Search For Single Top Quark Production
Optimized for s-channel Production
With 3.2 fb-1 CDF Run II Data
Authors : Koji Nakamura, Shinhong Kim for CDF Collaboration.
Documentation
cdf public note 9712
Result
s-channel Cross Section : σs = 1.5+0.9-0.7 pb.
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Abstract
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We present an updated result of s-channel-optimized search for single top quark production using 3.2 fb-1
of data accumulated with the CDF detector.
From the previous result with 2.7 fb-1, we additionally used 500 pb-1 of new data and applied a cut to suppress QCD background event.
As a result, we found that the probability (p-value) that the candidate events originate from a background
fluctuation in the absence of single top s-channel production is 0.022,
which is equivalent to 2.01σ deviations in Gaussian statistics, and this excess
corresponds to the single top s-channel cross section of 1.49+0.92-0.75 pb.
An observed value of |Vtb| is 1.32+0.27-0.26.
We also set the 95% CL. upper limit of σs=3.49 pb for the s-channel production cross section.
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Introduction
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Top quarks are predominantly produced in pairs via the strong interaction,
in pp collisions at Tevatron with √s=1.96 TeV.
Top quark has a weak isospin 1/2 composing a weak isospin doublet
with a bottom quark. It predict not only top quark pair production
via strong interaction but also singly produced together with a bottom quark
via weak interaction. However, finding single top quark production is challenging
since it is rarely produced(σsingletop=2.9 pb)
against background processes with the same final state like W+jets
and tt. Since the signal to background ratio is small,
The mesurement of electroweak single top production probes the W-t-b vertex,
provides a direct determination of Cabbibo-Kobayashi-Maskawa (CKM) matrix element
|Vtb| and offers a source of almost 100% polarized top quarks.
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Event Selection and Expected Number of Events
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The data are collected with an inclusive lepton trigger that requires
an electron or muon with ET>18 GeV (PT>18 GeV/c for the
muon) or missing ET>35 GeV plus jets trigger.
Since candidate events for this analysis are selected by requiring a W + 2jets and 3jet events topology where W decays leptonically,
W→eνe or W→μνμ, from this inclusive lepton dataset we select events offline
with a reconstructed isolated electron ET (muon PT) greater
than 20 GeV, missing ET>25 GeV and 2 jets with ET>20 GeV and |η|<2.8.
Both of the two jets should be identified as a b-jet
using SecVtx or JetProb tagging algorithm.
The background surviving these selection are tt, W + heavy-flavorjets,
i.e. W+bb, W+cc, W+c and diboson events WW, WZ, and ZZ.
Instrumental backgrounds originate from mis-tagged W+jets events (W events
with light-flavor jets, i.e. with u, d, s-quark and gluon content,
misidentified as heavy-flavor jets) and from nonW + jets events
(multi-jet events where one jet is erroneously identified as a lepton).
Following table lists the expected event yield for W + 2-btagged jets events
and W + 2-btagged jets + 1 jet events.
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Kinematics Fitter
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There are two significant ambiguity in calculating the reconstructed Mlνb. One comes from the uncertainty on
assigning a b-jet correctly to the b-jet from top quark decay. The other is the neutrino energy. Transverse energy of the
neutrino can be measured indirectly as the missing transverse energy, but neutrino momentum z-component can not be measured.
The purpose of the kinematics fitter is to evaluate the neutrino momentum and find the right b-jet assignment.
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Analysis Method
In order to discover single top production, and to measure its rate with a highest precision, we must take advantage of as many
differences between the signal and the background as possible. To this end, a variety of quantities which can be computed from
the reconstructed event variables have been investigated for their ability to separate the signal from the background.
No single variable encodes all conceivable separation, and so a likelihood is proposed to combine several variables together into
one discriminant to compute the cross section limits or to discover the signal.
The Likelihood is constructed by first forming template histogram of each variable, separately for the signal and for the several
background, denoted Pmifor variable i for the background event class m and signal. Since about 75% of the background
are W + HF and ttbar, we used only W + HF and ttbar for the background template. Likelihood is defined as :
We list the input variable shapes comparison between expected and observed as below
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Systematic Uncertainty
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The estimation of systematic uncertainties on the signal and background predictions are key ingredients in procedure
to search for single top production and the measurement of the cross section. As the systematic uncertainty,
we consider uncertainties in the predicted rate of signal and background processes, and uncertainties in the shapes of
the histogram templates. Here shows summary of the systematic uncertainties.
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Result
Likelihood Discriminant distribution
Cross section Measurement
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The result of the binned maximum likelihood fit is shown below. All sources of systematic uncertainties (normalization and shape) are included in the result.
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s-channel Cross Section : σs = 1.5+0.9-0.7 pb.
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Measurement of |Vtb| and Limit
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The CKM matrix element |Vtb| can be calculated very simply. Because the cross section is
proportional to the square of |Vtb|, |Vtb| is just the square root of the measured cross
section divided by the expected cross section from the Standard Model cross section.
It is also possible to set a lower limit on |Vtb|. A flat prior is assumed in |Vtb|2, which
is proportional to the measured cross section, and it is required to lie between zero and one
inclusive.
