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Simultaneous Measurement of the Ratio R=B(t→Wb)/B(t→Wq) and the Top-Quark Pair Production Cross Section with the D0 Detector at s=1.96TeV

V. M. Abazov1, Brad Abbott2, M. Abolins3  +560 moreInstitutions (84)
14 May 2008-Physical Review Letters (American Physical Society)-Vol. 100, Iss: 19, pp 192003-192007

AbstractWe present the first simultaneous measurement of the ratio of branching fractions, R = B(t --> Wb)/B(t --> Wq), with q being a d, s, or b quark, and the top-quark pair production cross section sigma(t (t) over bar) in the lepton plus jets channel using 0.9 fb(-1) of p (p) over bar collision data at root s = 1.96 TeV collected with the D0 detector. We extract R and sigma(t (t) over bar) by analyzing samples of events with 0, 1, and >= 2 identified b jets. We measure R = 0.97(-0.08)(+0.09) (stat + syst) and sigma(t (t) over bar) = 8.18(-0.84)(+0.90) (stat + syst) +/- 0.50(lumi) pb, in agreement with the standard model prediction.

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Summary

  • The lepton isolation criteria are based on calorimeter and tracking information.
  • It combines variables that characterize the presence and properties of secondary vertices and tracks with high impact parameter inside the jet.
  • Single top-quark production is modeled with the SINGLETOP [14] event generator.
  • Additional corrections [10] are applied to the reconstructed objects to improve the agreement between data and simulation.
  • The authors further subtract diboson, single top quark and Z jets contribu- tions, normalized to the next-to-leading-order cross sections [16] .
  • Only the four highest-p T jets are considered for these variables to reduce the sensitivity to soft radiation.
  • The authors evaluate it for each physics process considered and build corresponding template distributions consisting of ten bins.
  • The discriminant shape for the multijet background is obtained from a sample of data events where the lepton fails the isolation criteria.
  • The systematic uncertainties are incorporated in the fit using nuisance parameters [7] , each represented by a Gaussian term in the likelihood.
  • The largest uncertainty comes from the limited statistics.
  • Part of the im-provement results from a fourfold reduction in the systematic uncertainties due to b-tagging, which is mostly absorbed by the R measurement.

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Simultaneous Measurement of the Ratio R Bt ! Wb=Bt ! Wq and the Top-Quark Pair
Production Cross Section with the D0 Detector at

s
p
1 : 96 TeV
V. M. Abazov,
36
B. Abbott,
76
M. Abolins,
66
B. S. Acharya,
29
M. Adams,
52
T. Adams,
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E. Aguilo,
6
S. H. Ahn,
31
M. Ahsan,
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G. D. Alexeev,
36
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*
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PHYSICAL REVIEW LETTERS
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0031-9007=08=100(19)=192003(7) 192003-1 © 2008 The American Physical Society

