Measurement of the tt̄ production cross section in pp̄ collisions at √s = 1.96 TeV in dilepton final states

(1)

www.elsevier.com/locate/physletb

Measurement of the t

¯t production cross section in p ¯p collisions

at

√

s

= 1.96 TeV in dilepton final states

DØ Collaboration

V.M. Abazov

ai

_{, B. Abbott}

bt

_{, M. Abolins}

bk

_{, B.S. Acharya}

ac

_{, M. Adams}

ax

_{, T. Adams}

av

_,

M. Agelou

r

_{, J.-L. Agram}

s

_{, S.H. Ahn}

ae

_{, M. Ahsan}

be

_{, G.D. Alexeev}

ai

_{, G. Alkhazov}

am

_,

A. Alton

bj

_{, G. Alverson}

bi

_{, G.A. Alves}

b

_{, M. Anastasoaie}

ah

_{, T. Andeen}

az

_{, S. Anderson}

ar

_,

B. Andrieu

q

_{, Y. Arnoud}

n

_{, A. Askew}

av

_{, B. Åsman}

an

_{, A.C.S. Assis Jesus}

c

_,

O. Atramentov

bc

_{, C. Autermann}

u

_{, C. Avila}

h

_{, F. Badaud}

m

_{, A. Baden}

bg

_{, B. Baldin}

aw

_,

P.W. Balm

ag

_{, S. Banerjee}

ac

_{, E. Barberis}

bi

_{, P. Bargassa}

bx

_{, P. Baringer}

bd

_{, C. Barnes}

ap

_,

J. Barreto

b

_{, J.F. Bartlett}

aw

_{, U. Bassler}

q

_{, D. Bauer}

ba

_{, A. Bean}

bd

_{, S. Beauceron}

q

_,

M. Begalli

c

_{, M. Begel}

bp

_{, A. Bellavance}

bm

_{, S.B. Beri}

aa

_{, G. Bernardi}

q

_{, R. Bernhard}

aw,1

_,

I. Bertram

ao

_{, M. Besançon}

r

_{, R. Beuselinck}

ap

_{, V.A. Bezzubov}

al

_{, P.C. Bhat}

aw

_,

V. Bhatnagar

aa

_{, M. Binder}

y

_{, C. Biscarat}

ao

_{, K.M. Black}

bh

_{, I. Blackler}

ap

_{, G. Blazey}

ay

_,

F. Blekman

ap

_{, S. Blessing}

av

_{, D. Bloch}

s

_{, U. Blumenschein}

w

_{, A. Boehnlein}

aw

_,

O. Boeriu

bb

_{, T.A. Bolton}

be

_{, F. Borcherding}

aw

_{, G. Borissov}

ao

_{, K. Bos}

ag

_{, T. Bose}

bo

_,

A. Brandt

bv

_{, R. Brock}

bk

_{, G. Brooijmans}

bo

_{, A. Bross}

aw

_{, N.J. Buchanan}

av

_,

D. Buchholz

az

_{, M. Buehler}

ax

_{, V. Buescher}

w

_{, S. Burdin}

aw

_{, S. Burke}

ar

_{, T.H. Burnett}

bz

_,

E. Busato

q

_{, C.P. Buszello}

ap

_{, J.M. Butler}

bh

_{, J. Cammin}

bp

_{, S. Caron}

ag

_{, W. Carvalho}

c

_,

B.C.K. Casey

bu

_{, N.M. Cason}

bb

_{, H. Castilla-Valdez}

af

_{, S. Chakrabarti}

ac

_,

D. Chakraborty

ay

_{, K.M. Chan}

bp

_{, A. Chandra}

ac

_{, D. Chapin}

bu

_{, F. Charles}

s

_{, E. Cheu}

ar

_,

D.K. Cho

bh

_{, S. Choi}

au

_{, B. Choudhary}

ab

_{, T. Christiansen}

y

_{, L. Christofek}

bd

_{, D. Claes}

bm

_,

B. Clément

s

_{, C. Clément}

an

_{, Y. Coadou}

e

_{, M. Cooke}

bx

_{, W.E. Cooper}

aw

_{, D. Coppage}

bd

_,

M. Corcoran

bx

_{, A. Cothenet}

o

_{, M.-C. Cousinou}

o

_{, B. Cox}

aq

_{, S. Crépé-Renaudin}

n

_,

D. Cutts

bu

_{, H. da Motta}

b

_{, M. Das}

bf

_{, B. Davies}

ao

_{, G. Davies}

ap

_{, G.A. Davis}

az

_{, K. De}

bv

_,

P. de Jong

ag

_{, S.J. de Jong}

ah

_{, E. De La Cruz-Burelo}

bj

_{, C. De Oliveira Martins}

c

_,

S. Dean

aq

_{, J.D. Degenhardt}

bj

_{, F. Déliot}

r

_{, M. Demarteau}

aw

_{, R. Demina}

bp

_{, P. Demine}

r

_,

D. Denisov

aw

_{, S.P. Denisov}

al

_{, S. Desai}

bq

_{, H.T. Diehl}

aw

_{, M. Diesburg}

aw

_{, M. Doidge}

ao

_,

H. Dong

bq

_{, S. Doulas}

bi

_{, L.V. Dudko}

ak

_{, L. Duflot}

p

_{, S.R. Dugad}

ac

_{, A. Duperrin}

o

_,

J. Dyer

bk

_{, A. Dyshkant}

ay

_{, M. Eads}

ay

_{, D. Edmunds}

bk

_{, T. Edwards}

aq

_{, J. Ellison}

au

_,

(2)

