13 th Nordic LHC Physics Workshop Helsinki October 26, 2006
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Transcript of 13 th Nordic LHC Physics Workshop Helsinki October 26, 2006
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Measurement of the top quark properties with jet flavour and charge taggingChristophe Clément (CERN/ Stockholm University)
13th Nordic LHC Physics WorkshopHelsinkiOctober 26, 2006
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B(t→Wb)
• B(t→Wb)=1 usually assumed by CDF and D analyses
• B(t→Wb) might deviate from unity:– Additional quark singlets or doublets– ”Pollution” of top sample by non-top
process!– Non-SM processes in the production– Non-SM in the decay (H+,...)
• Experimentally B(t→Wb) affects number of b-jets need to experimentally discriminate b/w t→Wb and t→Wqlight
Vtb unconstrained without 3x3 unitarity constraint
October 16, 2006 C.Clément CERN EP seminar 9/39October 16, 2006 C.Clément CERN EP seminar 9/39
Vtb
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• Select a top-enriched sample– e+jets and +jets channels– larges statistics, good S/B
• # events with 0, 1 and ≥2 b-jets– B(t→Wb)– b-tagging efficiency– Jet identification efficiency– Probability to tag background
Deriving B(t→Wb) experimentally....
October 16, 2006 C.Clément CERN EP seminar 10/39October 16, 2006 C.Clément CERN EP seminar 10/39
t
t
W
Wb-jets?light jets?
Missing transverse
energyOne high pT
isolated lepton
light, c-jets
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• .
• W+jets
• Z+jets
• WW, WZ, ZZ
• single top
• multijet
Lepton + jets sample composition
October 16, 2006 C.Clément CERN EP seminar 11/39October 16, 2006 C.Clément CERN EP seminar 11/39
t t
True isolated lepton processes
Ntrue
fake isolated lepton processes
Nfake
Fake isolated electron Jets with leading /πo , convertions, γwith random tracks,...
Fake isolated inside jets from heavy flavor or in flight decaysDetermine Ntrue , Nfake on a
statistical basis- from data by deriving the probability for jets to fake isolated leptons
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• .
• W+jets
• Z+jets
• WW, WZ, ZZ
• single top
• multijet
Lepton + jets sample composition
October 16, 2006 C.Clément CERN EP seminar 12/39October 16, 2006 C.Clément CERN EP seminar 12/39
t t
Ntrue
Nfake
•Small•derived from MC•Lepton, jet efficiencies calibrated on data• σfrom data or NLO
Nother
Nbefore tag = Ntt + NWj + Nfake +
Nother
Nn-tags = Pntt (B(t→Wb)) Nn
tt + PnWj Nn
wj + N’nfake + Pn
other Nn
other
n-tags = 0, 1, 2Fit B(t→Wb), Ntt, Nwj to the Nn-tags
Tagging probability
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Explicitely reconstruct displaced secondary vertices: Secondary Vertex Tagger
1. Taggable jets are: Calorimeter jets with pT>15 GeV, ||<2.5 ΔR(calo jet, track jet)=0.5, ≥ 2 tracks in ΔR=0.5, Δz<2cm ≥ 1 hit in the innermost tracking detector, pT>0.5 GeV ≥ 1 track with pT>1 GeV
Decouple b-tagging from experimental issues
2. Tagged jets: Are taggable jets Contain a SV, χ2
Lxy > n σLxy
Similar algorithm used by CDF
How to identify b-quark jets...
October 16, 2006 C.Clément CERN EP seminar 13/39October 16, 2006 C.Clément CERN EP seminar 13/39
Lxy=
Taggability,tagger independent
B-tagging efficiency
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b-tagging efficiency
From di-jet data: extract b-tagging efficiency for muonic b-jets
We need the b-tagging efficiency for ”all kinds of b-jets”
PPbbtagtag(E(ETT,,) =) =
b,MCb,MC ------------------------------------ bb→→, data, data Taggability Taggability C Ctaggabilitytaggability(b) (b) bb→→,MC,MC
b→b→, data, data
bb
Transform semi-muonic b-tag efficiency into inclusive one
October 16, 2006 C.Clément CERN EP seminar 14/39October 16, 2006 C.Clément CERN EP seminar 14/39
Similar techniques described inPhys. Rev. D71, 052003 (2005)
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• Fit B(t→Wb) and Ntt simultaneously to 12 bins: e/+ 3/≥4 jets, 0/1/≥2 tags
• B(t→Wb) is constrained by relative 0/1/≥2 tags populations
• Very poor S/B in 0-tag sample, Nttbar~√Nobs
• Perform measurement of Ntt in 0-tag as additional constrain.
