1. First Evidence for Electroweak Single Top Quark Production
XLIInd Rencontres de Moriond
QCD and High Energy Hadronic Interactions
La Thuile, 17-24th March 2007
First Evidence for Electroweak Single Top Quark Production
Leonard Christofek
on behalf of the D0 Collaboration
Moriond 2007
Leonard Christofek, Brown University

2. Single Top Theory and Motivation
s-channel (tb)
t-channel (tqb)
Associated
production (tW)
NLO = 0.88 ± 0.11 pb
NLO = 1.98 ± 0.25 pb
NLO ~ 0.21 pb
Too small at Tevatron
Study the Wtb coupling in top quark production
measure |Vtb| directly (no assumption of 3 families)
test unitarity of CKM matrix
Cross section sensitive to new physics
Background to SM Higgs production
Test of techniques to extract small signal from dominant background
Moriond 2007
Leonard Christofek, Brown University

3. Leonard Christofek, Brown University
Window to new physics
s-channel (tb)
t-channel (tqb)
Associated
production c,
W’, b,
New heavy boson,
New strong dynamics
Flavor changing
neutral currents
(FCNC)
Modified
Wtb coupling
Each new process would enhance single top quark production!
Moriond 2007
Leonard Christofek, Brown University

4. Leonard Christofek, Brown University
Event Signature
High pT jet
High pT lepton
Missing transverse energy
High pT jet (“b-tagged”)
High pT jet (“b-tagged”)
One isolated high transverse momentum lepton
Missing transverse energy
Two to four high transverse momentum jets
One or two jets associated with b-quark:
Labeled as a “b-tagged” jet
Moriond 2007
Leonard Christofek, Brown University

5. Leonard Christofek, Brown University
Backgrounds
Top pair production normalized to NNLO cross section
Multijet background modeled using data with a non-isolated lepton and jets
W+jets (Wbb, Wcc, Wjj) and multijet background normalized to data before b-tagging
Moriond 2007
Leonard Christofek, Brown University

6. Leonard Christofek, Brown University
Analysis Strategy
Binned Likelihood
Cross Section or Limits
Moriond 2007
Leonard Christofek, Brown University

7. Leonard Christofek, Brown University
Background Model
Source of Uncertainty
Size
Top pair normalization
18%
W+jet & multijet norm.
18-28%
Integrated luminosity 6%
Trigger modeling
3-6%
Lepton ID corrections
2-7%
Other small components
Few%
Jet energy calibration
1-20%
“b”-tagged jet modeling
2-16%
Signal acceptances:
tb=(3.2 ± 0.4)% tqb=(2.1 ± 0.3)%
Single top signal smaller than total background uncertainty
Cross section uncertainty dominated by statistical uncertainty
Moriond 2007
Leonard Christofek, Brown University

8. Bayesian Neural Networks
Use 24 variables for training networks (input nodes).
Train network on signal and background simulated events:
Signal tends to one and background tends towards zero (output node).
Average many different networks for stability (hidden nodes).
Less prone to over-training.
Input Nodes
Hidden Nodes
Output Node
tqb
Wbb
Moriond 2007
Leonard Christofek, Brown University

9. Leonard Christofek, Brown University
Matrix Element
Use full kinematical information contained in event (i.e. the
four vectors from the lepton and jets).
Compute the probability for that event configuration to occur.
Backgrounds: Wbb, Wcg, Wgg, Wbbg
Moriond 2007
Leonard Christofek, Brown University

10. Leonard Christofek, Brown University
Decision Trees
Goal: recover events that fail a simple cut-based analysis
Use 49 variables for training: most discriminating variables M(alljets), M(W,b-tag1), cos(b-tag1,lepton), Q(lepton)*(untagged1)
Decision tree output for each event = leaf purity: NS/(NS+NB)
Train network on signal and background simulated events:
Signal tends to one and background tends towards zero
Boosting: retrain 20 times to improve “weak classifier”
Moriond 2007
Leonard Christofek, Brown University

11. Expected and Observed Results
Bayesian NN
Matrix Element
Decision Trees
Exp.
Obs.
(tb+tqb)[pb]
5.0 ± 1.9
4.9 ± 1.4
Significance
1.3
2.4
1.8
2.9
2.1
3.4
First evidence for single top quark production!
SM compatibility is 11%.
Moriond 2007
Leonard Christofek, Brown University

12. Leonard Christofek, Brown University
Discriminant Output
Moriond 2007
Leonard Christofek, Brown University

13. Leonard Christofek, Brown University
Extracting |Vtb|
Assuming standard model production:
Pure V-A and CP conserving interaction: f1R = f2L = f2R = 0.
|Vtd|2 + |Vts|2 << |Vtb|2 or B(t Wb) ~ 100%.
0.68 < |Vtb| < 1
at 95%CL (f1L = 1)
|Vtb f1L| = 1.3 ± 0.2
Moriond 2007
Leonard Christofek, Brown University

14. Leonard Christofek, Brown University
Combination
Highly correlated.
Correlation matrix
 = 4.8 ± 1.3 pb
Significance = 3.5 
Preliminary
Moriond 2007
Leonard Christofek, Brown University

15. Leonard Christofek, Brown University
Summary
First evidence for single top quark production.
With 3.5  significance.
4.8 ± 1.3 pb
First direct measurement of | Vtb |
Now starting to analyzing full 2 fb-1 data set.
Onto observation!
Moriond 2007
Leonard Christofek, Brown University

16. Backup Transparencies
Moriond 2007
Leonard Christofek, Brown University

17. Leonard Christofek, Brown University
Outline
Theory and motivation for single top
Event signature
Backgrounds
Multivariate analyses:
Bayesian Neural Network, Matrix Elements, Decision Trees
Cross section and signal significance
Combination of measurements
Measurement of the CKM element |Vtb|
Summary
Moriond 2007
Leonard Christofek, Brown University

18. Discriminating Variables
Moriond 2007
Leonard Christofek, Brown University

19. Leonard Christofek, Brown University
Single Top Monte Carlo
Based on COMPHEP
Reproduces NLO kinematic distributions
PYTHIA for parton hadronization
Moriond 2007
Leonard Christofek, Brown University

20. Systematic Uncertainties
Full treatment of systematics. Each systematic is turned off and
the cross section reevaluated. The contribution of each systematic
is estimated by taking subtracting these two widths in quadrature.
Moriond 2007
Leonard Christofek, Brown University