1. Top quark angular distribution results (LHC)
Thorsten Wengler, CERN
On behalf of the ATLAS and CMS collaborations
TOP 2012
September 16-21, 2012
Winchester, UK

2. All results based on pp collisions at √s = 7 TeV collected by CMS and ATLAS in 2011
LHCb
ALICE
LHC ring:
27 km circumference
ATLAS

3. Why the top is such a celebrity
Because it lives fast and dies young
no time for hadronisation in 5x10-25 s, properties can be determined e.g. from angular distribution of decay products
Why the top is such a celebrity
Because it has exciting kids
High pT leptons, miss ET, high pT (b)jets, facilitate triggering, selection, systematic studies
Because it’s the Heavy-Weight champion
Probe of EWSB sector, direct tests of new physics in production (e.g. resonances) and decay (e.g. H+, FCNC) 3

4. What do we test here?
The angular distributions of the decay products contain a lot of information:
Top spins correlated?
SM says YES
W+/W- decay modes:
Single- or di-leptonic
channels used here
W fractional helicities?
SM says F0~0.7,
FL~0.3, FR~0
Wtb vertex?
Tops polarized?
SM says ~NO
Event selection / background treatment: follows common tt selection procedures 4

5. W helicity θ* W polarization states
longitudinal left-handed right-handed
SM (NNLO, ~% rel. uncert.)
F0 = FL = FR =
PRD 81 (2010) b W t l νl θ*
angle between the charged lepton in the W rest frame and W momentum in top rest frame.
F0 + FL + FR = 1

6. W helicity Template fit to observed cos θ* distribution 1.04 fb-1
Single- and di-lepton channels
3 signal templates F0 = 1, FL = 1 and FR = 1 plus background templates
Sensitive to shape related uncertainties
ISR/FSR, jet reconstruction
1.04 fb-1
JHEP 1206 (2012) 088
Angular asymmetries on unfolded cos θ* distribution
Single- and di-lepton channels
Sensitive to background normalisation

7. W helicity Likelihood fit to cos θ* distribution
CMS PAS TOP
Single-lepton (μ) channel
Data sample 2.2 fb-1
Templates built by event-by-event reweighting of generated cos θ* plus separate BKGS contributions
Two types of fits, ‘2D’ and ‘3D’
∫ L dt = 2.2 fb-1
‘3D’-Fit
Free parameters: F0, FL, Ftt
Constraint: FR = 1 – F0 – FL
‘2D’-Fit
Free parameters: F0, Ftt
Constraints: FR = 0, FL = 1 – F0
Ftt: normalisation factor of tt component in # expected events entering likelihood

8. W helicity ‘3D’-Fit (main result) ‘2D’-Fit (consistent)
F0 = ± 0.074(stat.) ± 0.047(syst.)
FL = ± 0.045(stat.) ± 0.029(syst.)
FR = ± (stat.) ± 0.044(syst.)
F0 = ± 0.034(stat.) ± 0.050(syst.)
FL = ± (stat.) ± 0.050(syst.)
FR set to 0
Combined result
F0 = 0.67 ± 0.03(stat.) ± 0.06 (syst.)
FL = 0.32 ± 0.02(stat.) ± 0.03 (syst.)
FR = 0.01 ± 0.01(stat.) ± 0.04 (syst.)
With FR = 0 (consistent)
F0 = 0.66 ± 0.03(stat.) ± 0.04 (syst.)
FL = 0.34 ± 0.03(stat.) ± 0.04 (syst.)

9. W helicity  anomalous couplings
Agreement of helicity fractions with SM
Constraints on new physics contributions to Wtb vertex
=Vtb=1
= 0 in SM
Nucl. Phys. B 812 (2009) 181
Nucl. Phys. B 821 (2009) 215
∫ L dt = 2.2 fb-1
(95% CL)
vary
only gR
Phys. Rev. D 83 (2011)
ATLAS: for FR=0

10. Di-lepton channel is most promising
tt Spin correlations θ*
Phys. Lett. B539, 235 (2002)
Amount of spin information carried by daughter particle
1S0
tt spins correlated at production
Di-lepton channel is most promising
Measured value of A depends on quantisation axis
Example above: tt direction in ttCM frame helicity basis

11. tt Spin correlations SM e.g. H+, b’ e.g. KK gravitons, Z’
The SM spin correlation may be modified by new physics
e.g. H+, b’
e.g. KK gravitons, Z’