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38
(D0 Collaboration)
1
Universidad de Buenos Aires, Buenos Aires, Argentina
2
LAFEX, Centro Brasileiro de Pesquisas
´
sicas, Rio de Janeiro, Brazil
3
Universidade do Estado do Rio de Janeiro, Rio de Janeiro, Brazil
4
Universidade Federal do ABC, Santo Andre
´
, Brazil
5
Instituto de
´
sica Teo
´
rica, Universidade Estadual Paulista, Sa
˜
o Paulo, Brazil
6
University of Alberta, Edmonton, Alberta, Canada, Simon Fraser University, Burnaby, British Columbia, Canada,
York University, Toronto, Ontario, Canada,
and McGill University, Montreal, Quebec, Canada
7
University of Science and Technology of China, Hefei, People’s Republic of China
8
Universidad de los Andes, Bogota
´
, Colombia
9
Center for Particle Physics, Charles University, Prague, Czech Republic
10
Czech Technical University, Prague, Czech Republic
11
Center for Particle Physics, Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic
12
Universidad San Francisco de Quito, Quito, Ecuador
13
LPC, Univ Blaise Pascal, CNRS/IN2P3, Clermont, France
14
LPSC, Universite
´
Joseph Fourier Grenoble 1, CNRS/IN2P3, Institut National Polytechnique de Grenoble, France
15
CPPM, IN2P3/CNRS, Universite
´
de la Me
´
diterrane
´
e, Marseille, France
16
LAL, Univ Paris-Sud, IN2P3/CNRS, Orsay, France
17
LPNHE, IN2P3/CNRS, Universite
´
s Paris VI and VII, Paris, France
18
DAPNIA/Service de Physique des Particules, CEA, Saclay, France
19
IPHC, Universite
´
Louis Pasteur et Universite
´
de Haute Alsace, CNRS/IN2P3, Strasbourg, France
20
IPNL, Universite
´
Lyon 1, CNRS/IN2P3, Villeurbanne, France and Universite
´
de Lyon, Lyon, France
21
III. Physikalisches Institut A, RWTH Aachen, Aachen, Germany
22
Physikalisches Institut, Universita
¨
t Bonn, Bonn, Germany
23
Physikalisches Institut, Universita
¨
t Freiburg, Freiburg, Germany
24
Institut fu
¨
r Physik, Universita
¨
t Mainz, Mainz, Germany
25
Ludwig-Maximilians-Universita
¨
tMu
¨
nchen, Mu
¨
nchen, Germany
26
Fachbereich Physik, University of Wuppertal, Wuppertal, Germany
27
Panjab University, Chandigarh, India
28
Delhi University, Delhi, India
29
Tata Institute of Fundamental Research, Mumbai, India
30
University College Dublin, Dublin, Ireland
PRL 100, 192003 (2008)
PHYSICAL REVIEW LETTERS
week ending
16 MAY 2008
192003-2

31
Korea Detector Laboratory, Korea University, Seoul, Korea
32
SungKyunKwan University, Suwon, Korea
33
CINVESTAV, Mexico City, Mexico
34
FOM-Institute NIKHEF and University of Amsterdam/NIKHEF, Amsterdam, The Netherlands
35
Radboud University Nijmegen/NIKHEF, Nijmegen, The Netherlands
36
Joint Institute for Nuclear Research, Dubna, Russia
37
Institute for Theoretical and Experimental Physics, Moscow, Russia
38
Moscow State University, Moscow, Russia
39
Institute for High Energy Physics, Protvino, Russia
40
Petersburg Nuclear Physics Institute, St. Petersburg, Russia
41
Lund University, Lund, Sweden, Royal Institute of Technology and Stockholm University, Stockholm, Sweden,
and Uppsala University, Uppsala, Sweden
42
Physik Institut der Universita
¨
tZu
¨
rich, Zu
¨
rich, Switzerland
43
Lancaster University, Lancaster, United Kingdom
44
Imperial College, London, United Kingdom
45
University of Manchester, Manchester, United Kingdom
46
University of Arizona, Tucson, Arizona 85721, USA
47
Lawrence Berkeley National Laboratory and University of California, Berkeley, California 94720, USA
48
California State University, Fresno, California 93740, USA
49
University of California, Riverside, California 92521, USA
50
Florida State University, Tallahassee, Florida 32306, USA
51
Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
52
University of Illinois at Chicago, Chicago, Illinois 60607, USA
53
Northern Illinois University, DeKalb, Illinois 60115, USA
54
Northwestern University, Evanston, Illinois 60208, USA
55
Indiana University, Bloomington, Indiana 47405, USA
56
University of Notre Dame, Notre Dame, Indiana 46556, USA
57
Purdue University Calumet, Hammond, Indiana 46323, USA
58
Iowa State University, Ames, Iowa 50011, USA
59
University of Kansas, Lawrence, Kansas 66045, USA
60
Kansas State University, Manhattan, Kansas 66506, USA
61
Louisiana Tech University, Ruston, Louisiana 71272, USA
62
University of Maryland, College Park, Maryland 20742, USA
63
Boston University, Boston, Massachusetts 02215, USA
64
Northeastern University, Boston, Massachusetts 02115, USA
65
University of Michigan, Ann Arbor, Michigan 48109, USA
66
Michigan State University, East Lansing, Michigan 48824, USA
67
University of Mississippi, University, Mississippi 38677, USA
68
University of Nebraska, Lincoln, Nebraska 68588, USA
69
Princeton University, Princeton, New Jersey 08544, USA
70
State University of New York, Buffalo, New York 14260, USA
71
Columbia University, New York, New York 10027, USA
72
University of Rochester, Rochester, New York 14627, USA
73
State University of New York, Stony Brook, New York 11794, USA
74
Brookhaven National Laboratory, Upton, New York 11973, USA
75
Langston University, Langston, Oklahoma 73050, USA
76
University of Oklahoma, Norman, Oklahoma 73019, USA
77
Oklahoma State University, Stillwater, Oklahoma 74078, USA
78
Brown University, Providence, Rhode Island 02912, USA
79
University of Texas, Arlington, Texas 76019, USA
80
Southern Methodist University, Dallas, Texas 75275, USA
81
Rice University, Houston, Texas 77005, USA
82
University of Virginia, Charlottesville, Virginia 22901, USA
83
University of Washington, Seattle, Washington 98195, USA
(Received 10 January 2008; published 14 May 2008)
We present the first simultaneous measurement of the ratio of branching fractions, R Bt !
Wb=Bt ! Wq, with q being a d, s,orb quark, and the top-quark pair production cross section
t
t
in the lepton plus jets channel using 0:9fb
1
of p
p collision data at

