Transcript B(t →Wb)/B(t →Wq) Measurement top General Meeting, may 6, …

Measurement of the top quark
properties with jet flavour and
charge tagging
Christophe Clément (CERN/ Stockholm University)
13th Nordic LHC Physics Workshop
Helsinki
October 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+,...)
Vtb unconstrained
without 3x3 unitarity constraint
Vtb
• Experimentally B(t→Wb) affects number of
b-jets  need to experimentally
discriminate b/w t→Wb and t→Wqlight
October 16, 2006
C.Clément CERN EP seminar
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Deriving B(t→Wb) experimentally....
• Select a top-enriched sample
– e+jets and +jets channels
– larges statistics, good S/B
Missing
transverse
energy
W
One high pT
isolated lepton
t
• # events with 0, 1 and ≥2 b-jets
– B(t→Wb)
– b-tagging efficiency
– Jet identification efficiency
– Probability to tag background
t
b-jets?
light jets?
W
light, c-jets
October 16, 2006
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Lepton + jets sample composition
True isolated
lepton processes
Ntrue
fake isolated
lepton processes
Nfake
•
•
•
•
•
•
.t t
W+jets
Z+jets
WW, WZ, ZZ
single top
multijet
Fake isolated electron
Jets with leading /πo , convertions,
γwith random tracks,...
Determine Ntrue , Nfake on a statistical basis
- from data by deriving the probability
for jets to fake isolated leptons
October 16, 2006
Fake isolated 
 inside jets from heavy flavor or
in flight decays
C.Clément CERN EP seminar
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Lepton + jets sample composition
Ntrue
Nfake
Nbefore tag =
Nn-tags
•
•
•
•
•
•
.t t
W+jets
Z+jets
WW, WZ, ZZ
single top
multijet
Ntt
+
NWj
•Small
•derived from MC
•Lepton, jet efficiencies
calibrated on data
• σfrom data or NLO
Nother
+ Nfake +
Nother
= Pntt (B(t→Wb)) Nntt + PnWj Nnwj + N’nfake + Pnother Nnother
n-tags = 0, 1, 2
Tagging probability
Fit B(t→Wb), Ntt, Nwj to the Nn-tags
October 16, 2006
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How to identify b-quark jets...
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
Taggability,
tagger independent
Decouple b-tagging from
experimental issues
2. Tagged jets:
Are taggable jets
Contain a SV, χ2
Lxy > n σLxy
B-tagging
efficiency
Lxy=
Similar algorithm used by CDF
October 16, 2006
C.Clément CERN EP seminar
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b-tagging efficiency
From di-jet data: extract b-tagging efficiency for muonic b-jets
b→, data
We need the b-tagging efficiency for ”all kinds of b-jets”
b
Similar techniques described in
Phys. Rev. D71, 052003 (2005)
Pbtag(ET,) =
b,MC
------------------  b→, data  Taggability  Ctaggability(b)
b→,MC
Transform semi-muonic b-tag efficiency into inclusive one
October 16, 2006
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3 jets
Number of b-tagged jets
•
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.
≥4 jets
Number of b-tagged jets
October 16, 2006
≥4 jets, 0 tag
Topological discriminant
C.Clément CERN EP seminar
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No 0-tag
Confidence contour
plots in R, Ntt
B(t→Wb)
B(t→Wb)
Phys. Lett. B 639 (2006)
B(t→Wb) = 1.03+0.19-0.17
68% CL : B(t→Wb)>0.78 |Vtb|>0.88
95% CL : B(t→Wb)>0.61 |Vtb|>0.78
October 16, 2006
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Top Quark Charge
OR
?
Lift ambiguity present in all top analyses!
t→W+b or ”t”→W-b
Test exotic models...
t
+2/3
-1/3
Q1
mixing
Q4
M(Q4)~175GeV
-4/3
October 16, 2006
Mtop~270GeV
b
C.Clément CERN EP seminar
Phys.Rev. D65 (2002)
053002
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Ingredients
OR
Q jet
q . pTi
track i i
track i
p Ti
?
0.6
0.6
+
kinematic fit = Qtop
What is the expected shape of Qtop for
 SM top
”top” with 4e/3 charge?
October 16, 2006
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Analysis strategy



Discriminate between
|Qtop| = 2e/3 and |Q”top”|=4e/3
S/B~10
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|

Double tagged events
tt
Compare the observed |Qtop|
with expected SM and exotic distributions
October 16, 2006
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Jet Charge Algorithm
pTi,qi
Compute jet charge only for b-tagged jets
(2 per events)
Jet charge =
Algorithm:
Q jet
qp

p
i
i
i
a
Ti
a
Ti
Optimizaton on MC gives a=0.6
Sum over tracks with

pT>0.5GeV,

ΔR(track, jet) <0.5 of the jet axis
Derive expected shape of Qjet from data
with minimal input from simulation
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Jet Charge Performance in Data
_
Tag and probe method in ”pure” bb events

Tight di-jet sample
>3.0
Ideal case: sign of q = sign of qb
In reality:
_ _
_

Is it pure bb ? cc ? flavor excitation? g→ bb ?