12. tt Spin correlations θ*
Mahlon, Parke, PRD D81 (2010)
NLOW: Bernreuther, Si, Nucl. Phys. B837, 90 (2010) θ*
Determination requires full event reconstruction
Difficult in particular for di-lepton channel  under-constrained kinematics
due to 2 neutrinos
Mahlon, Parke (2010): use azimuthal angle Δϕ between charged leptons in lab frame
 Method: Fit templates of correlated and uncorrelated sample (+BKGS template) to data

13. tt Spin correlation Theor. Uncert. ~ 1% CMS PAS TOP-12-004
PRL108 (2012)
CMS PAS TOP
Theor. Uncert. ~ 1%
Nucl. Phys. B 837, 90 (2010)

14. tt Spin correlation
CMS also provides unfolded cos(θl+,n) cos(θl-,n) and Δϕl+l- distributions
 Direct comparisons to parton-level theory
Requires full event reconstruction!
Nuc. Phys. B 837 (2010) 90
Good agreement with SM
[SVD/RooUnfold]

15. tt Spin correlation First observation of top spin correlations
Results by channel
5.1 σ against zero spin correlations
First observation of top spin correlations
Asymmetries in addition to template fits:
 Needs full event reconstruction
Good agreement with SM,
also at Mtt > 450 GeV

16. Top polarization Now just looking at one top at a time … θ
Spin analysing power of the lepton ~1 as above
Top polarisation along axis θ
Angle between lepton direction in top rest frame and top direction in
ttCM frame [helicity basis]
Full event reconstruction required
For di-lepton channel can cross check one side against the other

17. Top Polarization θl+ θl- SM: P = 0
CMS PAS TOP
di-lepton channel
Event reconstruction with analytical matrix weighting technique
θl+
Pn at parton-level:
−0.009 ± 0.029(stat) ± 0.041(syst)
Unfolded cos(θl+) distribution
Compatible with
SM prediction
SM: P = 0
Unfolding with CVD/RooUnfold
Data: ± 0.012
MC: ± 0.002
Reconstruction level cos(θl+) and cos(θl-)
θl-
Data: ± 0.012
MC: ± 0.002

18. Top Polarization single-lepton [e,μ] channel
ATLAS-CONF
single-lepton [e,μ] channel
Kinematic fit for event reconstruction
 permutation with highest event probability
e channel
Template fit using αl p = +/- 1 obtained by reweighting the cos(θl)
μ channel
with κl P = αl p = 2f -1
(CMS)
(ATLAS)
SM: f = 0.5

19. Summary W helicity fractions CMS tt spin correlations CMSSM
CMS
0.24 ±0.02 ±0.08
CMS
W helicity fractions
tt spin correlations
ATLAS+CMS combination
under way
ATLAS result
superimposed
Pn= −0.009 ± ± 0.041
Limits on anomalous couplings
SM: P=0
top polarization

20. Additional Slides

21. W helicity
reconstructed W
reconstructed top
Data/MC comparisons

22. W helicity
Systematic uncertainties
Selected events data vs MC

23. W helicity
Data/MC comparisons for Single- and Di-lepton channels

24. W helicity
Selected events data vs MC and systematic uncertainties on the combined helicity fractions

25. W helicity Unfolded cos θ* distributions
Summary of measured helicity fractions

26. Spin correlations Reconstructed Δϕ Data/MC comparison per channel
combined

27. Spin correlations
Reconstructed Δϕ
Fit results
per channel
combined

28. Spin correlations Event yields in data and simulation
Summary of uncertainties on the measured value of fSM

29. Spin correlations
Reconstruction level asymmetry distributions

30. Spin correlations
Uncertainty on the fraction of events with spin correlation Δƒ as predicted by the fit.
Uncertainty on the unfolded values of AΔΦ and Ac1c2.

31. Top Polarization Data/MC Comparison for sample of b-tagged jets
Reconstruction level distributions for cos(θl+) and cos(θl-)

32. Top Polarization
Unfolding [SVD/RooUnfold] for acceptance and resolution effects
Acceptance matrix
Smearing matrix
Measured distribution
after background subtraction
Parton level distribution

33. Top Polarization Observed and simulated data yields
Systematic uncertainties

34. Top Polarization Systematic uncertainty on ƒ
Single channel and single charge results for f

35. Top Polarization Electron channel Muon channel
Data/MC comparisons of distributions and number of selected events

36. Top Polarization Electron channel Muon channel
Data/MC comparisons of distributions of the reconstructed neutrino

37. Top Polarization Electron channel Muon channel
Data/MC comparisons of reconstructed cos (θl) distributions