hep-ex0111047.extracted_bib
\bibitem{lq}
%H. Georgi and S. Glashow, Phys. Rev.
%Lett. \bf 32\rm , 438 (1974); J.C. Pati and 
%A. Salam, Phys. Rev. D \bf 10\rm , 275 (1974).
%
%
\bibitem{lqhiggs}
%P.H. Frampton, Mod. Phys. Lett. \bf A7\rm, 
%\rm 559 (1992).  
%
%
\bibitem{lqcomp}
%J. L. Hewett and T. G. Rizzo, Phys. Rep. \bf 183\rm, 
%193 (1989); E. Accomando \em et al.\rm , Phys. Rep. 
%\bf 299\rm, 1 (1998). 
%
%
\bibitem{lqtechni} 
%E. Eichten and K. Lane, \em hep-ph/9609297 \rm (1996); 
%\em hep-ph/9609298 \rm (1996). 
%
%
\bibitem{rpvio}
%D. Choudhury and S. Raychaudhuri, Phys. Lett. B\bf 401\rm ,
%367 (1997); G. Altarelli \em et al.\rm , Nucl. Phys. B\bf 506\rm, 
%3 (1997).
%
%
\bibitem{mc} 
%\rm J. Bl$\rm\ddot{u}$mlein, E. Boos, and A. Kryukov, 
%Z. Phys. C \bf 76\rm, 137 (1997). 
% 
%
\bibitem{generation}
%H.-U. Bengtsson {\it et al.},
%Phys. Rev. Lett. \bf 55\rm , 2762 (1985).
% 
%
\bibitem{thesis} 
%C. Hays, Ph.D. thesis, Columbia University, 
%2001 (unpublished). 
%
%
\bibitem{d01a}
%D. Acosta and S. Blessing, Ann. Rev.
%Nucl. Part. Sci. \bf 49\rm , 389 (1999).
%
%
\bibitem{cdfg23}
%T. Affolder {\it el al.}, Phys. 
%Rev. Lett. \bf 85\rm , 2056 (2000).
%
%
\bibitem{detector}
%D\O\ Collaboration, B. Abbott
%{\it et al.}, Nucl. Instrum. Methods Phys. Res. 
%A \bf 338\rm , 185 (1994).
%
%
\bibitem{pythia}
%T. Sj$\rm\ddot{o}$strand, Comp. Phys. Comm. 
%\bf 82\rm , 74 (1994); v. 6.127.
%
%
\bibitem{vecbos}
%F.A. Berends {\it et al.}, Nucl.
%Phys. \bf B357\rm , 32 (1991).
%
%
\bibitem{ttbar}
%D\O\ Collaboration, B. Abbott {\it et al.},
%Phys. Rev. D \bf 60\rm , 012001 (1999).
%
%
\bibitem{singlet}
%M.C. Smith and S. Willenbrock, Phys. Rev.
%D\bf 54\rm , 6696 (1996); T. Stelzer, Z. Sullivan, and 
%S. Willenbrock, Phys. Rev. D \bf 56\rm , 5919 (1997); \bf 58\rm ,
%094021 (1998).
%
%
\bibitem{herwig}
%G.~Marchesini {\it et al.}, Comput. Phys. Commun. {\bf 67}, 465 
%(1992).
%
%
\bibitem{comphep}
%A. Pukhov {\it et al.}, hep-ph/9908288; v. 3.0.
%
%
\bibitem{jetnet}
%C. Peterson, T. R$\rm\ddot{o}$gnvaldsson, and L. 
%L$\rm\ddot{o}$nnblad, CERN-TH.7135/94 (1993); v. 3.4.
%
%
\bibitem{kraemer} 
%\rm M. Kr$\rm\ddot{a}$mer {\it et al.}, 
%Phys. Rev. Lett. \bf 79\rm, 
%\rm 341 (1997). 
%
%
\bibitem{vlqxsec} 
%\rm J. Bl$\rm\ddot{u}$mlein, E. Boos, and A. Kryukov, 
%hep-ph/9811271. 
% 
%
\bibitem[\dag]{name}

\bibitem[*]{lehner}
Visitor from University of Zurich, Zurich, Switzerland.

