Transcript Top Quark Properties and Search for Single Top Quark at the Tevatron

Top Quark Properties
and
Search for Single Top Quark
at the Tevatron
Meenakshi Narain
Boston University
Presented at EPS 2005
Top Quark at the Tevatron
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Top quark discovered a decade ago
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(in 1995)!
Run I (1992-1996)
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s = 1.8 TeV
Integrated luminosity
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8.2 fb-1
Design
120 pb-1
Run II (2001-present)
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s = 1.96 TeV
3 fold increase performance since June’03
Integrated luminosity by June ‘05:
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Delivered >1fb-1
On tape ~800pb-1
Analyzed up to ~350 pb-1
5.1 fb-1
4.1 fb-1
We are here
Base
World’s only top factory!
Top Quark Physics
• Top is very massive
Mtop (world average)= 172.7  2.9 GeV
– It probes physics at much higher energy scale than the other fermions.
• Top decays before hadronizing
top ~ 10-24 sec
– momentum and spin information is passed to its decay products.
– No hadron spectroscopy.
• Top mass constrains the Higgs mass
– Mtop, enters as a parameter in the
calculation of radiative corrections to other
Standard Model observables
– it is also related, along with the mass of
the W boson, to the that of the Higgs boson.
The Top Properties Tour
Top Width
Top Charge
W helicity
Top Spin
Top Mass
CP Violation
Anomalous Couplings
Production X-Section
Production Kinematics
Resonance Production
Top Spin Polarization
Y
|Vtb|
Rare/non SM decays
Branching Fractions
Top Quark Decay Properties
• Does top quark decay 100% of the times to Wb?
– B(t Wb)
• Search for exotic decay modes of the top quark
– t H+b
• Properties of the W-t-b vertex
– W Helicity
– Top quark Charge
Is B(t Wb) ~ 100%?
• Within the SM, assuming unitarity of the CKM matrix,
B(tWb)~1.
• An observation of a B(tWb) significantly different than
unity would be a clear indication of new physics:
– non-SM top decay, non-SM background to top decay, fourth
fermion generation,..
Measurement of B(tWb)/B(tWq)
• B(tWb) can be accessed directly in single top production.
Top decays give access to B(tWb)/B(tWq):
| Vtb |2
B(t  Wb)
2
R


|
V
|
~ 0.998
tb
B(t  Wq ) | Vts |2  | Vtd |2  | Vtb |2
In the SM
• R can be measured by comparing the number of ttbar
candidates with 0, 1 and 2 jets tagged.
– In the 0-tag bin, a discriminant variable exploiting the differences in
event kinematics between ttbar and background is used.
Lepton+jets and dilepton (~160 pb-1)
Lepton+jets (~230 pb-1)
R  1.0300..19
17 ( stat  syst )
R  0.64 @ 95% (Bayes) CL
DØ Run II Preliminary
hep-ex/0505091
R  1.1200..27
23 ( stat  syst )
R  0.61 @ 95% (F & C) CL
Results consistent with the SM prediction
Exotic Decays of the top quark
• Since R is about 1
– Top quark decays to a b-quark  t X+b
• Is X = W+ ?
OR
• could X = H+ ?.
– as predicted by generic 2Higgs Doublet
Models?
Search for
+
tH b
• If MH±<mt-mb
– then t H+b competes with t W+b
– results in B(tWb)<1.
• H decays are different than W decays
– affect (tt) measurements in different channels (dileptons, lepton+jets,
lepton+tau).
• Perform simultaneous fit
– to the observation in all channels and
– determine model-dependent exclusion region in (tan, MH±).
Are the other Properties of the
Top Quark as Expected?
W-t-b Vertex: W helicity
Top Charge
W helicity in Top quark Decays
• Large top quark mass:
– Are there new interactions at energy
scales near EWSB?
