Single Top search
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Transcript of Single Top search
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Single Top search- Significance study using S+J B-tagging
K. NakamuraUniv. of Tsukuba
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Motivation
|Vtb|
|Vtb|
t-channel
s-channel
Produced electroweakly couplingDirect access to W-t-b vertex
singletop~|Vtb|2Top-polarization study and V-A structure of EW top interaction
Background study for SM Higgs searchExactly the same final state as WH->WbbBackground of all channels to use W+njet background
Probe b-quark PDF (t-channel)Non-SM phenomena
Heavy W’ bosonAnomalous W-t-b coupling ……
But…Cross section is small (~40% of tt)Larger background cause of W+2jets bin
Not established a significant evidence yet …
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Current Study and CDF NOTE
Signal Acceptance and Monte Carlo based backgrounds (CDF 7863)Validation of single-top signal samples (CDF 7701)
Event selection and MC validation
Anlysis with 0.7 fb-1 (CDF 8112)Non-W and W+njets (n=1,2,3) background estimationNeural network b-tagger (CDF 7816)Kinematics fitter for top quark reconstruction (CDF 749)Construct discriminates with Neural network with Likelihood ratio
Unit : pbLimit : 95%CL
theory Likelihood Ratio
Neural Network
t-channel 1.98±0.21 < 3.1
s-channel 0.88±0.07 < 3.2
t+s comb. 2.86±0.28 < 5.8 < 3.4
Result
Phys. Rev. D 71, 012005 (2005)
Projections for Single Top Sensitivity
(CDF 8286)NEW
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Backgrounds
MC based background
Data based backgroundNon-W MistagsWbb Wcc Wc
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Event Selection
Standard tight lepton selection of lepton+jet group.Detail is in appendix
Lepton ID
Et>20 GeVIsolated Conversion Veto
ElectronPt>20 GeV/cIsolated Cosmic Veto
Muon
Jet Energy CorrectionLevel4 Jet Energy Correction – not to apply abs correction
Et>15 GeV, Eta < 2.8
Et>20 GeVEt is corrected with lepton and Level4 jets
MET
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Signal/Background Expectation
@1fb-1
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The 2 jets bin networks have 14 input variables. The 14 input variables for 2 jet bin networks are shown in backup slide
NN discriminants
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NN b-tagger
This NN b-tagger is applied to jets that are already tagged by SecVtx tagger.
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Likelihood Function Analysis
t-channel
s-channel
Likelihood function uses 7 variables
Likelihood function uses 6 variables
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|Vtb|
s-channel
|Vtb|
t-channel
Motivation of our analysis
CDF NOTE 8112
Large amount of Background for this channel.About half of the background comes from Mistags and Wc(c).
Double tag is the powerful method to rejectthese background for s-channel !
But, statistics of the signal is limited by low efficiency of double tag.
We plan to apply “tight” + “loose” b-tagging S+JSecVtx JetProb
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Signal Sample
Generator : Mad EventEvent : single top s-channel LO Top Mass : Mt=175 GeV Name : ttop0oNew
Background Sample
Generator : Alpgen + pythiaEvent : W(->e)+nP W(->mu)+nP (n = 0,1,2,3,4)Name : ltop0n - ltop4n ltop0m – ltop4m
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SecVtx
Use Jprob<0.05
30% events in 2-jet bin can be tagged as s+s or s+j. >>> which corresponds to 46% of at least 1 secvtx tagged events in 2-jet bin.
Estimated b-tag performance of each method for s-channel
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W+nP Background with MC sampleMistag Matrix
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Over estimate SS+SJ
2 S
Under estimate SS+SJ
i
Siε
ji
Sj
Si εε
At least 1 S
ji
Sj
Si
Sj
Ji
Jj
Si εεεεεε
ji
Sj
Ji
Jj
Si εεεε
Null-corr 100%-corr
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W(->e)+nP Background n=0,1,2,3,4over
under
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overunder
W+nP Normalized to 0.7 fb-1
~1/40
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0.022BS
0.59B
S
4.8 (2.5%)
s-channel significance
7.3 (46%) s-channel26.0 (46%)
0.74 (25%)
26.7
99.7 (46%)
2.2 (2.5%)
11.4 (14%)
113.3
0.56(2.5%)0.050B
S
0.61B
S
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Summary Double tag including s+j tagging has a potential to improve the s/n ratio and significance.
30% events in 2-jet bin can be tagged as s+s or s+j tag The fraction of s+s or s+j tagged events is 46% of at least 1 tag events According to very rough estimation,
Detail of signal selection will be confirmed. We will examine some kinematics variables for background rejection. We will estimate the sensitivity for single-top production as a function of integrated luminosity……
Plan
0.022BS
0.59B
S
0.050BS
0.61B
S improveCDFNOTE Double tag
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Backup
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CEM ele ID PHX ele ID
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ID
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MC based background
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Non-W background
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Mistag background
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W+HF background
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NN input
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t-channel Likelihood valuables
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Lepton IDStandard tight lepton selection of lepton+jet group.
Detail is in appendix
Et>20 GeVIsolated Conversion Veto
Electron
Pt>20 GeV/cIsolated Cosmic Veto
Muon
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Jet Energy CorrectionLevel4 Jet Energy Correction – not to apply abs correction
Et>15 GeV, Eta < 2.8
MET
Et>20 GeV
Et is corrected with lepton and Level4 jets
Et
Phi
Et/Pt()
L4 Jet … 0.8L5 Jet … ~1
Phi/Phi()
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Gen info after geometrical cuts
muonelectronlepton
L5 MET correction
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Event Selection
(96%)(90%)(25%)(24%)(24%)(24%)(23%)(21%)
Cross checkThis is t-channell result but…
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B-taggingPerformance of SecVtx and JetProb b-tagging
Number of events of under histogram include W+njet (n>0) final state
For the statistics of the signal double SecVtx tag is applied, about 70% statistics are increased to use S+J tagging.
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Jet multiplicity distribution of b-tag performanceSecVtx JetProb
Cross checkJet multiplicity distribution after b tag (CDF 7863)
19.2% 64.7% 13.6% 2.1% 0.3%
Good agreement !?
Use Jprob<0.05 cuts for Jet Prob.
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overunder
W(->)+nP Background n=0,1,2,3,4