J. Elmsheuser

y

_{, V.D. Elvira}

aw

_{, S. Eno}

bg

_{, P. Ermolov}

ak

_{, O.V. Eroshin}

al

_{, J. Estrada}

aw

_,

H. Evans

bo

_{, A. Evdokimov}

aj

_{, V.N. Evdokimov}

al

_{, J. Fast}

aw

_{, S.N. Fatakia}

bh

_,

L. Feligioni

bh

_{, A.V. Ferapontov}

al

_{, T. Ferbel}

bp

_{, F. Fiedler}

y

_{, F. Filthaut}

ah

_{, W. Fisher}

bn

_,

H.E. Fisk

aw

_{, I. Fleck}

w

_{, M. Fortner}

ay

_{, H. Fox}

w

_{, S. Fu}

aw

_{, S. Fuess}

aw

_{, T. Gadfort}

bz

_,

C.F. Galea

ah

_{, E. Gallas}

aw

_{, E. Galyaev}

bb

_{, C. Garcia}

bp

_{, A. Garcia-Bellido}

bz

_{, J. Gardner}

bd

_,

V. Gavrilov

aj

_{, A. Gay}

s

_{, P. Gay}

m

_{, D. Gelé}

s

_{, R. Gelhaus}

au

_{, K. Genser}

aw

_{, C.E. Gerber}

ax

_,

Y. Gershtein

av

_{, D. Gillberg}

e

_{, G. Ginther}

bp

_{, T. Golling}

v

_{, N. Gollub}

an

_{, B. Gómez}

h

_,

K. Gounder

aw

_{, A. Goussiou}

bb

_{, P.D. Grannis}

bq

_{, S. Greder}

c

_{, H. Greenlee}

aw

_,

Z.D. Greenwood

bf

_{, E.M. Gregores}

d

_{, Ph. Gris}

m

_{, J.-F. Grivaz}

p

_{, L. Groer}

bo

_,

S. Grünendahl

aw

_{, M.W. Grünewald}

ad

_{, S.N. Gurzhiev}

al

_{, G. Gutierrez}

aw

_{, P. Gutierrez}

bt

_,

A. Haas

bo

_{, N.J. Hadley}

bg

_{, S. Hagopian}

av

_{, I. Hall}

bt

_{, R.E. Hall}

at

_{, C. Han}

bj

_{, L. Han}

g

_,

K. Hanagaki

aw

_{, K. Harder}

be

_{, A. Harel}

z

_{, R. Harrington}

bi

_{, J.M. Hauptman}

bc

_,

R. Hauser

bk

_{, J. Hays}

az

_{, T. Hebbeker}

u

_{, D. Hedin}

ay

_{, J.M. Heinmiller}

ax

_{, A.P. Heinson}

au

_,

U. Heintz

bh

_{, C. Hensel}

bd

_{, G. Hesketh}

bi

_{, M.D. Hildreth}

bb

_{, R. Hirosky}

by

_{, J.D. Hobbs}

bq

_,

B. Hoeneisen

l

_{, M. Hohlfeld}

x

_{, S.J. Hong}

ae

_{, R. Hooper}

bu

_{, P. Houben}

ag

_{, Y. Hu}

bq

_,

J. Huang

ba

_{, V. Hynek}

i

_{, I. Iashvili}

au

_{, R. Illingworth}

aw

_{, A.S. Ito}

aw

_{, S. Jabeen}

bd

_,

M. Jaffré

p

_{, S. Jain}

bt

_{, V. Jain}

br

_{, K. Jakobs}

w

_{, A. Jenkins}

ap

_{, R. Jesik}

ap

_{, K. Johns}

ar

_,

M. Johnson

aw

_{, A. Jonckheere}

aw

_{, P. Jonsson}

ap

_{, A. Juste}

aw

_{, D. Käfer}

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_{, M.M. Kado}

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_,

S. Kahn

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_{, E. Kajfasz}

o

_{, A.M. Kalinin}

ai

_{, J. Kalk}

bk

_{, D. Karmanov}

ak

_{, J. Kasper}

bh

_,

D. Kau

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_{, R. Kaur}

aa

_{, R. Kehoe}

bw

_{, S. Kermiche}

o

_{, S. Kesisoglou}

bu

_{, A. Khanov}

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_,

A. Kharchilava

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_{, Y.M. Kharzheev}

ai

_{, H. Kim}

bv

_{, T.J. Kim}

ae

_{, B. Klima}

aw

_{, M. Klute}

v

_,

J.M. Kohli

aa

_{, J.-P. Konrath}

w

_{, M. Kopal}

bt

_{, V.M. Korablev}

al

_{, J. Kotcher}

br

_{, B. Kothari}

bo

_,

A. Koubarovsky

ak

_{, A.V. Kozelov}

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_{, J. Kozminski}

bk

_{, A. Kryemadhi}

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_,

S. Krzywdzinski

aw

_{, Y. Kulik}

aw

_{, A. Kumar}

ab

_{, S. Kunori}

bg

_{, A. Kupco}

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_{, T. Kurˇca}

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_,

J. Kvita

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_{, S. Lager}

an

_{, N. Lahrichi}

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_{, G. Landsberg}

bu

_{, J. Lazoflores}

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_{, A.-C. Le Bihan}

s

_,

P. Lebrun

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_{, W.M. Lee}

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_{, A. Leflat}

ak

_{, F. Lehner}

aw,1

_{, C. Leonidopoulos}

bo

_{, J. Leveque}

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_,

P. Lewis

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_{, J. Li}

bv

_{, Q.Z. Li}

aw

_{, J.G.R. Lima}

ay

_{, D. Lincoln}

aw

_{, S.L. Linn}

av

_,

J. Linnemann

bk

_{, V.V. Lipaev}

al

_{, R. Lipton}

aw

_{, L. Lobo}

ap

_{, A. Lobodenko}

am

_,

M. Lokajicek

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_{, A. Lounis}

s

_{, P. Love}

ao

_{, H.J. Lubatti}

bz

_{, L. Lueking}

aw

_{, M. Lynker}

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_,

A.L. Lyon

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_{, A.K.A. Maciel}

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_{, R.J. Madaras}

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_{, P. Mättig}

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_{, C. Magass}

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_,

A. Magerkurth

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_{, A.-M. Magnan}

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_{, N. Makovec}

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_{, P.K. Mal}

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_{, H.B. Malbouisson}

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_,

S. Malik

bm

_{, V.L. Malyshev}

ai

_{, H.S. Mao}

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_{, Y. Maravin}

aw

_{, M. Martens}

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_,

S.E.K. Mattingly

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_{, A.A. Mayorov}

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_{, R. McCarthy}

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_{, R. McCroskey}

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_{, D. Meder}

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A. Melnitchouk

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_{, A. Mendes}

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_{, M. Merkin}

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_{, K.W. Merritt}

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_{, A. Meyer}

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_{, J. Meyer}

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M. Michaut

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_{, H. Miettinen}

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_{, J. Mitrevski}

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_{, J. Molina}

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_{, N.K. Mondal}

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_{, R.W. Moore}

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T. Moulik

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_{, G.S. Muanza}

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_{, M. Mulders}

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_{, L. Mundim}

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_{, Y.D. Mutaf}

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_{, E. Nagy}

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_,

M. Narain

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_{, N.A. Naumann}

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_{, H.A. Neal}

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_{, J.P. Negret}

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_{, S. Nelson}

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_{, P. Neustroev}

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_,

(3)

