The search for the electroweak production of top quarks is entering an
exciting time: with the ever increasing luminosity furnished by the Tevatron
ppbar Collider at Fermilab and sophisticated analysis techniques we are now
able to approach the expected standard model production cross section for this
new mode of production. Using 370pb-1 of D0 data and likelihoods as
discriminants to extract the signal from the large backgrounds, we set upper
limits on the standard model production cross section in the s-channel of 5.0
pb and in the t-channel of 4.4 pb, at 95% confidence level.Comment: 5 pages, 3 figures. Contributed to the proceedings of the 34th
  International Meeting on Fundamental Physics. El Escorial, Spain, April 200

Garcia-Bellido, Aran

English

arXiv

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LOOKING FOR SINGLE TOP QUARKS IN DØ DATA
ARAN GARCIA-BELLIDO
Physics Department, University of Washington, BOX 351560
Seattle, WA 98195-1560, U.S.A.
E-mail: aran@fnal.gov
The search for the electroweak production of top quarks is entering an exciting
time: with the ever increasing luminosity furnished by the Tevatron pp¯ Collider at
Fermilab and sophisticated analysis techniques we are now able to approach the
expected standard model production cross section for this new mode of production.
Using 370 pb−1 of DØ data and likelihoods as discriminants to extract the signal
from the large backgrounds, we set upper limits on the standard model production
cross section in the s-channel of 5.0 pb and in the t-channel of 4.4 pb, at 95%
confidence level.
1 Introduction
Top quarks have been observed so far produced in tt¯ pairs in collisions at
Fermilab’s proton-antiproton Tevatron Collider. Pair production occurs pre-
dominantly when two incoming quarks exchange a gluon (qq¯ → g → tt¯): that
is, they are mediated by the strong force. But another mode of production,
which has not been detected yet, is predicted by the standard model (SM),
when the incoming quarks exchange a W -boson and as a result a single top
quark is produced along with a b-quark or other light quarks. This is an elec-
troweak top quark production, also referred as single top quark production.
There are two main modes of W boson exchange with top quark production
observable at the Tevatron: s-channel and t-channel, as depicted in Fig. 1.
At NNLO, the predicted cross sections are 0.88 and 1.98 pb respectively.
q
q– ′
W*
t
b–
q
q′
W
t
b
g
b–
Figure 1. The two dominant production modes of single top quarks at the Tevatron: s-
channel (left) and t-channel (right) exchange of a W boson.
The observation and study of this new production mechanism is impor-
tant for the following reasons: a measurement of the cross section can yield
the first direct measurement of the CKM matrix element |Vtb| without any as-
sumptions on the number of families in the SM; new particles can enhance the
production cross section in the s-channel, and anomalous couplings can alter
the production through the t-channel, thus this channel offers a very sensitive
window into new physics; this channel can be used to test the V-A nature of
the SM; and not least important, this channel requires a good knowledge of
the W+jets background and is itself a background to Higgs boson searches.
For more details of the analyses described here, see Ref. 1.
2 Signal and Background Topologies
The signal topology we are interested in arises from the top quark decaying to
Wb and the W decaying leptonically to electrons or muons. We are looking
for one isolated high pT lepton, missing transverse energy, and at least one
b-quark jet (two in the case of the s-channel, and usually only one in the
t-channel, since the b from gluon splitting tends to get lost in the beam-pipe).
The main background is W+jets production: these events tend to be a little
bit more pencil-like and tend to have smaller total reconstructed energy. But
the cross section for W+jets is at least two orders of magnitude bigger than
that of single top. The second largest background is tt¯ production in the ℓ+jets
or dilepton decay modes. These events tend to have more total reconstructed
energy (having two heavy top quarks in them) and be more spherical than
single top. The tt¯ cross section is around twice that of single top.
The W+jets background is modeled using Alpgen 2, but it is normalized
to the data. The tt¯ background is also modeled using Alpgen and normalized
to the NNLO cross section of 6.7 pb. The QCD background is obtained
from data and the fraction present in the signal data is obtained from the
same normalization of W+jets to signal data. For single top, we employ
CompHEP 3, which effectively describes all angular correlations between the
final state particles at NLO.
3 Analyzed Data and Event Selection
Data collected with the DØ detector 4 between August 2002 and October
2004 corresponding to nearly 370 pb−1 have been analyzed in the search for
s- and t-channel single top quark signals.
We apply loose selection cuts to the data and the Monte Carlo to extract
signal-like physics: we require to have 2, 3 or 4 jets in the event, the leading one
with a pT > 25 GeV and |η| < 2.5, the rest with pT > 15 GeV and |η| < 3.4;
we additionally require one isolated electron or muon with pT > 15 GeV
and |η(e)| < 1.1 or |η(µ|) < 2.0; and the transverse missing energy to be
6ET > 15 GeV.
Finally, we require the presence of at least one jet identified as coming
from a b quark. We use the Jet LIfetime Probability (JLIP) algorithm based
on the fact that jets originated from a b quark exhibit tracks with large impact
parameters in the transverse plane and thus allow to separate jets originated
close to the primary vertex or not. For each jet we have a continuous distri-
bution and decide to “tag” the jet as a tight b jet if PJLIP < 0.3% or loose b
jet if PJLIP < 1%. The samples are separated into events with one tight and
