1. ATLAS实验Single top产生截面测量
冯 存 峰

2. Single Top Physics Provides a direct probe of the Wtb coupling
Sensitive to many models of new physics (FCNC,W’, b’)
Determines the quark mixing matrix element Vtb without assumptions about the number of quark generations
Feynman diagrams of single-top quark production processes
t-channel (64.6pb)
Wt-channel (15.7pb)
s-channel (4.6pb)

3. ATLAS and LHC status
2011 LHC proton-proton collisions with √s=7TeV have finished
ATLAS detector has collected about 5 fb-1 pp data
High-quality physics results will based on the (full) 2011 data
New software release  17.0.X
Understanding of the performance with this release now
High pileup machine
<mu> was increased to ~15 after August technical stop

4. ATLAS Single top status
t-channel cross section measurement
s-channel cross section measurement
Wt-channel discovery
Search for new physics: FCNC, W’, b’…
Rel 16 papers:
Cut-based and MVA analysis!!
t-channel:
Cut-based: ATL-COM-PHYS (approved)
NN: ATL-COM-PHYS (approved)
BDT: ATL-COM-PHYS (conditional)
Wt di-lepton: ATL-COM-PHYS
W’ : ATL-COM-PHYS (conditional)
FCNC: ATL-COM-PHYS (approved)
Combination: ATL-COM-PHYS

5. Contribution T-channl lepton+jets analysis
Wt-channel dilepton analysis
Event generation and validation
Background estimation
Multivariable analysis
Systematic uncertainties estimation

6. Event generation and validation
Generate and validate the POWHEG generator for single top Wt-channel at parton level
POWHEG Wt cross section validation

7. Fake lepton background estimation
Measure the electron/muon real/fake rate for single-top group
Measure the real rate with Z->ll control samples
Measure the fake rate with QCD rich region with iteration method
Estimate the QCD background on t-channel analysis
Matrix method (2x2 matrix)
Estimate the QCD and W+jets background on Wt-channel
Matrix method (4x4 matrix, single fake and double fake)
t-channel fakes
Wt-channel fakes
(~1% level)

8. Multivariable analysis on t-channel
Uses 14 discriminating variables both on 2 and 3 jets bin
BDT response and overtraining check
BDT training parameters
ATL-COM-PHYS

9. Multivariable analysis on Wt-channel
Training on 1 jet bin
Backgrounds include ttbar, Diboson, Zee, Zmm, Ztautau (no fakes, 1% level)
Normalization factors for Zee and Zmm are applied prior to training
Uses 22 discriminating variables
BDT performance in 1-jet events

10. Systematic uncertainties estimation
Estimate the systematic uncertainties on Wt dilepton analysis for EPS conference and Wt discovery paper
Lepton scale factors uncertainties
PDF uncertainties
Pileup uncertainties
(ATLAS-CONF )

11. Look at Wt at parton level with MadGraph
Ttbar is the most difficult background to be rejected in Wt analysis
Have a fraction about 90% in Wt signal region
Large systematic uncertainties affect the Wt measurement
Look for powerful discriminating variables on parton level with MadGraph

12. Track Jet study for low pt Jet
Ttbar pass Wt dilepton event selection in 1 jet bin for a un-found low pt jet
Try to find the low pt b-jet with jets reconstructed from tracks
Ttbar background could be reduced in Wt dilepton channel
Signifience 1.2 -> 1.5 (based on EPS results)

13. Next year T channel measurement almost ready now.
More work needed for Wt channel, but should be ready within next year.
New physics topic need to define.