V-A coupling
• helicity of the W boson:
gWtb  |Vtb| (V-A)
– examines the nature of the tbW vertex
– provides a stringent test of Standard
Model
F0= 0.7
F-= 0.3
F+=0
W0 Longitudinal fraction W- Left-Handed fraction W+ Right-Handed fraction
F+
FF0
0
+1/2
t
-1/2
+1/2
W
t
b
b
t
W
b
W
+1/2
+1
+1/2
-1/2
+1
W helicity
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In the Standard Model (with mb=0):
3
3
3
w(cos l b )  F  (1  cos l b ) 2  F0  (1  cos 2 l b )  F  (1  cos l b ) 2
8
8
8
SM: F-= 0.3
Left-handed
Longitudinal
Right-handed
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The PT of the lepton has information about the
helicity of the W boson:
– longitudinal: leptons are emitted perpendicular to the
W (harder lepton PT)
– left-handed: leptons are emitted opposite to W boson
(softer lepton PT)
F0= 0.7
F+=0
W Helicity
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Likelihood analysis of PT spectrum
Consider dilepton channels
Fix F0=0.7, measure F+ (F-=1-F0-F+)
Binned likelihood and estimate F+
using Bayesian method
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Likelihood analysis of cos *
Consider lepton+jets channels
Fix F0=0.7, measure F+ (F-=1-F0-F+)
Two analysis: topological and b-tag
Lepton+jets (1 b-tag)
L=230 pb-1
hep-ex/0505031
D: F  0.13  0.20( stat )  0.06( syst )
F  0.28 @ 95%CL
CDF : F0  0.27 
0.35
0.21
( stat  syst )
F0  0.88 @ 95%CL
Run I best (DØ 125 pb-1): F0=0.560.31
D0 (combined)
F+  0.04  0.11( stat )  0.06( sys )
F  0.25 @ 95%CL( stat  sys )
Run I best (CDF 109 pb-1): F+<0.18 @ 95% CL
Results consistent with the SM prediction: F0=0.7, F+=0
Measurement of top quark Charge
• Is it the Standard Model top ?
OR
• An exotic doublet of quarks (Q1, Q4)
– with charges (-1/3,-4/3) and M ~ 175 GeV/c2
– while M(top) ~ 274 GeV/c2
• W.-F. Chang et al.,hep-ph/9810531
• q = -4/3 is consistent with EW data,
– new b-couplings improve the EW fit
• (E. Ma et al. , hep-ph/9909537)
Top Quark Charge Measurement
• Goal: discriminate between
– |Qtop| = 2e/3 and |Q”top”| = 4e/3
• Top quark charge is given by the sum
of the charge of its decay products
• Determine:
– Charge of W (lepton)
– Charge of b-jet Qjet = qi pTia/  pTia
• (here, a=0.6)
– Associate b-jets to correct W (charged
lepton)
• The charge of the quark is correlated
with the charge of the highest pT
hadron during hadronization
Top Quark Charge
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We need an observable and an expectation for
the ”2/3” and ”4/3” scenarios
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Consider only lepton+jets double-tagged events
– Two top quarks in the event  measure the charge
”twice”
Q1
SM
 ql  qb
qb = b lept. side
SM
  ql  qB
qB = b hadr. side
Q2
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The exotic scenario is obtained by permuting the
charge of the tagged jets
Q1
EX
 ql  qB
qB = b hadr. side
EX
  ql  qb
qb = b lept. side
Q2
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qb and qB are taken from the data derived jet
charge templates
• Results coming soon...
qB
qb
qb
qB
qB
Top Quark Production Properties
• Since top decay properties look quite
consistent with SM predictions….
• What about its production?
– Could it be a “t-prime”?
– Search for t’t’ production (t’ Wq)
– Could the ttbar pair originate from the decay of a
resonance?
– Model independent search for narrowresonance Xtt used
to exclude a leptophobic X boson:
Run I search of X with G=1.2%M:
MX>560 GeV @ 95% CL (DØ) and MX>480 GeV @ 95% CL (CDF)
• What about single top production?
Search for Single Top Quark
Search for Single Top
s-channel
t-channel
NLO = 0.88pb ± 8%
NLO = 1.98pb ± 11%
hep-hp/207055 (Harris, Laenen, Phaf, Sullivan, Weinzierl)
PRD 63, 014018 (2001)
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Measure production cross sections
Direct measurement of |Vtb| (s  |Vtb|2)
Top spin physics (~100% polarized top quark)
s- and t-channels sensitive to different New
Physics
– Irreducible background to associated Higgs
production
– Exotic Models (FCNC, Top Flavor, 4th Gen)
(t-channel) (pb)
• Electroweak Production of top quark:
SM
3Dtheo
(s-channel) (pb)
Single Top Status
t-channel
s-channel
q
W
t
d
u
W
q'
b
b
t
• Cross sections:
– NLO calculation:
– Run I 95% CL limits, DØ:
CDF:
– Run II CDF 95% CL limits:
s-channel
0.88pb (±8%)
< 17pb
< 18pb
< 14pb
t-channel
s+t
1.98pb (±11%)
< 22pb
< 13pb
< 14pb
< 10pb
• Other Standard Model production mode (Wt) negligible
< 18pb
Signature & Backgrounds
Signal for s and t channel
mostly similar
(t-channel)
• Lepton + Missing ET + Jets
• t-channel extra b tends to
be forward
• Similar to top pair
production, but with less
jets
Harder Signal To Find
Backgrounds
•W/Z + jets Production
•Fake Leptons
•Top Pair Production
•WW, WZ, Z, etc.