no loose b jets, and events with one tight b jet and one loose b jet. Allowing
the second b tagged jet to be looser than the first, increases our acceptance
and somewhat reduces the uncertainty coming from b tagging in these small
samples.
4 Discriminant Variables and Likelihood discriminants
At this point in the analysis, cross-check samples enriched in W+jets and tt¯
events only are utilized to convince ourselves that the background modeling is
correct not only in the signal region but also in other regions. Figure 2 shows
some of the variables in the signal region used later in the analysis. Good
agreement is found in all of them between data and background modeling.
Single top is a difficult signal to extract from the large backgrounds, as
can be seen from Fig. 2. Since no single distribution allows to separate the
signals from the backgrounds, multivariate analysis techniques are best suited
to efficiently examine a large portion of the phase space trying to identify and
isolate regions were the signal sticks out. For this analysis we have employed
likelihood discriminants with mostly uncorrelated variables as input. Here is
where we benefit from the loose selection cuts: the likelihood discriminants do
a better job of extracting the signal than any optimization of simple stringent
cuts on basic object distributions.
The likelihood discriminant is defined as: L(~x) =
Psignal(~x)
Psignal(~x)+Pbackground(~x)
,
where Psignal(~x) and Pbackground(~x) are the probability density functions of
signal and background events as derived from the product of input template
distributions. We have performed this analysis in separate channels for elec-
tron/muon, single/double tagged, s-channel/t-channel signals and W+jets/tt¯
backgrounds giving a total of 16 different likelihoods. Given the different
 (GeV)
T
Leading jet p50 100 150 200 250 300 350
Ev
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ld
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Multijet
Data
DØ Run II Preliminary, 370 pb-1
 (GeV)
T
 + Lepton p
T
Missing E
0 50 100 150 200 250 300 350
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100E
ve
nt
s t-channel (x10)
s-channel (x10)
bjjbν->ltt
bνblν->ltt
jjνWZ->l
jjνWW->l
W+bb
W+jj
Qcd
Data
DØ Run II Preliminary, 370 pb-1
non b-tagged jetηLepton charge x 
-4 -3 -2 -1 0 1 2 3 4
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DØ Run II Preliminary, 370 pb-1
Reconstructed top mass using the leading b-tagged jet (GeV)
100 200 300 400 500 600
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Multijet
Data
DØ Run II Preliminary, 370 pb-1
Figure 2. Distributions for data and background with the electron and muon channels
combined of: leading jet pT, the 6ET+lepton pT, the charge of the lepton times the eta of
the untagged jet, and the invariant mass of the reconstructed W and the leading tagged
jet.
topologies of the main backgrounds and signals, we thus benefit from the dif-
ferent separation power that each likelihood can accomplish in each channel
and then can easily combine them for the extraction of the limit. The output
of some of the likelihoods can be seen in Fig. 3.
5 Results and conclusions
We use two-dimensional distributions of the output likelihoods of tt¯ vs.
W+jets for each signal, and use the shape information on each bin of that
plane, taking into account the correlations in the errors in each bin, in a
Bayesian analysis with a Poisson distribution for the observed counts in data
and flat priors for the signal cross sections. The priors for the signal ac-
ceptance and the backgrounds are multivariate gaussians. The observed (ex-
pected) 95% confidence level upper limits are 5.0 (3.3) pb for the s-channel
and 4.4 (4.3) pb for the t-channel.
With more than 1 fb−1 of delivered luminosity and better object recon-
struction, calorimeter calibration, much reduced jet energy scale errors (which
improve our invariant mass calculation), better b-tagging techniques, and more
powerful separation techniques like matrix element and boosted decision trees,
we can expect the single top search at DØ to enter a very exciting time in the
/t-channel Likelihood filtertSingle tag                                    t
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
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DØ Run II Preliminary, 370 pb-1
Single tag                           W+jets/t-channel Likelihood filter
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
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/s-channel Likelihood filtertDouble tag                                  t
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Double tag                         W+jets/s-channel Likelihood filter
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Figure 3. Likelihood outputs for electron and muon channels combined in the single tagged
tqb/tt¯ (upper left) and tqb/W+jets (upper right) filters; and for double tagged tb/tt¯ (lower
left) and tb/W+jets (lower right) filters.
near future. We are at the same time looking for physics beyond the standard
model in this final state topology coming from a new massiveW ′ boson 5 and
flavor changing neutral currents 6.
Acknowledgments
The author would like to thank the organizers for the hospitable and enjoyable
atmosphere achieved throughout the meeting.
References
1. V.M. Abazov et al, Phys. Lett. B 622, 265 (2005), hep-ex/0505063;
V.M. Abazov et al, submitted to Phys. Rev. D. Fermilab-PUB-06/069-
E, hep-ex/0604020; V.M. Abazov et al, DØ Public Note 4871.
2. M.L. Mangano, M. Moretti, F. Piccinini, R. Pittau, and A.D. Polosa, J.
High Energy Physics 0307, 001 (2003).
3. E. Boos et al, Nucl. Instrum. Methods 534, 250 (2004).
4. V.M. Abazov et al, submitted to NIM A, Fermilab-PUB-05/341-E.
5. V.M. Abazov et al, submitted to Phys. Lett. B. Fermilab-Pub-06/257-E,
hep-ex/0607102.
6. V.M. Abazov et al, in preparation.


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