V. O’Dell

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_{, D.C. O’Neil}

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_{, V. Oguri}

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_{, N. Oliveira}

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_{, N. Oshima}

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_,

G.J. Otero y Garzón

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_{, P. Padley}

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_{, N. Parashar}

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_{, S.K. Park}

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_{, J. Parsons}

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_,

R. Partridge

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_{, N. Parua}

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_{, A. Patwa}

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_{, G. Pawloski}

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_{, P.M. Perea}

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_{, E. Perez}

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P. Pétroff

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_{, M. Petteni}

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_{, R. Piegaia}

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_{, M.-A. Pleier}

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_{, P.L.M. Podesta-Lerma}

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V.M. Podstavkov

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_{, Y. Pogorelov}

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_{, M.-E. Pol}

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_{, A. Pompoš}

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_{, B.G. Pope}

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_,

W.L. Prado da Silva

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_{, H.B. Prosper}

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_{, S. Protopopescu}

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_{, J. Qian}

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_{, A. Quadt}

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B. Quinn

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_{, K.J. Rani}

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_{, K. Ranjan}

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_{, P.A. Rapidis}

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_{, P.N. Ratoff}

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_{, S. Reucroft}

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_,

M. Rijssenbeek

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_{, I. Ripp-Baudot}

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_{, F. Rizatdinova}

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_{, S. Robinson}

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_{, R.F. Rodrigues}

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_,

C. Royon

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_{, P. Rubinov}

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_{, R. Ruchti}

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_{, V.I. Rud}

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_{, G. Sajot}

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_{, A. Sánchez-Hernández}

af

_,

M.P. Sanders

bg

_{, A. Santoro}

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_{, G. Savage}

aw

_{, L. Sawyer}

bf

_{, T. Scanlon}

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_{, D. Schaile}

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_,

R.D. Schamberger

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_{, H. Schellman}

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_{, P. Schieferdecker}

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_{, C. Schmitt}

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C. Schwanenberger

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_{, A. Schwartzman}

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_{, R. Schwienhorst}

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_{, S. Sengupta}

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H. Severini

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_{, E. Shabalina}

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_{, M. Shamim}

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_{, V. Shary}

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_{, A.A. Shchukin}

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_,

W.D. Shephard

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_{, R.K. Shivpuri}

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_{, D. Shpakov}

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_{, R.A. Sidwell}

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_{, V. Simak}

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_,

V. Sirotenko

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_{, P. Skubic}

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_{, P. Slattery}

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_{, R.P. Smith}

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_{, K. Smolek}

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_{, G.R. Snow}

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_,

J. Snow

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_{, S. Snyder}

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_{, S. Söldner-Rembold}

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_{, X. Song}

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_{, L. Sonnenschein}

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_,

A. Sopczak

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_{, M. Sosebee}

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_{, K. Soustruznik}

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_{, M. Souza}

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_{, B. Spurlock}

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_,

N.R. Stanton

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_{, J. Stark}

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_{, J. Steele}

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_{, K. Stevenson}

ba

_{, V. Stolin}

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_{, A. Stone}

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_,

D.A. Stoyanova

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_{, J. Strandberg}

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_{, M.A. Strang}

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_{, M. Strauss}

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_{, R. Ströhmer}

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_,

D. Strom

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_{, M. Strovink}

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_{, L. Stutte}

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_{, S. Sumowidagdo}

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_{, A. Sznajder}

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_{, M. Talby}

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_,

P. Tamburello

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_{, W. Taylor}

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_{, P. Telford}

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_{, J. Temple}

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_{, M. Tomoto}

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_{, T. Toole}

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_,