s
p
1:96 TeV collected with the D0
detector. We extract R and
t
t
by analyzing samples of events with 0, 1, and 2 identified b jets. We
PRL 100, 192003 (2008)
PHYSICAL REVIEW LETTERS
week ending
16 MAY 2008
192003-3

measure R 0:97
0:09
0:08
stat syst and
t
t
8:18
0:90
0:84
stat syst0:50lumi pb, in agreement with
the standard model prediction.
DOI: 10.1103/PhysRevLett.100.192003 PACS numbers: 13.85.Lg, 12.15.Hh, 13.85.Qk, 14.65.Ha
Within the standard model (SM) the top quark decays to
a W boson and a down-type quark q (q d, s, b) with a
rate proportional to the squared Cabibbo-Kobayashi-
Maskawa (CKM) matrix element, jV
tq
j
2
[1]. Under the
assumption of three fermion families and a unitary 3 3
CKM matrix, the jV
tq
j elements are severely constrained,
with jV
tb
j0:999100
0:000034
0:000004
[2]. However, in several
extensions of the SM the 3 3 CKM submatrix would
not appear unitary and jV
tq
j elements can significantly
deviate from their SM values. This would affect the rate
for single top-quark production via the electroweak inter-
action [3] and the ratio R of the top-quark branching
fractions, which can be expressed in terms of the CKM
matrix elements as
R
Bt ! Wb
Bt ! Wq
j V
tb
j
2
j V
tb
j
2
jV
ts
j
2
jV
td
j
2
:
A precise measurement of R is therefore a necessary
ingredient for performing direct measurements of the
jV
tq
j elements via the combination with future measure-
ments of the single top-quark production in s and t chan-
nels [4], free of assumptions about the number of quark
families or the unitarity of the CKM matrix.
We report the first simultaneous measurement of R and
the top-quark pair (t
t) production cross section
t
t
. R was
measured by the CDF and D0 collaborations [5,6]. The
simultaneous measurement of R and
t
t
, in contrast to
previous measurements [7,8], allows one to extract
t
t
without assuming Bt ! Wb1, and to achieve a higher
precision on both quantities by exploiting their different
sensitivity to systematic uncertainties.
The current measurement is based on data collected with
the D0 detector [9] between August 2002 and
December 2005 at the Fermilab Tevatron p
p collider at

