13 th nordic lhc physics workshop helsinki october 26, 2006
DESCRIPTION
Measurement of the top quark properties with jet flavour and charge tagging Christophe Clément (CERN/ Stockholm University). 13 th Nordic LHC Physics Workshop Helsinki October 26, 2006. V tb unconstrained without 3x3 unitarity constraint. B( t →Wb ). B( t → Wb )=1 usually assumed by - PowerPoint PPT PresentationTRANSCRIPT
Measurement of the top quark properties with jet flavour and charge taggingChristophe Clément (CERN/ Stockholm University)
13th Nordic LHC Physics WorkshopHelsinkiOctober 26, 2006
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
• 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
• .
• 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
• .
• 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
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
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)
• 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
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
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
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
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
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
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
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
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
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
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
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
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)
Complement slides
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%
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
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 ...
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
• 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
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
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
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
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
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
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
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
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
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
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