J. Torborg

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_{, S. Towers}

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_{, T. Trefzger}

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_{, S. Trincaz-Duvoid}

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_{, B. Tuchming}

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_{, C. Tully}

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_,

A.S. Turcot

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_{, P.M. Tuts}

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_{, L. Uvarov}

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_{, S. Uvarov}

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_{, S. Uzunyan}

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_{, B. Vachon}

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_,

P.J. van den Berg

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_{, R. Van Kooten}

ba

_{, W.M. van Leeuwen}

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_{, N. Varelas}

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_,

E.W. Varnes

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_{, A. Vartapetian}

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_{, I.A. Vasilyev}

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_{, M. Vaupel}

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_{, P. Verdier}

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_,

L.S. Vertogradov

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_{, M. Verzocchi}

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_{, F. Villeneuve-Seguier}

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_{, J.-R. Vlimant}

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E. Von Toerne

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_{, M. Vreeswijk}

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_{, T. Vu Anh}

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_{, H.D. Wahl}

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_{, L. Wang}

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_{, J. Warchol}

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G. Watts

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_{, M. Wayne}

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_{, M. Weber}

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_{, H. Weerts}

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_{, N. Wermes}

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_{, A. White}

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V. White

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_{, D. Whiteson}

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_{, D. Wicke}

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_{, D.A. Wijngaarden}

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_{, G.W. Wilson}

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S.J. Wimpenny

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_{, J. Wittlin}

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_{, M. Wobisch}

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_{, J. Womersley}

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_{, D.R. Wood}

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_,

T.R. Wyatt

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_{, Q. Xu}

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_{, N. Xuan}

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_{, S. Yacoob}

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_{, R. Yamada}

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_{, M. Yan}

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_{, T. Yasuda}

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_,

Y.A. Yatsunenko

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_{, Y. Yen}

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_{, K. Yip}

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_{, H.D. Yoo}

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_{, S.W. Youn}

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_{, J. Yu}

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A. Yurkewicz

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_{, A. Zabi}

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_{, A. Zatserklyaniy}

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_{, M. Zdrazil}

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_{, C. Zeitnitz}

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_{, D. Zhang}

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_,

X. Zhang

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_{, T. Zhao}

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_{, Z. Zhao}

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_{, B. Zhou}

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_{, J. Zhu}

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_{, M. Zielinski}

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_{, D. Zieminska}

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_,

A. Zieminski

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_{, R. Zitoun}

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_{, V. Zutshi}

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_{, E.G. Zverev}

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a_{Universidad de Buenos Aires, Buenos Aires, Argentina} b_{LAFEX, Centro Brasileiro de Pesquisas Físicas, Rio de Janeiro, Brazil}

c_{Universidade do Estado do Rio de Janeiro, Rio de Janeiro, Brazil} d_{Instituto de Física Teórica, Universidade Estadual Paulista, São Paulo, Brazil}

(4)

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

f_{Institute of High Energy Physics, Beijing, People’s Republic of China} g_{University of Science and Technology of China, Hefei, People’s Republic of China}

h_{Universidad de los Andes, Bogotá, Colombia}

i_{Center for Particle Physics, Charles University, Prague, Czech Republic} j_{Czech Technical University, Prague, Czech Republic}

k_{Center for Particle Physics, Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic} l_{Universidad San Francisco de Quito, Quito, Ecuador}

m_{Laboratoire de Physique Corpusculaire, IN2P3-CNRS, Université Blaise Pascal, Clermont-Ferrand, France} n_{Laboratoire de Physique Subatomique et de Cosmologie, IN2P3-CNRS, Universite de Grenoble 1, Grenoble, France}

o_{CPPM, IN2P3-CNRS, Université de la Méditerranée, Marseille, France} p_{IN2P3-CNRS, Laboratoire de l’Accélérateur Linéaire, Orsay, France}

q_{LPNHE, IN2P3-CNRS, Universités Paris VI and VII, Paris, France} r_{DAPNIA/Service de Physique des Particules, CEA, Saclay, France}

s_{IReS, IN2P3-CNRS, Université Louis Pasteur, Strasbourg, and Université de Haute Alsace, Mulhouse, France} t_{Institut de Physique Nucléaire de Lyon, IN2P3-CNRS, Université Claude Bernard, Villeurbanne, France}

u_{III Physikalisches Institut A, RWTH Aachen, Aachen, Germany} v_{Physikalisches Institut, Universität Bonn, Bonn, Germany} w_{Physikalisches Institut, Universität Freiburg, Freiburg, Germany}

x_{Institut für Physik, Universität Mainz, Mainz, Germany} y_{Ludwig-Maximilians-Universität München, München, Germany} z_{Fachbereich Physik, University of Wuppertal, Wuppertal, Germany}

aa_{Panjab University, Chandigarh, India} ab_{Delhi University, Delhi, India}

ac_{Tata Institute of Fundamental Research, Mumbai, India} ad_{University College Dublin, Dublin, Ireland}

ae_{Korea Detector Laboratory, Korea University, Seoul, Republic of Korea} af_{CINVESTAV, Mexico City, Mexico}

