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Top quark angular distribution results (LHC)

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

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Top quark angular distribution results (LHC)

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

  3. Why the top? • 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 Tops polarized? SM says ~NO Wtb vertex? Event selection / background treatment: follows common tt selection procedures 4

  5. W helicity longitudinal left-handed right-handed W polarization states SM (NNLO, ~% rel. uncert.) F0 = 0.687 FL = 0.311 FR = 0.0017 PRD 81 (2010) 111503 F0 + FL + FR = 1 l θ* b angle between the charged lepton in the W rest frame and W momentum in top rest frame. W t νl

  6. W helicity Template fit to observed cos θ* distribution • 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-11-020 • 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 = 0.567 ± 0.074(stat.) ± 0.047(syst.) FL = 0.393 ± 0.045(stat.) ± 0.029(syst.) FR = 0.040 ± 0.035 (stat.) ± 0.044(syst.) F0 = 0.643 ± 0.034(stat.) ± 0.050(syst.) FL = 0.357 ± 0.034 (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 Nucl. Phys. B 812 (2009) 181 Nucl. Phys. B 821 (2009) 215 =Vtb=1 = 0 in SM ∫ L dt = 2.2 fb-1 (95% CL) vary only gR ATLAS: for FR=0 Phys. Rev. D 83 (2011) 034006

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

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

  12. tt Spin correlations NLOW: Bernreuther, Si, Nucl. Phys. B837, 90 (2010) θ* Mahlon, Parke, PRD D81 (2010) 074024 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 CMS PAS TOP-12-004 PRL108 (2012) 212001 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 Asymmetries in addition to template fits: Results by channel  Needs full event reconstruction 5.1 σ against zero spin correlations First observation of top spin correlations Good agreement with SM, also at Mtt > 450 GeV

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

  17. Top Polarization di-lepton channel CMS PAS TOP-12-016 Event reconstruction with analytical matrix weighting technique θl+ Unfolding with CVD/RooUnfold Data: 0.040 ± 0.012 MC: 0.049 ± 0.002 Reconstruction level cos(θl+) and cos(θl-) θl- Compatible with SM prediction Unfolded cos(θl+) distribution Data: 0.048 ± 0.012 MC: 0.046 ± 0.002 Pnat parton-level: −0.009 ± 0.029(stat) ± 0.041(syst) SM: P = 0

  18. Top Polarization single-lepton [e,μ] channel ATLAS-CONF-2012-133 Kinematic fit for event reconstruction  permutation with highest event probability e channel Template fit using αl p = +/- 1 obtained by reweighting the cos(θl) (CMS) (ATLAS) with κl P = αl p = 2f -1 SM: f = 0.5 μ channel

  19. Summary CMS SM W helicity fractions tt spin correlations ATLAS result superimposed ATLAS+CMS combination under way CMS 0.24 ±0.02 ±0.08 Limits on anomalous couplings Pn= −0.009 ± 0.029 ± 0.041 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 Muon channel Electron 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

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