October 16, 2006 C.Clément CERN EP seminar 16/39October 16, 2006 C.Clément CERN EP seminar 16/39
3 jets
≥4 jets ≥4 jets, 0 tag
Number of b-tagged jets
Number of b-tagged jets Topological discriminant
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October 16, 2006 C.Clément CERN EP seminar 23/39October 16, 2006 C.Clément CERN EP seminar 23/39
Phys. Lett. B 639 (2006)
No 0
-tag
B(t→Wb) = 1.03+0.19-0.17
Confidence contour plots in R, Ntt
B(t→Wb) B(t→Wb)
68% CL : B(t→Wb)>0.78 |Vtb|>0.8895% CL : B(t→Wb)>0.61 |Vtb|>0.78
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OROR
??
Top Quark Charge
October 16, 2006 C.Clément CERN EP seminar 25/39October 16, 2006 C.Clément CERN EP seminar 25/39
Lift ambiguity present in all top analyses!t→W+b or ”t”→W-b
Test exotic models...t
bQ1
Q4
+2/3
-1/3
-4/3mixing
M(Q4)~175GeV
Mtop~270GeV
Phys.Rev. D65 (2002) 053002
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OROR ??
Q jettrack i
qi . pTi0.6
track ipTi
0.6 kinematic fit++ == Qtop
Ingredients
What is the expected shape of Qtop for SM top”top” with 4e/3 charge?
October 16, 2006 C.Clément CERN EP seminar 26/39October 16, 2006 C.Clément CERN EP seminar 26/39
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Analysis strategy Discriminate between |Qtop| = 2e/3 and |Q”top”|=4e/3
Two |Qtop| per event
Use the pure sample-- lepton+4≥jets events with 2 SVT
Compute the jet charge of the 2 b-tagged jets
Associate the b-jets to correct W boson (charged lepton)
Combine the 2 jet charges and the lepton charge to derive the 2 |Qtop|
Compare the observed |Qtop| with expected SM and exotic distributions
Double tagged events
October 16, 2006 C.Clément CERN EP seminar 27/39October 16, 2006 C.Clément CERN EP seminar 27/39
t t
S/B~10
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Jet Charge Algorithm
Compute jet charge only for b-tagged jets - (2 per events)
Jet charge =
Optimizaton on MC gives a=0.6Sum over tracks with
pT>0.5GeV, ΔR(track, jet) <0.5 of the jet axis
Algorithm:
i
aT
i
aTi
jet
i
i
p
pqQ
October 16, 2006 C.Clément CERN EP seminar 28/39October 16, 2006 C.Clément CERN EP seminar 28/39
Derive expected shape of Qjet from data
with minimal input from simulation
pTi,qi
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Jet Charge Performance in Data
Tag and probe method in ”pure” events
In reality: Is it pure ? ? flavor excitation? g→ ? B→ B →D → → B→ light hadrons → Charge misidentification
Tight di-jet sample
>3.0
October 16, 2006 C.Clément CERN EP seminar 30/39October 16, 2006 C.Clément CERN EP seminar 30/39
_bb
Ideal case: sign of q = sign of qb
_bb
_cc
_bb
´ _Bo→ Bo
Charge flipping processes
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Is the triple tag sample pure ”bb”? The fraction of c-jets in the triple tag sample is determined by pTrel fit of the
order of a few percents,
Flavor excitation/ splitting?