ag_{FOM-Institute NIKHEF and University of Amsterdam/NIKHEF, Amsterdam, The Netherlands} ah_{Radboud University Nijmegen/NIKHEF, Nijmegen, The Netherlands}

ai_{Joint Institute for Nuclear Research, Dubna, Russia} aj_{Institute for Theoretical and Experimental Physics, Moscow, Russia}

ak_{Moscow State University, Moscow, Russia} al_{Institute for High Energy Physics, Protvino, Russia} am_{Petersburg Nuclear Physics Institute, St. Petersburg, Russia}

an_{Lund University, Lund, Royal Institute of Technology and Stockholm University, Stockholm, and Uppsala University, Uppsala, Sweden} ao_{Lancaster University, Lancaster, United Kingdom}

ap_{Imperial College, London, United Kingdom} aq_{University of Manchester, Manchester, United Kingdom}

ar_{University of Arizona, Tucson, AZ 85721, USA}

as_{Lawrence Berkeley National Laboratory and University of California, Berkeley, CA 94720, USA} at_{California State University, Fresno, CA 93740, USA}

au_{University of California, Riverside, CA 92521, USA} av_{Florida State University, Tallahassee, FL 32306, USA} aw_{Fermi National Accelerator Laboratory, Batavia, IL 60510, USA}

ax_{University of Illinois at Chicago, Chicago, IL 60607, USA} ay_{Northern Illinois University, DeKalb, IL 60115, USA}

az_{Northwestern University, Evanston, IL 60208, USA} ba_{Indiana University, Bloomington, IN 47405, USA} bb_{University of Notre Dame, Notre Dame, IN 46556, USA}

bc_{Iowa State University, Ames, IA 50011, USA} bd_{University of Kansas, Lawrence, KA 66045, USA} be_{Kansas State University, Manhattan, KA 66506, USA}

bf_{Louisiana Tech University, Ruston, LA 71272, USA} bg_{University of Maryland, College Park, MD 20742, USA}

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bi_{Northeastern University, Boston, MA 02115, USA} bj_{University of Michigan, Ann Arbor, MI 48109, USA} bk_{Michigan State University, East Lansing, MI 48824, USA}

bl_{University of Mississippi, University, MS 38677, USA} bm_{University of Nebraska, Lincoln, NE 68588, USA}

bn_{Princeton University, Princeton, NJ 08544, USA} bo_{Columbia University, New York, NY 10027, USA} bp_{University of Rochester, Rochester, NY 14627, USA} bq_{State University of New York, Stony Brook, NY 11794, USA}

br_{Brookhaven National Laboratory, Upton, NY 11973, USA} bs_{Langston University, Langston, OK 73050, USA} bt_{University of Oklahoma, Norman, OK 73019, USA}

bu_{Brown University, Providence, RI 02912, USA} bv_{University of Texas, Arlington, TX 76019, USA} bw_{Southern Methodist University, Dallas, TX 75275, USA}

bx_{Rice University, Houston, TX 77005, USA} by_{University of Virginia, Charlottesville, VA 22901, USA}

bz_{University of Washington, Seattle, WA 98195, USA}

Received 26 May 2005; received in revised form 25 July 2005; accepted 27 August 2005 Available online 15 September 2005

Editor: L. Rolandi

Abstract

We present a measurement of the top quark pair (t¯t) production cross section in p ¯p collisions at√s= 1.96 TeV using

events with two charged leptons in the final state. This analysis utilizes an integrated luminosity of 224–243 pb−1collected with the DØ detector at the Fermilab Tevatron Collider. We observe 13 events in the e+e−, eµ and µ+µ−channels with an expected background of 3.2± 0.7 events. For a top quark mass of 175 GeV, we measure a t ¯t production cross section of

PACS: 13.85.Lg; 13.85.Qk; 14.65.Ha

The top quark was discovered [1] in 1995 at the Fermilab Tevatron Collider in p¯p collisions at√s=

1.8 TeV. Its observation completed the third quark weak isospin doublet suggested by the absence of fla-vor changing neutral current interactions[2]and mea-surement of the b quark weak isospin [3]. By virtue of its large mass (mt= 178.0 ± 4.3 GeV[4]), the top

quark could decay into exotic particles, e.g., a charged Higgs boson[5]. Such decays would lead to a mea-sured t¯t production cross section (σ_t_¯t) apparently de-pendent on the t¯t final state. It is therefore necessary to precisely measure σ_t_¯t in all decay channels and compare it with the standard model prediction. The

E-mail address:[email protected](C. Clément). 1 _{Visitor from University of Zurich, Zurich, Switzerland.}

increased luminosity and higher collision energy of

√

s= 1.96 TeV at the Run II of Tevatron permit

sub-stantially more accurate measurement of σt¯tin all final

states.

In the SU(2)× U(1) electroweak model with one Higgs doublet[6], each top quark of a t¯t pair is ex-pected to decay approximately 99.8% of the time to a W boson and a b quark [7]. Dilepton final states arise when both W bosons decay leptonically. These occur along with two energetic jets resulting from the hadronization of the b quarks and missing transverse energy (E

/

T) from the high transverse

mo-mentum (pT) neutrinos. In this Letter, we present a

measurement of σ_t_¯t with 224–243 pb−1 of p¯p col-lider data at √s= 1.96 TeV collected with the

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and µ+µ−final states. The electrons and muons may originate either directly from a W boson or indi-rectly from a W → τν decay. The corresponding t ¯t branching fractions (B) are 1.58%, 3.16%, and 1.57%

[7] for the e+e−, eµ, and µ+µ− channels, respec-tively.

