1. Measurement of the Single Top Production Cross Section at CDF
Koji Nakamura Univ. of Tsukuba
On Behalf of the CDF Collaboration
PHENO Wisconsin Madison
29 Apr. 2008

2. Top Quark at Tevatron Electroweak single top production allows
Top quarks are predominantly produced in pairs by the strong interaction
Discovered in 1995 at Tevatron
Studying properties like cross section, mass, charge, W helicity
~ 6.7 pb
~ 2.9 pb
Electroweak single top production allows
The theoretical cross section is 0.4xsttbar
Evidence was seen by D0 and CDF experiment
29 April 2008
PHENO 2008
(Dec.2006)
(Aug. 2007)

3. Why Single Top Quark? Production rate is proportional to |Vtb|2
st = (1.98 ± 0.25) |Vtb|2 pb
ss = (0.88 ± 0.11) |Vtb|2 pb
Top Polarization study
Single top quarks are 100% polarized in SM
Can test this with angular distribution of top decay
Probe Non Standard Model phenomena
Can search for heavy W’ boson or H±
Technical Motivation
Test of the methodology for Higgs search
(the same final state as the WHlνbb signal)
29 April 2008
PHENO 2008

4. Single top at CDF Event topology 2 or 3 high Pt jets (Pt>20 GeV)
Singletop production with decay
into lepton + 2 jets final state
One high Pt lepton
(Pt>20 GeV)
Large
Missing Energy
(Et>25 GeV)
Singletop
Signal is hidden under the huge bkg
 Multivariate analyses are needed
Top pair
Dominant process of 4 jets bin
counting method is possible
29 April 2008
PHENO 2008

5. Analysis strategy Signal Model Background Model CDF Data Set
Blind analysis
Event Selection
Split in sub set
of different purity
W+2jets
W+3jets
1 tag
≥2 tag
1 tag
≥2 tag
Multivariate Analysis
Combined Analysis
Both s- and t-channel as the signal
Fixed st / ss Ratio to SM
Sensitive to Discovery and |Vtb|
Boosted
Decision Tree
Matrix Element
Discriminant
Separate Search
Ether s- or t-channel is the signal
Not fixed st / ss Ratio
Sensitive to New Physics
Likelihood
Function
Neural Network
29 April 2008
PHENO 2008

6. Matrix Element Neural Network Likelihood Function
Boosted Decision Tree
Matrix element:
Different for each process.
extension of a cut based analysis
≥ 20 variables used for training
Optimize series
of binary cuts
Parton
distribution
functions
Transfer functions:
jet energy measurement
Calculate for each leaf purity as BDT output …
Neural Network
Discriminant
Likelihood Function
i=variable index
k=sample index
29 April 2008
PHENO 2008
Use variables

7. Systematic Uncertainties
Dominant Systematic uncertainty
is the normalization of data based bkg estimation.
All rate and shape variation
systematic uncertainty
are included cross section
and significance fit
29 April 2008
PHENO 2008

8. Cross section measurement
Result I
Neural Network
Likelihood Function
Matrix Element
Boosted Decision Tree
All results are consistent
with SM prediction
29 April 2008
PHENO 2008

9. Significance Result II Neural Network Likelihood Function
Matrix Element
Boosted Decision Tree
29 April 2008
PHENO 2008

10. Combination I Result III Super-Discrimintant
Combine ME, NN, LF analyses into one by using discriminants as inputs to a neural net.
“Neuro-
Evolution of
Augmenting
Topologies”
29 April 2008
PHENO 2008

11. Combination II (cross check)
Result IV
Asymmetric Iterative BLUE
Generate correlated PEs and measure correlations.
Parameterize uncertainties as function of single top s
Iterate to avoid biases.
LF-ME 58.9% ME-NN 60.8% LF-NN 74.1%
How correlated single analyses are :
“Best
Linear
Unbiased
Estimator”
Original BLUE :
L. Lyons NIM A (1988)
29 April 2008
29 April 2008
PHENO 2008
PHENO 2008 11

12. CKM matrix element |Vtb|
Result V
The cross section measurement is also converted directly into
a measurement of |Vtb| :
Assuming no anomalous coupling
Superdiscriminant:
|Vtb|=0.88 ± 0.14 (exp.) ± 0.07 (theory)
Result is consistent
with |Vtb|~1
(|Vtd|,|Vts| << |Vtb|)
29 April 2008
PHENO 2008

13. S- channel optimized search
Result VI
This is a first result of the optimization search
Used double tag to separate s- and t-ch
 use “Loose” second tag to increase statistics
Likelihood base signal background separator
t-ch was treated as a background.
Result :
Cross section Best Fit
95% CL. Upper limit
Exp. Limit : 2.36 pb
(bkg. Only)
Obs. Limit : 2.77 pb
Result is consistent with SM prediction
29 April 2008
PHENO 2008

14. Conclusion √∫Ldt dependence.
Precise measurement of cross section and |Vtb| :
|Vtb|=0.88 ± 0.14 (exp.) ± 0.07 (theory)
Significance extrapolation assuming
√∫Ldt dependence.
Extrapolation of Expected Error on|Vtb|.
29 April 2008
PHENO 2008

15. Thank you!
29 April 2008
PHENO 2008

16. Expected candidate events
29 April 2008
PHENO 2008

17. Acceptance Gain for Muon
29 April 2008
PHENO 2008

18. A Neural Network b-tagging tool
29 April 2008
PHENO 2008