DO Timeline

1
Measurements of the W Helicity in Top Quark Decays
Kenneth Johns University of Arizona for the DØ
and CDF Collaborations
2
W Helicity

• The heavy mass of the top quark makes it a prime
  target for searches of physics beyond the
  Standard Model
• Measurement of the W helicity is a measurement of
  the tbW vertex
• Top quark lifetime lt hadronization time
• V-A weak interaction determines the top quark
  decay in SM

b
b
3
W Helicity

• In the mb0 limit,
• Finite mb and O(as) corrections change the above
  values by lt 2
• We look for new physics by searching
• for F0 ? 0.7 (assuming F0)
• for F gt 0 (assuming F00.7 )
• F is indirectly constrained to a few percent by
  b?s? data (e.g. FujikawaYamada, PRD 49 (1994)
  5890)

4
W Helicity

• The angular decay distribution for unpolarized
  top
• w(cos?) 3/8(1cos?)2F 3/8(1-cos?)2F-
  3/4(sin2?)F0

5
W Helicity

• The angular factors are also reflected in the
  shape of the lepton PT distribution
• The lepton PT spectrum for F will be harder than
  that for F0
• The lepton PT spectrum for F- will be softer than
  that for F0

6
W Helicity Measurements
Expt Published? Method
CDF Run I PRL 2000 106 pb-1 PTlepton
CDF Run I PRD 2005 109 pb-1 Mlb2
DØ Run I N 125 pb-1 Matrix Element
CDF Run II N 162 pb-1 PTlepton
CDF Run II N 162 pb-1 Mlb2
DØ Run II N 163 pb-1 cos(?)
7
W Helicity Measurements
Expt Method Samples Notes
CDF Run I PTlepton LJ (w/wo b-tag) LL (eµ) SVT, SLT
CDF Run I Mlb2 LJ (w b-tags) LL (eµ) SVT
DØ Run I ME LJ 4 jets only
CDF Run II PTlepton LJ w b-tags LL SVT 3, 4 jets
CDF Run II Mlb2 LJ w b-tag SVT
DØ Run II cos(?) LJ w/wo b-tag SVT
SVT Secondary Vertex Tag SLT Soft Lepton Tag
8
Matrix Element Method

• ME method offers the possibility of increased
  statistical precision by using all measured
  quantities in an event
• Write the probability density
• Include background
• Form a likelihood

9
Matrix Element Details

• Mttbar
• qqbar only (no gg)
• 4 jets only (no NLO)
• Mbkg
• Wjets only
• Selection cut on Pbkg used to reduce background
• Ensemble tests are used to estimate bias

10
Matrix Element Results

• Assuming mt 175 GeV, F0 0.60 0.30 (stat)
• Uncertainty in mt is accounted for by integrating
  L(F0,mt) over mt
• Including the remaining systematic errors gives
• F0 0.56 0.31 (statmt) 0.07 (sys)

mt
F0
11
PTlepton Method

• PTlepton is sensitive to the W helicity
• Charged leptons tend to be emitted opposite to WL
  direction
• Charged leptons tend to be emitted transverse to
  W0 direction
• F 0 (hence measure F0)
• Select LJ b-tag and LL events
• Determine backgrounds ala cross section analyses
• Construct PTlepton PDFs for signal and
  background
• Construct unbinned
• Including bias correction
• Estimate systematic uncertainties using ensemble
  testing
• Method of Feldman-Cousins is used to make a
  coherent statement about the true F0 given an
  estimated F0

12
PTlepton Details

• Signal and background composition

LJ
LL
13
PTlepton Combined Results
14
PTlepton LL Results
15
Mlb2 Method

• This method exploits the approximation
• A kinematic ?2 is used to match a reconstructed
  jet with the b parton
• Top-specific corrections derived from Monte Carlo
  are used to convert jet energies into parton
  energies
• F0 is extracted using a binned maximum likelihood
  fit
• Again, the results are interpreted using
  Feldman-Cousins confidence belts

16
Mlb2 Details

• Systematic errors for the LJ data (CDF)

Source ?F0
Background shape 0.12
Top mass uncertainty 0.09
Jet energy scale 0.06
PDF uncertainty 0.04
MC modeling 0.03
ISR/FSR 0.02
SVT b-tagging 0.01
MC statistics 0.01
Total 0.17
17
Mlb2 Results
18
Mlb2 Run I Results

• Similar to the Run II analysis
• b-tagged jets are chosen to form Mlb2
• Neyman construction for upper limit

19
Cos(?) Method

• Use topological likelihood to determine signal
  and background contributions
• Use kinematic fit (assuming mt175 GeV) to select
  b-jet associated with leptonically decaying W
• Selects correct b-jet 57 of the time
• Produce cos(?) templates using Monte Carlo
• Perform binned likelihood fit to data
• Use Bayesian approach to set a confidence
  interval
• Use ensemble tests for systematic errors