The DØ detector has a silicon microstrip tracker and a central fiber tracker located within a 2 T su-perconducting solenoidal magnet[8]. The surrounding liquid-argon/uranium calorimeter has a central cryo-stat covering pseudo-rapidities|η| up to 1.1,2and two end cryostats extending coverage to |η| ≈ 4 [9]. A muon system[10]resides beyond the calorimetry, and consists of a layer of tracking detectors and scintilla-tion trigger counters before 1.8 T toroids, followed by two similar layers after the toroids. Luminosity is mea-sured using plastic scintillator arrays located in front of the end cryostats. The trigger and data acquisition systems are designed to accommodate the high lumi-nosities of Run II. The data used in this analysis were collected by requiring two leptons (e or µ) in the hard-ware trigger and one or two leptons in the softhard-ware triggers[8].

To extract the t¯t signal, we select events with two high-pT isolated leptons, largeE

/

T, and at least two

jets. We further improve the signal to background ratio by selecting events with kinematics compatible with

t¯t events. To derive the cross section we determine

the overall efficiency (including trigger, geometrical, and event selection efficiencies) for t¯t and the number of expected background events. We distinguish two categories of backgrounds: “physics” and “instrumen-tal”. Physics backgrounds are processes in which the charged leptons arise from electroweak boson decays and theE

/

T originates from high pT neutrinos. This

signature arises in Z/γ∗→ τ+τ−where the τ leptons decay leptonically, and W W/W Z (diboson) produc-tion. Instrumental backgrounds are defined as events in which (a) a jet or a lepton within a jet fakes the iso-lated lepton signature, or (b) theE

/

T originates from

misreconstructed jet or lepton energies or from noise in the calorimeter.

2 _{Rapidity y and pseudo-rapidity η are defined as} func-tions of the polar angle θ and parameter β as y(θ, β) ≡

1

2ln[(1 + β cos θ)/(1 − β cos θ)]; η(θ) ≡ y(θ, 1), where β is the ra-tio of a particle’s momentum to its energy.

The electrons used in the analysis are defined as clusters of calorimeter cells for which (a) the frac-tion of energy deposited in the electromagnetic secfrac-tion of the calorimeter has to be at least 90% of the to-tal cluster energy, (b) the energy is concentrated in a narrow cone and isolated from further calorimeter en-ergy, (c) the shape of the shower is compatible with that of an electron, (d) the electron matches a charged track in the tracking system. In order to further re-move backgrounds we use (e) a discriminant that se-lects prompt isolated electrons based on the tracking system and calorimeter information [11]. Electrons which fulfill criteria (a) to (e) are referred to as “tight” electrons. For background calculations we introduce “loose” electrons for which only (a) and (b) are re-quired. The muons considered in the analysis are de-fined as tracks reconstructed in the three layers of the muon system, with a matching track in the tracking system. The energy deposited in the calorimeter inside a hollow cone around the muon must be less than 12% of the muon pT. To further remove background, the

sum of the charged track momenta in a cone around the muon track has to be smaller than 12% of the muon

pT. Muons that fulfill all these criteria are referred to

as “tight” muons. For background calculations, we in-troduce “loose” muons for which the isolation criteria are relaxed.

Jets are reconstructed with a fixed cone of radius

R = 0.53 _{and must be confirmed by the} indepen-dent calorimeter trigger readout. Jet energy calibration is applied to the jets [13]. TheE

/

T is equal in

mag-nitude and opposite in direction to the vector sum of all significant calorimeter cell transverse energies. It is corrected for the transverse momenta of all isolated muons, as well as for the corrections to the electron and jet energies.

Event selections for each channel are optimized to minimize the expected statistical uncertainty on the cross section. We select events with at least two jets with pj_T > 20 GeV and|y| < 2.5 (see footnote2) and two leptons with p_T > 15 GeV. Muons are accepted

in the region|η| < 2.0, while electrons must be within

|η| < 1.1 or 1.5 < |η| < 2.5. The two leptons are re-3 _{Jets are defined using the iterative seed-based cone algorithm} with R =( φ)2_{+ ( η)}2_{= 0.5 (where φ is the azimuthal} an-gle), including mid-points as described in Section 3.5 (p. 47) of[12].

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quired to be of opposite signs in the e+e−and µ+µ−

channels.

A cut onE

/

T is crucial to reduce the otherwise

large Z/γ∗ background. This background is partic-ularly severe in the e+e− and µ+µ− channels. Due to different resolutions in electron energies and muon momenta, the optimization leads to different selec-tions in the three channels. In the eµ channel, we requireE

/

T > 25 GeV and φ(E

/

T, µ) > 0.25, where

φ(E

/

T, µ) is the azimuthal angle between theE

/

T

and the muon. The latter gives additional rejection against Z/γ∗→ ττ background in events with two jets. In the e+e−channel, we veto events with dielec-tron invariant mass 80 Mee 100 GeV and require

/

ET > 35 GeV (E

/

T > 40 GeV) for Mee> 100 GeV

(Mee< 80 GeV). In the µ+µ− channel, we accept

events withE

/

T > 35 GeV. This cut is tightened at low

and high values of φ(E

/

T, µ1) where µ1denotes the

leading pT muon. Events with φ(E

/

T, µ1) > 175◦

are removed.

