1. W Charge Asymmetry at CDF
Kevin McFarland
University of Rochester
on behalf of CDF collaboration
Ph.D. thesis of Bo-Young Han
DIS 2008, UC-London, 8 April 2008

2. K. McFarland, W Charge Asymmetry
Outline
Introduction and Analysis Technique
Signal, Backgrounds and Corrections
Uncertainties
Preliminary results
8 April 2008
K. McFarland, W Charge Asymmetry

3. W Charge Asymmetry and PDFs
At Tevatron, W± are produced primarily by uproton (danti-proton) and uanti-proton (dproton)
W charge Asymmetry u d
u quarks carry more momentum than d quarks
8 April 2008
K. McFarland, W Charge Asymmetry

4. Lepton Charge Asymmetry
CDFII, 170 pb-1
Previous work from TeVatron has measured the charge asymmetry vs. lepton η in W→ℓν
This observable is a convolution of the W charge asymmetry and V-A W decay angular distribution
Lepton charge asymmetry
W charge asymmetry
8 April 2008
K. McFarland, W Charge Asymmetry

5. K. McFarland, W Charge Asymmetry
Lepton and W Rapidity
Lepton prefers to decay against boost
8 April 2008
K. McFarland, W Charge Asymmetry

6. K. McFarland, W Charge Asymmetry
Analysis Technique
Measure W± rapidity
Use W mass constraint
solve eqn answer :
Weight the two solutions
This method must be iterated to remove input bias
shown it does not depend on assumed charge asymmetry
can’t measure !!!
Probability of angular distribution
Differential W cross section
8 April 2008
K. McFarland, W Charge Asymmetry

7. Complications: Detector Effects
true rapidity
weighting factor
response matrix Y1 Y2
wt1
wt2 YW YW
FSR
depends on physics input →should be iterated
doesn’t depends on physics input →do not need to be iterated
Response matrix to correct for detector acceptance, smearing and final state effects
QED, W→τν→eννν
8 April 2008
K. McFarland, W Charge Asymmetry

8. Complications: W Production from the sea
Sign of V-A angular bias flips when W± is produced from anti-quarks
take this fraction as an input
ratio of two angular distributions at each rapidity
8 April 2008
K. McFarland, W Charge Asymmetry

9. What is input, and what is measured?
Inputs from theory:
and
Output from iteration:
method documented in B.Y.Han et al., arXiv:hep-ph/
8 April 2008
K. McFarland, W Charge Asymmetry

10. K. McFarland, W Charge Asymmetry
Outline
Introduction and Analysis Technique
Signal, Backgrounds and Corrections
Uncertainties
Preliminary results
8 April 2008
K. McFarland, W Charge Asymmetry

11. K. McFarland, W Charge Asymmetry
Summary of Data Issues
Require one high ET electron and missing ET to tag the neutrino
ETe>25(20) GeV in central (plug) detector
missing ET>25 GeV
Detector response, including missing ET, tuned on Z→e+e- sample
Highlight backgrounds (primarily jets faking electrons) and charge mismeasurement
also, trigger and detector acceptance and smearing
8 April 2008
K. McFarland, W Charge Asymmetry

12. Data and Simulation Kinematic Distributions
Red : MC
Blue : data
Scale and resolution tuned on Z→e+e-
8 April 2008
K. McFarland, W Charge Asymmetry

13. K. McFarland, W Charge Asymmetry
Backgrounds
Measure jet backgrounds directly in data
use extra energy “isolation” around electron to separate, and fit shape to background fraction
Illustrative fit (one of many) shown at right
use jet sample to predict measured “yW” and charge from this sample
uncertainty is ~0.15% of total sample
A number of minor backgrounds (real Ws) from simulation
electron
8 April 2008
K. McFarland, W Charge Asymmetry

14. Electron Charge Identification
Charge identification is crucial for this measurement
Forward tracking has fewer points at shorter lever arm
Determine directly from (background subtracted)
8 April 2008
K. McFarland, W Charge Asymmetry

15. K. McFarland, W Charge Asymmetry
Outline
Introduction and Analysis Technique
Signal, Backgrounds and Corrections
Uncertainties
Preliminary results
8 April 2008
K. McFarland, W Charge Asymmetry

16. K. McFarland, W Charge Asymmetry
Systematic Effects
Detector response
energy scale and smearing for electron and recoil
efficiency to find electron and pass missing ET cut
Inputs
PDF uncertainties (CTEQ6 PDF error sets) for total W production and quark/anti-quark fractions
8 April 2008
K. McFarland, W Charge Asymmetry

17. K. McFarland, W Charge Asymmetry
Evaluation
Derive result with shifted parameters
Systematics dominate only if yW<0.2 or >2.6
largest detector effect: energy scale
largest PDF effect: qbar/q
largest selection effect: elec. cuts
charge fake rate uncertainty
8 April 2008
K. McFarland, W Charge Asymmetry

