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

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

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

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

W helicity θ* W polarization states longitudinal left-handed right-handed SM (NNLO, ~% rel. uncert.) F0 = 0.687 FL = 0.311 FR = 0.0017 PRD 81 (2010) 111503 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

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

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

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.)

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) 034006 ATLAS: for FR=0

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

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’

tt Spin correlations θ* Mahlon, Parke, PRD D81 (2010) 074024 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

tt Spin correlation Theor. Uncert. ~ 1% CMS PAS TOP-12-004 PRL108 (2012) 212001 CMS PAS TOP-12-004 Theor. Uncert. ~ 1% Nucl. Phys. B 837, 90 (2010)

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]

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

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

Top Polarization θl+ θl- SM: P = 0 CMS PAS TOP-12-016 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: 0.040 ± 0.012 MC: 0.049 ± 0.002 Reconstruction level cos(θl+) and cos(θl-) θl- Data: 0.048 ± 0.012 MC: 0.046 ± 0.002

Top Polarization single-lepton [e,μ] channel ATLAS-CONF-2012-133 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

Summary W helicity fractions CMS tt spin correlations CMS SM 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 ± 0.029 ± 0.041 Limits on anomalous couplings SM: P=0 top polarization

Additional Slides

W helicity reconstructed W reconstructed top Data/MC comparisons

W helicity Systematic uncertainties Selected events data vs MC

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

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

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

Spin correlations Reconstructed Δϕ Data/MC comparison per channel combined

Spin correlations Reconstructed Δϕ Fit results per channel combined

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

Spin correlations Reconstruction level asymmetry distributions

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.

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

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

Top Polarization Observed and simulated data yields Systematic uncertainties

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

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

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

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