s
p
1:96 TeV, corresponding to an integrated luminosity
of about 0:9fb
1
. We use the top-quark pair decay channel
t
t ! W
qW
q, with the subsequent decay of one W
boson into two quarks, and the other one into an electron
or muon and a neutrino, referred to as the lepton plus jets
( jets) channel. We select a data sample enriched in t
t
events by requiring 3 jets with transverse momentum
p
T
> 20 GeV and pseudorapidity jj < 2:5 [10], one iso-
lated electron (muon) with p
T
> 20 GeV and jj < 1:1
(jj < 2:0), and missing transverse energy E6
T
> 20 GeV
(e jets) or E6
T
> 25 GeV ( jets). The leading jet p
T
is required to exceed 40 GeV. Events containing a second
isolated lepton with p
T
> 15 GeV are rejected. The lepton
isolation criteria are based on calorimeter and tracking
information. Details of lepton, jets, and E6
T
identification
are described elsewhere [10].
We identify b jets using a neural-network tagging algo-
rithm [11]. It combines variables that characterize the
presence and properties of secondary vertices and tracks
with high impact parameter inside the jet. In the simula-
tion, we assign a probability for each jet to be b tagged
based on its flavor, p
T
, and . These probabilities are
determined from data control samples, and can be com-
bined to yield a probability for each t
t event to have 0, 1, or
2 b-tagged jets [7].
We split the jets sample into subsamples according
to lepton flavor (e or ), jet multiplicity (3 or 4 jets) and
number of identified b jets (0, 1 or 2), thus obtaining 12
disjoint data sets. We simultaneously fit R and
t
t
to the
observed number of 1 b tag and 2 b tag events, and, in 0
b tag events with 4 jets, to the shape of a discriminant D
that exploits kinematic differences between t
t signal and
background. We do not use a discriminant in events with 3
jets and 0 b tags, since the signal-to-background ratio is
about 5 times smaller.
The dominant background is the production of W bosons
in association with heavy and light flavor jets (W jets).
Smaller contributions arise from Z jets, diboson, and
single top-quark production. Multijet events enter the se-
lected sample if a jet is misidentified as an electron, or a
muon in a jet from a heavy quark or an in-flight pion or
kaon decay appears isolated.
We model W jets and Z jets processes with the
ALPGEN [12] leading-order generator for the matrix ele-
ment calculation and
PYTHIA [13] for parton showering and
hadronization. Diboson samples are generated with
PYTHIA. Single top-quark production is modeled with the
SINGLETOP [14] event generator. The t
t signal is simulated
with
PYTHIA for a top-quark mass of m
top
175 GeV and
includes three decay modes t
t ! W
bW
b, t
t !
W
bW
q
l
(or t
t ! W
q
l
W
b) and t
t ! W
q
l
W
q
l
,
where q
l
denotes a light down-type (d or s) quark. These
three decay modes are referred to as bb, bq
l
and q
l
q
l
.We
pass the generated events through a
GEANT-based [15]
simulation of the D0 detector. Additional corrections [10]
are applied to the reconstructed objects to improve the
agreement between data and simulation.
The determination of the background composition starts
with the evaluation of the multijet background for each jet
multiplicity and lepton flavor before b-jet tagging by
counting events in the corresponding control data samples
and applying the matrix method [7]. We estimate the
number of events with a lepton originating from a W or
Z boson decay by subtracting the multijet background from
the observed event yield before b tagging. We further
subtract diboson, single top quark and Z jets contribu-
PRL 100, 192003 (2008)
PHYSICAL REVIEW LETTERS
week ending
16 MAY 2008
192003-4