18. K. McFarland, W Charge Asymmetry
Outline
Introduction and Analysis Technique
Signal, Backgrounds and Corrections
Uncertainties
Preliminary results
8 April 2008
K. McFarland, W Charge Asymmetry

19. K. McFarland, W Charge Asymmetry
Uncertainties
Unfolding correlates (strongly) the statistical errors in nearby bins
Systematics also strongly correlated…
PDF fitters, please exercise caution…
8 April 2008
K. McFarland, W Charge Asymmetry

20. K. McFarland, W Charge Asymmetry
Input PDFs
We are in the process of documenting how result depends on input PDFs
PDF fitters can explicitly put this in (or ignore if small)
effect of increasing valence u+d by +5%
effect of increasing sea u+d by +5%
8 April 2008
K. McFarland, W Charge Asymmetry

21. CDF Preliminary Result (1fb-1)
Positive and negative yW agree, so fold
Compare to NNLO (MRST)
NLO error PDFs (CTEQ)
8 April 2008
K. McFarland, W Charge Asymmetry

22. CDF Preliminary Result (1fb-1)
Compare CTEQ6M and MRST2006 with PDFs and their uncertainties
8 April 2008
K. McFarland, W Charge Asymmetry

23. K. McFarland, W Charge Asymmetry
Conclusions
First direct measurement of W charge asymmetry
despite additional complication of multiple solutions, it works!
appears that it will have impact on d/u of proton
Looking forward to working with PDF fitting groups to incorporate
8 April 2008
K. McFarland, W Charge Asymmetry

24. K. McFarland, W Charge Asymmetry
Outline
Introduction and Analysis Technique
Signal, Backgrounds and Corrections
Uncertainties
Preliminary results
Backup
8 April 2008
K. McFarland, W Charge Asymmetry

25. K. McFarland, W Charge Asymmetry
Results
8 April 2008
K. McFarland, W Charge Asymmetry

26. K. McFarland, W Charge Asymmetry
Systematics
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K. McFarland, W Charge Asymmetry

27. K. McFarland, W Charge Asymmetry
Acceptance
8 April 2008
K. McFarland, W Charge Asymmetry

28. Plug Electron Tracking Efficiency
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K. McFarland, W Charge Asymmetry

29. QCD higher order corrections
Our weight factor is initialized by one prediction.
The parameterization of angular distribution :
The ratio of anti-quark and quark in proton : Q(yW,PWT) using
Differential cross-sections : dσ±/dyW
KNNLO = (dσNNLO/dyW) / (dσLO/dyW)
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K. McFarland, W Charge Asymmetry

30. K. McFarland, W Charge Asymmetry
f(PT) and g(PT)
8 April 2008
K. McFarland, W Charge Asymmetry

31. NNLO K-factor: (dσNNLO/dyW)/(dσLO/dyW)
NNLO dσ±/dyW (mrst2002 PDFs) from theoretical prediction.
8 April 2008
K. McFarland, W Charge Asymmetry

32. K. McFarland, W Charge Asymmetry
NNLO K-factor
W charge asymmetry measurement using K-factor.
8 April 2008
K. McFarland, W Charge Asymmetry

33. K. McFarland, W Charge Asymmetry
Z PT at LO (Pythia)
check if LO (Pythia) is tuned by DATA.
Using Zee samples : zewk6d(Pythia) vs bhel0d(data)
Looks MC agrees with data.
8 April 2008
K. McFarland, W Charge Asymmetry

34. K. McFarland, W Charge Asymmetry
W Pt tune at
Comparison of Pt distribution between NLO(Herwig) and LO(Pythia)
Using W→eν samples
Obtain the re-weight factor from the ratio of both
We are going to re-weight the event of NLO.
8 April 2008
K. McFarland, W Charge Asymmetry

35. K. McFarland, W Charge Asymmetry
Q(yW,PWT) (cont.)
10 < PT and 10 < PT < 20
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K. McFarland, W Charge Asymmetry

36. K. McFarland, W Charge Asymmetry
Q(yW,PWT) (cont.)
20 < PT < 40 and 40 < PT < 60
8 April 2008
K. McFarland, W Charge Asymmetry

37. K. McFarland, W Charge Asymmetry
Q(yW,PWT) (cont.)
60 < PT < 80 and 80 < PT < 200
8 April 2008
K. McFarland, W Charge Asymmetry

38. K. McFarland, W Charge Asymmetry
Q factor + K-factor
Even if QCD higher order correction makes small effect on the asymmetry, we believe that it describes better differential cross-section and angular distribution.
8 April 2008
K. McFarland, W Charge Asymmetry

39. K. McFarland, W Charge Asymmetry
Q(yW,PWT) using
W charge asymmetry measurement using NLO Q factor
Compare it with previous result using LO Q factor
8 April 2008
K. McFarland, W Charge Asymmetry