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Upper limit
No significant excess from the background was seen, so we set 95% CL. upper limit for s-channel cross section.
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95 % CL. upper limit : σs < 3.49 pb
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Hypothesis Test and Sensitivity Extrapolation
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We have calculate the signal significance of this result using a standard likelihood ratio technique.
In this approach, pseudo-experiments are generated from background only events.
The likelihood ratio is used as the test statistic. Left hand figure shows the distribution of test statistics
for the single-top+background hypothesis and the background only hypothesis in 100000 pseudo-experiments.
The Green line shows the median of the sig+bkg hypothesis which is used to determine the expected p-value.
Right hand figure shows sensitivity extrapolation for this analysis.
Green band is 1σ uncertainty and yellow band shows 2σ uncertainty.
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s- and t-channel 2 dimension Cross section fit
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The result of the binned maximum likelihood fit is shown below. All sources of systematic uncertainties (normalization and shape) are included in the result.
To evaluate s- and t-channel singletop cross section, we combined our analysis and another Likelihood analysis
which is optimized to t-channel.
We fit single top s-channel cross section and t-channel cross section simultaneously.
For the category which have W+2jet and both of 2 jets identified as b-jet, our Ls discriminant are used,
and for the others category, Lt discriminant are used.
Figure below shows measured cross section and it's 1σ and 2σ uncertainty as a two dimensional distribution.
Cross sections for the s-channel and t-channel are 0.9~pb and 1.2~pb respectively.
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Best Fit Cross Section : σs = 1.4 pb, σt = 1.0 pb.
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Kinematic plots
Signal Region
Tight Lepton 2jet SecVtx double tag
Extended Muon 2jet SecVtx double tag
Tight Lepton 3jet SecVtx double tag
Extended Muon 3jet SecVtx double tag
Tight Lepton 2jet SecVtx+JetProb double tag
Extended Muon 2jet SecVtx+JetProb double tag
Tight Lepton 3jet SecVtx+JetProb double tag
Extended Muon 3jet SecVtx+JetProb double tag
0tag Control Region
Tight Lepton 2jet double taggable but 0tag
Extended Muon 2jet double taggable but 0tag
Tight Lepton 3jet double taggable but 0tag
Extended Muon 3jet double taggable but 0tag
1tag Control Region
Tight Lepton 2jet double taggable but 1tag
Extended Muon 2jet double taggable but 1tag
Tight Lepton 3jet double taggable but 1tag
Extended Muon 3jet double taggable but 1tag
Tight Lepton 2jet SecVtx double tag
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Di-jet invariant mass.
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Transverse energy of Di-jet system.
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Invariant mass of the reconstructed top-quark.
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ΔR between two b-tagged jets.
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Transverse energy of leading jet.
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Pseud-rapidity of second leading jet.
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Lepton transverse momentum.
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leading jet η times lepton charge.
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Log likelihood Ratio of Matrix Element.
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Kinematic fitter output.
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Extended Muon 2jet SecVtx double tag
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Di-jet invariant mass.
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Transverse energy of Di-jet system.
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Invariant mass of the reconstructed top-quark.
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ΔR between two b-tagged jets.
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Transverse energy of leading jet.
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Pseud-rapidity of second leading jet.
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Lepton transverse momentum.
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leading jet η times lepton charge.
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Log likelihood Ratio of Matrix Element.
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Kinematic fitter output.
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Tight Lepton 3jet SecVtx double tag
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hadronic decayed top quark invariant mass by b-jet, LF jet, and invisible jet and MW constraint.
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Transverse momentum of non-b-tagged jet.
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Invariant mass of the reconstructed top-quark.
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ΔR between two b-tagged jets.
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Pseud-rapidity of leading jet.
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Pseud-rapidity of second leading jet.
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Pseud-rapidity of lepton
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cosine angle between lepton and d-type initial quark direction at top quark restframe.
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Kinematic fitter output.
Extended Muon 3jet SecVtx double tag
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hadronic decayed top quark invariant mass by b-jet, LF jet, and invisible jet and MW constraint.
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Transverse momentum of non-b-tagged jet.
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Invariant mass of the reconstructed top-quark.
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ΔR between two b-tagged jets.
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Pseud-rapidity of leading jet.
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Pseud-rapidity of second leading jet.
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Pseud-rapidity of lepton
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cosine angle between lepton and d-type initial quark direction at top quark restframe.
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Kinematic fitter output.
Tight Lepton 2jet SecVtx+JetProb double tag
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Di-jet invariant mass.
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Transverse energy of Di-jet system.
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Invariant mass of the reconstructed top-quark.
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ΔR between two b-tagged jets.
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Transverse energy of leading jet.
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Pseud-rapidity of second leading jet.
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Lepton transverse momentum.
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leading jet η times lepton charge.
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Log likelihood Ratio of Matrix Element.
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Kinematic fitter output.
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Extended Muon 2jet SecVtx+JetProb double tag
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Di-jet invariant mass.
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Transverse energy of Di-jet system.
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Invariant mass of the reconstructed top-quark.
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ΔR between two b-tagged jets.
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Transverse energy of leading jet.