20
Cos(?) Details

• Cos(?) for ttbar signal (b-tag, ejets channel)

F-0.3 F0.3
21
Cos(?) Results

• Topological analysis (no explicit b-tag)

22
Cos(?) Results

• b-tag analysis

23
Measurement Summary
Expt Method Result
CDF Run I PTlepton 106 pb-1 F0 0.91 0.37 0.13 F lt 0.28 (95CL)
CDF Run I Mlb2 109 pb-1 F lt 0.24 (95CL)
DØ Run I ME 125 pb-1 F0 0.56 0.31
CDF Run II PTlepton 162 pb-1 F0 lt 0.88 (95 CL) F0 0.27 0.31 -0.21
CDF Run II Mlb2 162 pb-1 F0 gt 0.25 (95 CL) F0 0.89 0.32 0.17
DØ Run II cos(?) 163 pb-1 F lt 0.24 (90CL) F lt 0.24 (90CL)
24
Conclusions

• Good effort in measuring the W helicity in top
  decay
• Variety of methods, variety of data samples
• All measurements are consistent with the SM
• CDF PTlepton spectrum in the LL sample is
  interesting
• Presently statistical errors are x2 systematic
  errors
• Very useful to combine results from DØ and CDF
• Dominant systematic errors arise from
  uncertainties in top quark mass, backgrounds, and
  jet energy scale
• Look forward to exploiting the full statistical
  power of Run II data
• Look forward to exploiting the top quark factory
  at the LHC

25
W Helicity

• Top decays

26
Matrix Element Details

• Systematic errors

Source s(F0)
Acceptance 0.05
Jet energy scale 0.01
Spin correlations 0.01
PDF 0.01
Signal model 0.02
Multiple interactions 0.006
QCD background 0.02
Subtotal 0.07
Statistical mass 0.31
Total 0.314
27
PTlepton Method
28
PTlepton LJ Results
29
PTlepton Details

• Systematic errors

Source ssys (LJLL)
Background normalization 0.10
Top mass uncertainty 0.11
ISR/FSR 0.05
PDF uncertainty 0.03
PTlepton shape uncertainty 0.02
Monte Carlo statistics 0.01
Acceptance correction 0.02
Trigger correction 0.02
Total 0.17
30
Feldman-Cousins

• The result of the maximum likelihood fit for F0
  can be outside the physical region
• The procedure of Feldman-Cousins can be used to
  construct a confidence interval in the physical
  region
• Ensemble tests are used to map true F0s to a
  distribution of estimated F0s using the
  Feldman-Cousins ordering principle
• Systematic errors can be included by adding in
  quadrature s(F0est) and s(sys)
• The resulting 2D figure then gives the confidence
  interval on true F0 for a measured (estimated) F0

31
Mlb2 Details

• Systematic errors

Source ?F0
Bkg shape 0.12
Top mass uncertainty 0.09
Jet energy scale 0.06
PDF uncertainty 0.04
MC modeling 0.03
ISR/FSR 0.02
SVT b-tagging 0.01
MC statistics 0.01
Total 0.17
32
Mlb2 Details

• Backgrounds
• 31 events observed

Background Total
QCD 3.41.0
Wjets (mistags) 2.80.6
Wbb 1.60.7
Wcc 0.60.3
Wc 0.70.3
WW/WZ 0.290.05
Single top 0.490.07
Total 9.91.7
Total ?2 acceptance 6.41.1
33
Cos(?) Details

• Systematic errors (topological)

• Systematic errors (b-tag)

Source s (F)
Top mass 0.06
Jet energy scale 0.06
Likelihood fit 0.02
Background model 0.01
Underlying event 0.06
MC statistics 0.01
Total 0.11
Source s (F)
Top mass 0.11
Jet energy scale 0.04
Background model 0.08
Signal model 0.05
Total 0.15
34
Cos(?) Details

• Cos(?) for tt signal (b-tag, ejets channel)

35
Cos (?) Details

• Signal and background are determined using a
  topological likelihood

b-tag
Channel tt Wjets QCD
µjets 9.6 2.7 2.0 1.4 0.7 0.4
ejets 14.2 3.4 6.6 1.8 0.6 0.3
topological
Channel tt Wjets QCD
µjets 11.3 1.3 17.6 1.2 2.1 0.5
ejets 25.9 1.5 20.3 1.5 2.7 0.5
b-tag (µjets)
36
Bayesian Limit

• DØ uses a Bayesian technique to set a confidence
  interval in the physical region of F
• Let xML be the result of the maximum likelihood
  fit
• If xML is outside the physical range (or close to
  the physical