tions, normalized to the next-to-leading-order cross sec-
tions [16]. The remaining data events are assumed to come
from t
t and W jets. In every step of the fitting procedure
used to extract
t
t
and R, we iteratively redetermine the
expected number of t
t events and reevaluate the W jets
background.
Since the probability to tag a t
t event depends on the jet
flavor, it depends on R. Assuming three t
t decay modes bb,
bq
l
and q
l
q
l
, the probability for a t
t event to pass our
selection criteria and to have nb-tagged jets is:
P
n
total
t
tR
2
AbbP
n
t
bb2R1 RAbq
l
P
n
t
bq
l
1 R
2
Aq
l
q
l
P
n
t
q
l
q
l
; (1)
where A (P
n
t
) describes the acceptance (tagging probabil-
ity) for each mode. Figure 1(a) shows P
n
t
as a function of R
for t
t events with 4 jets. Table I presents the sample
composition for the measured
t
t
and R 1.
The topological discriminant D [10] exploits the kine-
matic differences between t
t and W jets events to
achieve a better constraint on the number of t
t events in
the subsample with 4 jets and 0 b tags. We select
variables well described by the background model that
provide a good separation between t
t and W jets back-
ground. Only the four highest-p
T
jets are considered for
these variables to reduce the sensitivity to soft radiation.
The optimal set of variables is chosen to minimize the
expected statistical uncertainty on the fitted fraction of t
t
events. Because of differences in acceptance and sample
composition, the discriminants are constructed from differ-
ent sets of variables in the e jets and jets channels.
In the e jets channel we use five variables: the leading jet
p
T
, the maximum R [10] between two jets, A, C
M
, and
D
M
[17]. In the jets channel we use six variables: A,
D
M
, the scalar sum of the p
T
of jets and the muon, the
scalar sum of the p
T
of the third and fourth jet, the
transverse mass of all jets, and the ratio of the mass of
the three leading jets to the mass of the event, defined as the
invariant mass of all jets, the lepton and E6
T
.
The discriminant function is built using simulated W
jets and t
t events. We evaluate it for each physics process
considered and build corresponding template distributions
consisting of ten bins. For t
t we obtain a distribution for
each of the three decay modes. The shapes of the discrimi-
nant distributions for Z jets, diboson and single top
backgrounds are found to be similar to that of the W
jets events and we use the latter to model them. The
discriminant shape for the multijet background is obtained
from a sample of data events where the lepton fails the
isolation criteria.
We define a likelihood function as the product of
Poisson probabilities over all 30 subsamples and bins of
the discriminant. In each subsample the expected number
of events is estimated as a function of R and
t
t
.We
include 12 additional Poisson terms to constrain the multi-
jet background in each subsample. The systematic uncer-
tainties are incorporated in the fit using nuisance
parameters [7], each represented by a Gaussian term in
the likelihood. In this approach, each source of systematic
uncertainty is allowed to affect the central value of R and
t
t
during the fit, yielding a combined statistical and
TABLE I. Sample composition for the measured
t
t
and R
1. Total uncertainties are given.
N
jets
Sample 0 b tags 1 b tag 2 b tags
3 W jets 1394:4 65:1 102:5 9:48:3 1:2
Multijet 287:4 35:928:1 3:53:3 0:4
Other 254:0 35:229:4 3:55:2 0:7
t
t 109:7 6:6 143:3 5:154:3 4:3
Total 2045:5 82:
5 303:3 11:871:2 4:5
Observed 2050 294 76
4 W jets 188:2 38:017:3 3:81:8 0:4
Multijet 66:9 9:96:6 1:00:8 0:1
Other 62:2 11:88:0 1:41:7 0:3
t
t 83:8 9:4 126:4 11:464:2 4:5
Total 401
:1 42:1 158:3 12:169:5 4:5
Observed 389 179 58
R
probability
R
0.2
R
0 0.2 0.4 0.6 0.8 1
0
0.4
0.6
0.8
1
0 tag
1 tag
2 tags
DØ Run II
01 2
Number of events
Number of tagged jets
0
200
400
600
DØ Data
tt
other
W+jets
Multijet
-1
L=0.9 fb
Likelihood discriminant
Number of events
0
20
40
60
80
100
120
0 0.2 0.4 0.6 0.8 1
0
20
40
60
80
100
120
DØ Data
tt
other
W+jets
Multijet
-1
L=0.9 fb
)c()b()a(
FIG. 1. (a) Probability of t
t events to have 0, 1, and 2 b tags as a function of R for events with 4 jets; (b) predicted and observed
number of events in the 0, 1 and 2 b tag samples for the measured R and
t
t
for events with 4 jets and (c) predicted and observed
discriminant distribution in the 0 b tag sample with 4 jets.
PRL 100, 192003 (2008)
PHYSICAL REVIEW LETTERS
week ending
16 MAY 2008
192003-5

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