The final selection in the eµ channel requires H_T=

p1

T +

(p_Tj) > 140 GeV, where p1

T denotes the pT

of the leading lepton. This cut effectively rejects the largest backgrounds for this final state which arise from Z/γ∗ → τ+τ− and diboson production. The

e+e− analysis uses a cut on sphericity S = 3(1+

2)/2 > 0.15, where 1 and 2 are the two leading

eigenvalues of the normalized momentum tensor[14]. This requirement rejects events in which jets are pro-duced in a planar geometry through gluon radiation. The final selection applied in the µ+µ−channel fur-ther rejects the Z/γ∗→ µ+µ−background. We com-pute for each µ+µ−event the χ2of a fit to the Z→

µ+µ−hypothesis given the measured muon momenta and known resolutions. Selecting events with χ2> 2

is more effective than selecting on the dimuon invari-ant mass for this channel.

Signal acceptances and efficiencies are derived from a combination of Monte Carlo simulation (MC) and data. Top quark pair production is simulated us-ing ALPGEN[15]with mt= 175 GeV. PYTHIA[16]

is used for fragmentation and decay. B hadron and

τ lepton decays are modeled via EVTGEN[17]and TAUOLA [18], respectively. A full detector simula-tion using GEANT[19]is performed. Lepton trigger and identification efficiencies as well as lepton mo-mentum resolutions are derived from Z/γ∗→ +−

(= e, µ) data. These per-lepton normalization factors

and momentum smearings are applied to MC events to ensure the simulated samples provide an accurate de-scription of the data. The jet reconstruction efficiency, jet energy resolution andE

/

T resolution in the MC are

adjusted to their measured values in data.

To calculate the expected number of events from physics backgrounds, we use Z/γ∗→ τ+τ−and di-boson MC samples generated with PYTHIA and ALP-GEN, respectively. The Z/γ∗→ τ+τ−contribution is normalized to the cross section measured by DØ[20]. For the diboson processes, diboson+ 2 jets events are generated at leading order (LO) and are scaled by the ratio of the next-to-leading order to LO inclusive cross sections derived for diboson inclusive production[21]. Instrumental backgrounds are determined from the data. Fake electrons can arise from jets comprised es-sentially of a leading π0/η and an overlapping or

conversion-produced track. We estimate this back-ground by calculating the fraction fe of loose

elec-trons which appear as tight elecelec-trons in a control sam-ple dominated by fake electrons. In the e+e−channel the control sample consists of events that satisfied the trigger and have two loose electrons. In the eµ chan-nel the events in the control sample must satisfy the trigger and have one tight muon and one loose elec-tron. Contributions from processes with real electrons (W→ eν and Z/γ∗→ e+e−) are suppressed by re-quiringE

/

T < 10 GeV in both e+e−and eµ channels

and|Mee− MZ| > 15 GeV in the e+e−channel only.

We also veto events in which both loose electrons have a matching track. We observe that femeasured in the

e+e−and eµ control samples agree within statistical errors. The predicted number of events with a fake electron in the final sample is obtained by multiply-ing the number of e+e− (eµ) events with one loose electron and one tight electron (muon) by fe.

An isolated muon can be mimicked by a muon in a jet when the jet is not reconstructed. We measure the fraction fµ of loose muons that satisfy the tight

muon criteria in a control sample dominated by fake muons. In the µ+µ−channel the control sample is de-fined as events that have two loose muons. To suppress physics processes with real isolated muons the leading

pT muon is required to fail the tight muon criteria.

This cuts efficiently Z/γ∗→ µ+µ− events but also

W→ µν events where a second-leading muon might

arise from a muon in a jet. The number of events with a fake muon contributing to the final sample is estimated

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by counting the number of events with one tight muon and a loose muon and multiplying it by fµ. In the eµ

channel the contribution from events where both lep-tons are fake leplep-tons is already accounted for by using

fe. The remaining contribution from events with a real

electron and a fake muon, is determined by combining

feand a fake rate fµobtained on a control sample that

satisfies the eµ trigger.

The processes Z/γ∗ → +− ( = e, µ), while lacking high pT neutrinos, might have a significant

amount of measuredE

/

T due to limitedE

/

T resolution.

In the e+e−channel, this background is estimated by measuring aE

/

T misreconstruction rate on data and

ap-plying it to the simulation. We observe that theE

/

T

spectrum in e+e− events with 80 Mee 100 GeV

agrees well with theE

/

T spectrum observed in γ + 2

jets candidate events. We obtain the E

/

T

misrecon-struction rate in data as the ratio of the number of γ+2 jets events passing theE

/

T selection divided by the

number failing the selection. TheE

/

T

misreconstruc-tion rate is also consistent with Z/γ∗→ e+e−+2 jets

simulation. This rate is multiplied by the number of events that fail theE

/

T selections but pass all other

se-lections. In the µ+µ−channel, the expected contribu-tion of Z/γ∗→ µ+µ−background in the final sample is derived from events simulated with ALPGEN. Good agreement is observed between the data and the sim-ulation in the variablesE

/

T and φ(E

/

T, µ1). This

al-lows us to obtain the probability for a Z/γ∗→ µ+µ−

event to pass theE

/

T selection from the simulation.

The sample is normalized to the number of observed

Z/γ∗→ µ+µ− events in the data with 70 Mµµ

110 GeV before theE

/

T selection.