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Pseud-rapidity of second leading jet.
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Lepton transverse momentum.
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leading jet η times lepton charge.
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Log likelihood Ratio of Matrix Element.
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Kinematic fitter output.
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Tight Lepton 3jet SecVtx+JetProb double tag
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hadronic decayed top quark invariant mass by b-jet, LF jet, and invisible jet and MW constraint.
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Transverse momentum of non-b-tagged jet.
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Invariant mass of the reconstructed top-quark.
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ΔR between two b-tagged jets.
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Pseud-rapidity of leading jet.
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Pseud-rapidity of second leading jet.
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Pseud-rapidity of lepton
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cosine angle between lepton and d-type initial quark direction at top quark restframe.
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Kinematic fitter output.
Extended Muon 3jet SecVtx+JetProb double tag
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hadronic decayed top quark invariant mass by b-jet, LF jet, and invisible jet and MW constraint.
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Transverse momentum of non-b-tagged jet.
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Invariant mass of the reconstructed top-quark.
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ΔR between two b-tagged jets.
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Pseud-rapidity of leading jet.
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Pseud-rapidity of second leading jet.
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Pseud-rapidity of lepton
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cosine angle between lepton and d-type initial quark direction at top quark restframe.
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Kinematic fitter output.
Tight Lepton 2jet double taggable but 0tag
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Di-jet invariant mass.
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Transverse energy of Di-jet system.
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Invariant mass of the reconstructed top-quark.
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ΔR between two b-tagged jets.
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Transverse energy of leading jet.
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Pseud-rapidity of second leading jet.
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Lepton transverse momentum.
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leading jet η times lepton charge.
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Log likelihood Ratio of Matrix Element.
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Kinematic fitter output.
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Extended Muon 2jet double taggable but 0tag
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Di-jet invariant mass.
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Transverse energy of Di-jet system.
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Invariant mass of the reconstructed top-quark.
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ΔR between two b-tagged jets.
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Transverse energy of leading jet.
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Pseud-rapidity of second leading jet.
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Lepton transverse momentum.
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leading jet η times lepton charge.
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Log likelihood Ratio of Matrix Element.
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Kinematic fitter output.
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Tight Lepton 3jet double taggable but 0tag
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hadronic decayed top quark invariant mass by b-jet, LF jet, and invisible jet and MW constraint.
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Transverse momentum of non-b-tagged jet.
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Invariant mass of the reconstructed top-quark.
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ΔR between two b-tagged jets.
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Pseud-rapidity of leading jet.
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Pseud-rapidity of second leading jet.
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Pseud-rapidity of lepton
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cosine angle between lepton and d-type initial quark direction at top quark restframe.
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Kinematic fitter output.
Extended Muon 3jet double taggable but 0tag
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hadronic decayed top quark invariant mass by b-jet, LF jet, and invisible jet and MW constraint.
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Transverse momentum of non-b-tagged jet.
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Invariant mass of the reconstructed top-quark.
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ΔR between two b-tagged jets.
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Pseud-rapidity of leading jet.
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Pseud-rapidity of second leading jet.
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Pseud-rapidity of lepton
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cosine angle between lepton and d-type initial quark direction at top quark restframe.
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Kinematic fitter output.
Tight Lepton 2jet double taggable but 1tag
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Di-jet invariant mass.
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Transverse energy of Di-jet system.
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Invariant mass of the reconstructed top-quark.
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ΔR between two b-tagged jets.
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Transverse energy of leading jet.
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Pseud-rapidity of second leading jet.
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Lepton transverse momentum.
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leading jet η times lepton charge.
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Log likelihood Ratio of Matrix Element.
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Kinematic fitter output.
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Extended Muon 2jet double taggable but 1tag
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Di-jet invariant mass.
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Transverse energy of Di-jet system.
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Invariant mass of the reconstructed top-quark.
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ΔR between two b-tagged jets.
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Transverse energy of leading jet.
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Pseud-rapidity of second leading jet.
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Lepton transverse momentum.
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leading jet η times lepton charge.
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Log likelihood Ratio of Matrix Element.
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Kinematic fitter output.
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Tight Lepton 3jet double taggable but 1tag
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hadronic decayed top quark invariant mass by b-jet, LF jet, and invisible jet and MW constraint.
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Transverse momentum of non-b-tagged jet.
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Invariant mass of the reconstructed top-quark.
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ΔR between two b-tagged jets.
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Pseud-rapidity of leading jet.
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Pseud-rapidity of second leading jet.
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Pseud-rapidity of lepton
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cosine angle between lepton and d-type initial quark direction at top quark restframe.
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Kinematic fitter output.
Extended Muon 3jet double taggable but 1tag
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hadronic decayed top quark invariant mass by b-jet, LF jet, and invisible jet and MW constraint.
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Transverse momentum of non-b-tagged jet.
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Invariant mass of the reconstructed top-quark.
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ΔR between two b-tagged jets.
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Pseud-rapidity of leading jet.
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Pseud-rapidity of second leading jet.
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Pseud-rapidity of lepton
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cosine angle between lepton and d-type initial quark direction at top quark restframe.
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Kinematic fitter output.