B→

B →D_→ 
´

B o → Bo → 
B→ light hadrons →

Charge misidentification

October 16, 2006
Charge flipping processes
C.Clément CERN EP seminar
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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?
2 b-jets back to back dominate
>3.0
Phys. Rev. D 65, 094006 (2002)
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p.d.f.’s Qb, Qb-, Qc, Qc- from data...
P+ (Qjet) = (1-xc) (1-xflip)Pb (Qjet)+ (1-xc)xflip P-b + xc P-c
p.d.f of Qjet
in probe jet
Fraction of charge
flipping processes
30±1% from MC,
Cross checked on data
_
Fraction of cc derived
from pTrel spectrum
of  (1+2-1%)
Tight di-jet sample
+
Similar equation for P- (Qjet)
4 Unknown p.d.f’s
Pb, Pb-, Pc, PcOctober 16, 2006
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p.d.f.’s Qb, Qb-, Qc, Qc-from data...
P+ (Qjet) = 0.69 Pb (Qjet) + 0.30 Pb- + 0.01 P-c
Tight di-jet sample
P- (Qjet) = 0.30 Pb (Qjet) + 0.69 P-b + 0.01 Pc
P´+(Qjet) = 0.567 Pb (Qjet) + 0.243 P-b + 0.19 P-c
P´- (Qjet) = 0.243 Pb (Qjet) + 0.243 Pb- + 0.19 Pc
loose di-jet sample
-Correct for different
t→Wb and bb kinematics
October 16, 2006
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SM Top and Exotic Quark Charge Observables

Use kinematic fit to assign b-jets to correct W-bosons in
tt
MC
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
Q1
SM
Q2
Q1
SM
 ql  qb
  ql  qB
EX
 ql  qB
EX
  ql  qb
Q2
October 16, 2006
qB
qB
qb
qb
C.Clément CERN EP seminar
qb
qB
qb
qB
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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)
Admixture of Q4 and t-quark
σtt
- not excluded by
- fQ4 <0.52 at 68% C.L.
hep-ex/060844
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Conclusions
• World best/only measurement
B(t→Wb) = 1.03+0.19-0.17
Phys. Lett. B 639 (2006)
Exclude 4e/3 at 92%C.L. (91 % expected)
hep-ex/060844
• 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
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Complement slides
Top Pair Final States
 How top quark was discovered!
Lepton+jets
b
q
t
W
t
q’
”golden channel”
e+jets & +jets ~32%
l
W
 Used for most measured top
quark properties so far
v
b
Dilepton
b
l
W
t
t
v
All
hadronic
b
q
W
q’
t
W
b
v
ee+e+ ~5%
t
b
October 16, 2006
44%
l
W
q
q’
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Top Quark Decay
 In SM top decays via V-A charged current
ig
b
2 2
1
5
V tb tW
⇒ Mostly left handed b-quarks in the 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
October 26, 2006
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Illustration:
How data constraints
B(t→Wb) and σtt ...
R=1.0, σ=7pb
R=0.5, σ=7pb
Single Tag l+jets
Jet multiplicity
Double Tag l+jets
Jet multiplicity
October 16, 2006
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Pntt versus B(t→Wb)
• Probability to see n-tags in t t 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)
ttbar → l+jets
Take into account
”contamination” by
October 16, 2006
tt
→ ll
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Prediction 7pb & B(t→Wb)=1
•
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
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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|>0.88
95% CL : B(t→Wb)>0.61 |Vtb|>0.78
October 16, 2006
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• 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
Cross Section
The 0-tag sample
• Use topological properties of
events in 0-tag sample for
addititional constraint of Ntt in
the 0-tag sample.
October 16, 2006
C.Clément CERN EP seminar
Low B(t→Wb)
Large σtt
Preliminary result from R
summer 2004 (170pb-1)
No 0-tag sample used
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Nevents
Likelihood discriminant in l+4jets 0-tag sample
Data
Data and prediction for
7pb and R=1
Likelihood discriminant output
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Systematics on template shapes
Some systematic uncertainties
can affect the template
shapes...
Systematics on template
shapes on ttbar→ l+jet
JES
JetID
Jet energy resolution
W-modeling (W+jets)
Taggability
Tagging probabilities for b, c
and light jets
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Likelihood discriminant in 0-tag sample
l + 4 jets before tagging
1. Sphericity
S = 3(λ2+λ3)/2 , λ’s smallest eigenvalues of
momentum tensor M (ttbar S~1)
2. K’Tmin
K’Tmin = Rminjj/EWT with EWT = ElT + MET
3. Centrality
C=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)
October 16, 2006
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Results (230 pb-1)
≥4 jets
3 jets
≥4 jets, 0 tag
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b-tagging efficiency from data
”n-sample”:
1 jet with a 
Not -tagged
n
-tagged
n
Not -tagged,
SVT tagged
nSVT
-tagged,
SVT tagged
n ,SVT
”p-sample”:
2 b-2-b jets
1 jet with a 
Not -tagged
p
-tagged
p
Not -tagged,
SVT tagged
pSVT
-tagged,
SVT tagged
p,SVT
Systematic uncertainties
Jet charge specific
Kinematic fit
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Data Calibration Corrections
Discriminant Power
D
| ab  ab |
V
b
 Vb 
MC truth on tag side
Tag and probe
Method: data
Tag and probe
Method: Z→bb
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Why does it work?
The charge of the quark is correlated with the charge of
the 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
Charge of highest pT hadron
Thís is then smeared by
detector effects...
The original b-quark not always
In highest pT hadron
Charge of b-quark
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