Much worse than for pair production
because of lower jet multiplicity
Anything with a lepton + jets + ET signature
Discriminating Variables
• Object kinematics
– Jet pT for different jets
• Tagged, untagged,...
• Event kinematics
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–
H (total energy)
HT (transverse energy)
M (invariant mass)
MT (transverse mass)
Summing over various
objects in the event
• Angular variables
– Jet-jet separation
– Jet pseudorapidity (tchannel)
– Top quark spin
Separating Signal from Backgrounds
• Four analysis methods
Cut-Based
Neural Networks
Decision Trees
Likelihood
Discriminant
• Three (Cut, NN, DT) use the same structure:
– Optimize separately for s-channel and t-channel
• Optimize separately for electron and muon channel (same
variables)
– Focus on dominant backgrounds: W+jets, tt
• W+jets – train on tb-Wbb and tqb-Wbb
• tt – train on tb – tt  l + jets and tqb – tt  l + jets
– Based on same set of discriminating variables
➔ 8 separate sets of cuts/networks/trees
1. Cut-Based Analysis
• Cuts on sensitive variables
to isolate single top
– Separate optimizations for
s-channel and t-channel
– Loose cuts on energyrelated variables:
pT (jet1tagged)
H(alljets – jet1tagged)
H(alljets – jet1best)
HT (alljets)
M(toptagged)
M(alljets)
M(alljets – jet1tagged)
ŝ
Factor 2 improvement!
2. Neural Network
Analysis
Input Nodes: One for
full
dataset
electron
each variable xi
muon
=1 b-tag 2 b-tags =1 b-tag 2 b-tags
construct
networks
2d histograms, Wbb vs tt filter
Output Node:
linear combination
of hidden nodes
Hidden Nodes: Sigmoid
dependent on the input variables
Result
• No evidence for single top signal
Set 95% CL upper cross section limit
– Using Bayesian approach
– Combine all analysis channels
(e, m, =1 tag, 2 tags)
– Take systematics and
correlations into account
➔
Expected limit: set Nobs to
background yield
Expected/Observed limit:
s < 9.8 / 10.6 pb
t < 12.4 / 11.3 pb
Systematic uncertainty:
Neural Network Output
e+m
1 tag
e+m
1 tag
e+m
1 tag
e+m
1 tag
Result
• No evidence for single top signal
Set 95% CL upper cross section limit
– Using Bayesian approach and binned likelihood
➔
• Built from 2-d histogram of
Wbb NN vs tt NN
• Including bin-by-bin systematics and correlations
Expected/Observed limit:
s < 4.5 / 6.4 pb
t < 5.8 / 5.0 pb
3. Decision Tree Analysis
• Replace Neural Networks by Decision Trees
– single tree, ~100 nodes
Fail
HT>212
Mt<352
– Remaining analysis steps identical
• Same inputs
• Same filter configuration
• Binned likelihood analysis
Expected/Observed limit:
s < 4.5 / 8.3 pb
t < 6.4 / 8.1 pb
• Sensitivity comparable to Neural Network analysis
Pass
pt<31.6
purity
4. Likelihood Discriminant Analysis
• New Analysis based on 370pb-1 dataset
• Different btagging algorithm and selection
P ( x)
• Likelihood Discriminant: L 
signal
– Input Variables:
Psignal ( x)  Pbackground ( x)
Result
• …
Expected/Observed limit:
s < 3.3 / 5.0 pb
t < 4.3 / 4.4 pb
Best Limit !!!!
Comparison of LH with NN analysis
Single Top Summary
• Cross sections:
– NLO calculation:
– Run I 95% CL limits, DØ:
CDF:
– Run II CDF 95% CL limits:
s-channel
0.88pb (±8%)
< 17pb
< 18pb
< 14pb
t-channel
s+t
1.98pb (±11%)
< 22pb
< 13pb
< 14pb
< 10pb
< 18pb
– RunII DØ 95% Cl Limits:
(230 pb-1)
Cut Based
Decision Tree
Neural Network*
< 10.6pb
< 8.3pb
< 6.4pb
< 11.3pb
< 8.1pb
< 5.0pb
(370 pb-1) (new analysis)
Likelihood Discriminant
< 5.0pb
< 4.4pb
* = Accepted for publication, hep-ex/0505063
using only muon channel data
Sensitivity to non-SM Single Top
using only electron channel data
Conclusion
• Measurements of various top quark
properties are underway and will improve
with larger data sets
• The single top cross section limits and
sensitivity of the analyses are getting to a
level where we can expect to observe
single top quark production soon!.
Stay Tuned.