The number of observed events and estimated physics and instrumental backgrounds in the dilep-ton + 2 jets sample, the integrated luminosities and the × B for the t ¯t signal are given in Table 1 for each channel. We observe 5, 8 and 0 events in the

e+e−, eµ and µ+µ−channels, respectively. We esti-mate the probability to observe5, 8, and exactly 0 events in the e+e−, eµ, and µ+µ−channels as 22%, 43%, and 5%, respectively, using the measured σt¯tand

taking into account systematic uncertainties. By gen-erating pseudo-experiments we estimate that 20% of the possible outcomes have lower likelihoods than that of our observation. The significance of the observed

t¯t signal over the background is 3.8 standard

devia-tions.

Table 1

Expected signal (assuming mt= 175 GeV and σt¯t= 7 pb) and

background event yields for e+e−, eµ, and µ+µ− channels. In-strumental backgrounds include

/

ET and fake lepton backgrounds.

Total uncertainties are given

Channel e+e− eµ µ+µ− Integrated luminosity (pb−1) 243 228 224 Physics backgrounds 0.3± 0.1 0.7± 0.2 0.2± 0.1 Instrumental backgrounds 0.7± 0.1 0.2± 0.1 1.1+0.4_−0.3 Total background 0.9± 0.1 0.9± 0.2 1.4± 0.4 × B (10−3) 1.1+0.1_−0.2 3.2+0.4_−0.3 1.0± 0.1 Expected signal 1.9+0.2_−0.3 5.1+0.6_−0.5 1.6± 0.2 Total prediction 2.8± 0.3 6.1+0.6_−0.5 2.9± 0.6 Observed 5 8 0 Table 2

Summary of systematic uncertainties on σt¯t

Source σ_t_¯t(pb)

Jet energy calibration + 0.8–0.7

Jet identification + 0.3–0.6 Muon identification + 0.5–0.4 Electron identification ± 0.3 Trigger + 0.3–0.2 Other + 0.2–0.3 Total ± 1.1

To compute the cross section, we calculate in each channel the probability to observe the number of events seen in the data as a function of σ_t_¯tgiven the number of background events and the signal efficien-cies. The combined cross section is the value of σt¯t

that maximizes the product of the likelihoods in the three channels. The resulting top quark pair produc-tion cross secproduc-tion at√s= 1.96 TeV in dilepton final

states is

σ_t_¯t= 8.6+3.2_−2.7(stat)± 1.1(syst) ± 0.6(lumi) pb

for mt= 175 GeV, within errors of the standard model

theoretical prediction of 6.77± 0.42 pb [22] and in agreement with the recent result in Ref.[23]. We find

σ_t_¯talso consistent with measurements carried out in different final states[11,24]. The total systematic un-certainty is obtained by varying the background pre-diction and signal efficiencies within their uncertain-ties and taking into account correlations. The domi-nant systematic uncertainties are given inTable 2. In

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Fig. 1. Predicted and observed (a) number of events with 0, 1 and 2 or more jets with all other selections applied, (b)

/

ET and (c) leading lepton

pT in dilepton events after all selections. The Z/γ∗contribution includes e+e−, τ+τ−→ eµ, and µ+µ−final states. The t¯t prediction is

shown for σ_t_¯t= 7 pb.

addition, a 6.5% systematic uncertainty is assigned to the luminosity measurement[25]. The top quark mass affects the signal efficiency, resulting in a dependence of σ_t_¯ton mt given by dσt¯t/dmt= −0.08 pb/GeV for

mtin the range 160 GeV to 190 GeV.

Fig. 1(a) shows that the observed number of events with 0, 1, and 2 or more jets, with all other selections applied, is consistent with the prediction (assuming

σt¯t= 7 pb). Fig. 1(b) shows that the observed and

predictedE

/

T spectra after all selections agree well.

Other kinematic distributions in dilepton events are also well described by the sum of t¯t signal and back-ground contributions at various steps of the event se-lection.

The leading lepton pT spectrum in the t¯t

dilep-ton final states has recently been studied by the CDF Collaboration [26] and a mild excess has been ob-served at low transverse momenta. This is not con-firmed by our data, as shown in Fig. 1(c). To test agreement between data and the prediction, we gen-erate pseudo-experiments from the predicted leading lepton pT spectrum and use our measured σt¯tto

nor-malize the t¯t signal. We find that 31% of the pseudo-experiments are less consistent with the parent distri-bution than the data. We conclude that data agree well with the prediction.

In summary, we have measured the top quark pair production cross section at√s= 1.96 TeV in e+e−,

eµ and µ+µ− final states to be σt¯t= 8.6+3.2_−2.7(stat)±

1.1(syst)± 0.6(lumi) pb for mt= 175 GeV, in

agree-ment with the standard model prediction and with measurements in other final states.

Acknowledgements

We thank the staffs at Fermilab and collaborating institutions, and acknowledge support from the DOE and NSF (USA); CEA and CNRS/IN2P3 (France); FASI, Rosatom and RFBR (Russia); CAPES, CNPq, FAPERJ, FAPESP and FUNDUNESP (Brazil); DAE and DST (India); Colciencias (Colombia); CONACyT (Mexico); KRF (Korea); CONICET and UBACyT (Argentina); FOM (The Netherlands); PPARC (United Kingdom); MSMT (Czech Republic); CRC Program, CFI, NSERC and WestGrid Project (Canada); BMBF and DFG (Germany); SFI (Ireland); Research Corpo-ration, Alexander von Humboldt Foundation, and the Marie Curie Program.

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