\bibitem[\dag]{przybycien}
Visitor from Institute of Nuclear Physics, Krakow, Poland.
%
\vskip 0.25cm

\bibitem{lq}  
H. Georgi and S. Glashow, Phys. Rev.  
Lett. \bf 32\rm , 438 (1974); J.C. Pati and   
A. Salam, Phys. Rev. D \bf 10\rm , 275 (1974).  
%  

\bibitem{lqhiggs}  
P.H. Frampton, Mod. Phys. Lett. A \bf 7\rm,   
\rm 559 (1992).    
%  

\bibitem{lqcomp}  
J. L. Hewett and T. G. Rizzo, Phys. Rep. \bf 183\rm,   
193 (1989); E. Accomando \em et al.\rm , Phys. Rep.   
\bf 299\rm, 1 (1998).   
%  

\bibitem{lqtechni}   
E. Eichten and K. Lane, hep-ph/9609297; hep-ph/9609298, Proceedings of 1996
Snowmass Summer Study.   
% 

\bibitem{rpvio}  
D. Choudhury and S. Raychaudhuri, Phys. Lett. B \bf 401\rm ,  
367 (1997); G. Altarelli \em et al.\rm , Nucl. Phys. B \bf 506\rm,   
3 (1997).  
% 

\bibitem{mc}   
\rm J. Bl$\rm\ddot{u}$mlein, E. Boos, and A. Kryukov,   
Z. Phys. C \bf 76\rm, 137 (1997).   
%  

\bibitem{generation}  
H.-U. Bengtsson, W.-S. Hou, A. Soni, and D. H. Stork,  
Phys. Rev. Lett. \bf 55\rm , 2762 (1985).  
%   

\bibitem{thesis} 
C. Hays, Ph.D. thesis, Columbia University, 
2001 (unpublished). 
%   

\bibitem{d01a}  
B. Abbott \em et al.\rm (D\O\ Collaboration), Phys. Rev. Lett. \bf 80\rm , 
2051 (1998).
%  

\bibitem{d01avector}  
V. M. Abazov \em et al.\rm (D\O\ Collaboration), hep-ex/0105072.
%  

\bibitem{cdfg23}  
T. Affolder {\it et al.}, Phys.   
Rev. Lett. \bf 85\rm , 2056 (2000).  
%  

\bibitem{OPAL}  
G. Abbiendi {\it et al.}, Eur. Phys. J. C \bf 13\rm , 15 (2000). 
%  

\bibitem{detector}  
B. Abbott {\it et al.} (D\O\ Collaboration), Nucl. Instrum. Methods Phys. Res. 
A \bf 338\rm , 185 (1994).  
%  

\bibitem{geant}  
R. Brun and F. Carminati, CERN Program Library Long Writeup, W5013, 1993 
(unpublished).  We used version 3.15.
%  

\bibitem{pythia}  
T. Sj$\rm\ddot{o}$strand, Comput. Phys. Commun. \bf 82\rm , 74 (1994).  We used
version 6.127.  
%  

\bibitem{vecbos}  
F.A. Berends {\it et al.}, Nucl. Phys. B\bf 357\rm , 32 (1991).  
%  

\bibitem{ttbar}  
B. Abbott {\it et al.} (D\O\ Collaboration), Phys. Rev. D \bf 60\rm , 012001 
(1999).  
%  

\bibitem{singlet}  
M.C. Smith and S. Willenbrock, Phys. Rev. D \bf 54\rm , 6696 (1996); T. 
Stelzer, Z. Sullivan, and  S. Willenbrock, Phys. Rev. D \bf 56\rm , 5919 
(1997); {\sl ibid.} \bf 58\rm , 094021 (1998); V. M. Abazov {\it et al.}, 
Phys. Lett. B \bf 517\rm , 282 (2001).
%  

\bibitem{herwig}  
G.~Marchesini {\it et al.}, Comput. Phys. Commun. {\bf 67}, 465   
(1992).  
%  

\bibitem{comphep}  
A. Pukhov {\it et al.}, hep-ph/9908288.  We used version 3.0.  
%  

\bibitem{kraemer}   
\rm M. Kr$\rm\ddot{a}$mer {\it et al.},   Phys. Rev. Lett. \bf 79\rm,   
\rm 341 (1997).   
%  

\bibitem{vlqxsec}   
\rm J. Bl$\rm\ddot{u}$mlein, E. Boos, and A. Kryukov,   
hep-ph/9811271.   
%   

\bibitem{jetnet}  
C. Peterson, T. R$\rm\ddot{o}$gnvaldsson, and L.   
L$\rm\ddot{o}$nnblad, Comput. Phys. Commun. {\bf 81}, 185 (1994).  We used 
version 3.4.  
%  

\bibitem{bayes}  
I. Bertram {\it et al.}, Fermilab-TM-2104 (unpublished).
%  

\bibitem{d0g2}   
B. Abbott {\it et al.} (D\O\ Collaboration), Phys. Rev. Lett. {\bf 84}, 2088 
(2000).
%