>3.02 b-jets back to back dominate
Phys. Rev. D 65, 094006 (2002)
-
October 16, 2006 C.Clément CERN EP seminar 32/39October 16, 2006 C.Clément CERN EP seminar 32/39
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P+ (Qjet) = (1-xc) (1-xflip)Pb (Qjet)+ (1-xc)xflip Pb + xc Pc
- -
Fraction of derived from pT
rel spectrum of (1+2
-1%)
_ccFraction of charge
flipping processes30±1% from MC, Cross checked on data
Similar equation for P- (Qjet)
4 Unknown p.d.f’sPb, Pb, Pc, Pc
Tight di-jet sample
+
p.d.f of Qjet in probe jet
- -
p.d.f.’s Qb, Qb, Qc, Qc from data...--
October 16, 2006 C.Clément CERN EP seminar 33/39October 16, 2006 C.Clément CERN EP seminar 33/39
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p.d.f.’s Qb, Qb, Qc, Qc from data...--
October 16, 2006 C.Clément CERN EP seminar 34/39October 16, 2006 C.Clément CERN EP seminar 34/39
P+ (Qjet) = 0.69 Pb (Qjet) + 0.30 Pb + 0.01 Pc
P- (Qjet) = 0.30 Pb (Qjet) + 0.69 Pb + 0.01 Pc
P´+(Qjet) = 0.567 Pb (Qjet) + 0.243 Pb + 0.19 Pc
P´- (Qjet) = 0.243 Pb (Qjet) + 0.243 Pb + 0.19 Pc
- -
-
-
-
-
Tight di-jet sample
loose di-jet sample
-Correct for different
t→Wb and bb kinematics
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SM Top and Exotic Quark Charge Observables Use kinematic fit to assign b-jets to correct W-bosons in MC
BlSM
blSM
qqQ
qqQ
2
1
qb = qjet of the b-jet associated to the leptonic side of the event
qB = qjet of the b-jet asssociated to the hadronic side of the event
qB
qb
qB
qb
October 16, 2006 C.Clément CERN EP seminar 35/39October 16, 2006 C.Clément CERN EP seminar 35/39
t t
blEX
BlEX
qqQ
qqQ
2
1
qBqBqb
qb
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Admixture of Q4 and t-quark - not excluded by - fQ4 <0.52 at 68% C.L.
σtt
Result on ~0.4 fb-1 of D data 21 l + jets double tagged events 16 events with converged kinematic fit 32 measured top charges
Perform likelihood ratio test b/w 4e/3 and 2e/3 hypothesis
Exclude 4e/3 at 92%C.L. (91 % expected)
October 16, 2006 C.Clément CERN EP seminar 37/39October 16, 2006 C.Clément CERN EP seminar 37/39
hep-ex/060844hep-ex/060844
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Conclusions• World best/only measurement
• Acknowledge collaborators: J. Strandberg (Stockholm University, now at Michigan) and
P. Hansson (Kungliga Tekniska Högskolan)
• Calibration of b-tagging algorithm and jet charge algorithm on data was crucial to establish these measurements
• Calibration of efficiencies, fake rates from DATA is often at the heart of analyses at Tevatron and most tricky part.
• Probably even more so at LHC experiments, large leap forward, need to understand data before we can convince ourselves we
October 16, 2006 C.Clément CERN EP seminar 39/39October 16, 2006 C.Clément CERN EP seminar 39/39
B(t→Wb) = 1.03+0.19-0.17
Exclude 4e/3 at 92%C.L. (91 % expected)
hep-ex/060844
Phys. Lett. B 639 (2006)
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Complement slides
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How top quark was discovered!How top quark was discovered!
Used for most measured top Used for most measured top quark properties so farquark properties so far
tt W
W
Dileptonb
b
ll
vv
tt W
W
Lepton+jetsb
b
lq
vq’
tt W
W
All hadronic
b
b
q
q’q
q’
Top Pair Final StatesTop Pair Final States
October 16, 2006 C.Clément CERN EP seminar 8/39October 16, 2006 C.Clément CERN EP seminar 8/39
44%
ee+e+ ~5%
”golden channel”e+jets & +jets ~32%
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Top Quark DecayTop Quark Decay
3 quark generations + direct measurements of Vub and Vcb predict Vtb,~1 B(t→Wb)~1
SM predicts FCNC decays are tiny, t→Wq is dominant
In SM top decays via V-A charged current
⇒ Mostly left handed b-quarks in the decay
ig
2 2b 1 5 V tb tW
October 26, 2006 C.Clément Noridc LHC Workshop 5/39October 26, 2006 C.Clément Noridc LHC Workshop 5/39
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Single Tag l+jets
Double Tag l+jets
Jet multiplicity
Jet multiplicity
October 16, 2006 C.Clément CERN EP seminar 17/39October 16, 2006 C.Clément CERN EP seminar 17/39
R=1.0, σ=7pb R=0.5, σ=7pbIllustration:How data constraintsB(t→Wb) and σtt ...
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Pntt versus B(t→Wb)
• Probability to see n-tags in events (Pntt )
depends on the number of b-jets
• Pn tt = R2 Pn tag(tt→bb) + 2 R(1-R)Pn tag(tt→bql) + (1-R)2Pn
tag(tt→qlql)
t t
October 16, 2006 C.Clément CERN EP seminar 15/39October 16, 2006 C.Clément CERN EP seminar 15/39
ttbar → l+jets
Take into account ”contamination” by → llt t
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• Fit B(t→Wb) and Ntt simultaneously to 12 bins: e/+ 3/≥4 jets, 0/1/≥2 tags
• B(t→Wb) is constrained by relative 0/1/≥2 tags populations
• Very poor S/B in 0-tag sample, Nttbar~√Nobs
• Perform measurement of Ntt in 0-tag as additional constrain.
October 16, 2006 C.Clément CERN EP seminar 16/39October 16, 2006 C.Clément CERN EP seminar 16/39
Prediction 7pb & B(t→Wb)=1
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Lower limit on B(t→Wb) and |Vtb|
• Prior π(B(t→Wb))=0 outside [0,1]
• Monte Carlo integration over 191 nuisance parameters associated to systematic errors
• Provides a 2D p.d.f. for B(t→Wb) and Ntt
• Limit on |Vtb| can be derived using |Vtb|=√B(t→Wb) (SM)
68% CL : B(t→Wb)>0.78 |Vtb|>0.8895% CL : B(t→Wb)>0.61 |Vtb|>0.78
October 16, 2006 C.Clément CERN EP seminar 24/39October 16, 2006 C.Clément CERN EP seminar 24/39
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The 0-tag sample
• Ntt ~ √N(0-tag)
• Without further information on the 0-tag events low B(t→Wb) + large Ntt (σtt) can still be consistent with data
• Use topological properties of events in 0-tag sample for addititional constraint of Ntt in the 0-tag sample.
Cro
ss
Sect
ion
RPreliminary result from summer 2004 (170pb-1) No 0-tag sample used
October 16, 2006 C.Clément CERN EP seminar 18/39October 16, 2006 C.Clément CERN EP seminar 18/39
Low B(t→Wb)Large σtt
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Likelihood discriminant in l+4jets 0-tag sample
Data and prediction for
7pb and R=1
Data
October 16, 2006 C.Clément CERN EP seminar 20/39October 16, 2006 C.Clément CERN EP seminar 20/39
Likelihood discriminant output
Nevents
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Systematics on template shapes
Some systematic uncertainties can affect the template shapes...
Systematics on template shapes on ttbar→ l+jet
JESJetIDJet energy resolutionW-modeling (W+jets)TaggabilityTagging probabilities for b, c and light jets
October 16, 2006 C.Clément CERN EP seminar 21/39October 16, 2006 C.Clément CERN EP seminar 21/39
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1. SphericityS = 3(λ2+λ3)/2 , λ’s smallest eigenvalues of
momentum tensor M (ttbar S~1)
2. K’Tmin
K’Tmin = Rminjj/EW
T with EWT = El
T + MET
3. CentralityC=HT/H , HT is scalar sum of jets ET and
H is the sum of the jet energies.
4. H’T2
H’T2 = HT2/Hz, HT2 : pT sum of all jets but leading jet,
Hz is the scalar sum of all jets |Ez| plus |Ez| of the neutrino (W-assumption)
Likelihood discriminant in 0-tag samplel + 4 jets before tagging
October 16, 2006 C.Clément CERN EP seminar 19/39October 16, 2006 C.Clément CERN EP seminar 19/39
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Results (230 pb-1)
October 16, 2006 C.Clément CERN EP seminar October 16, 2006 C.Clément CERN EP seminar 22/39 22/39
3 jets ≥4 jets
≥4 jets, 0 tag
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b-tagging efficiency from data
”n-sample”:1 jet with a
Not -taggedn
-taggedn
Not -tagged, SVT taggednSVT
-tagged, SVT taggedn ,SVT
”p-sample”:2 b-2-b jets 1 jet with a
Not -taggedp
-taggedp
Not -tagged, SVT taggedpSVT
-tagged, SVT taggedp,SVT
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Systematic uncertainties
October 16, 2006 C.Clément CERN EP seminar 38/39October 16, 2006 C.Clément CERN EP seminar 38/39
Kinematic fit
Jet charge specific
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Data Calibration Corrections
Discriminant Power
bb
bb
VV
aaD
||
Tag and probe
Method: Z→bbOctober 16, 2006 C.Clément CERN EP seminar 31/39October 16, 2006 C.Clément CERN EP seminar 31/39
Tag and probe
Method: data
MC truth on tag side
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Why does it work?The charge of the quark is correlated with the charge ofthe highest pT hadron resulting of the hadronization
Simple study carried out with Pythia:Generated QCD 2→2 process, pT>15GeV
And look at hight pT b quarks produced in the process.
Usually large number of hadrons Produced, most quite modest pT
Thís is then smeared by detector effects...
The original b-quark not always In highest pT hadron
Charge of b-quark
Charge of highest p T hadron
October 16, 2006 C.Clément CERN EP seminar 29/39October 16, 2006 C.Clément CERN EP seminar 29/39