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ATLAS sensitivity to W polarization and top spin correlation in tt events

ATLAS sensitivity to W polarization and top spin correlation in tt events. TEV4LHC Workshop, CERN, April 28-30, 2005. F. Hubaut , E. Monnier, P. Pralavorio. CPPM/IN2P3 – Université de la Méditerranée Marseille, FRANCE hubaut@in2p3.fr. Introduction, Motivations

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ATLAS sensitivity to W polarization and top spin correlation in tt events

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  1. ATLAS sensitivity to W polarization and top spin correlation in tt events TEV4LHC Workshop, CERN, April 28-30, 2005 F. Hubaut, E. Monnier, P. Pralavorio CPPM/IN2P3 – Université de la Méditerranée Marseille, FRANCE hubaut@in2p3.fr • Introduction, Motivations • W polarization and sensitivity to tWb anomalous couplings • Spin correlation • Conclusions and perspectives W polarization and top spin correlation in ATLAS

  2. Top production and decay t+X 46% 20% m+jets e+jets e+e e+m m+m 14.6% 1.2% all hadronic 2.4% 1.2% 14.6% Strong Interaction tt Weak Interaction single top LHC σ~850 pb 10% qq, 90% gg Tevatron σ~7 pb 85% qq, 15% gg LHC σ~300 pb Tevatron σ~3 pb • BR (tWb) ~ 100 % in SM • tt final states (LHC, 1 year initial lumi.) • Full hadronic (3.7M): jets • Dileptonic (0.4M): ee, em, mm • Semileptonic(2.5M): e+jets,m+jets • ~ easy reconstruction • High statistics • High S/B W polarization and top spin correlation in ATLAS

  3. W polarization in top decay • Direct test of the V-A structure of the Wtb decay vertex • Search for anomalous couplings and new physics (e.g. V+A component) • Top decay: only significant source of longitudinal W bosons EWSB Production • Cross Section • Top Spin Polarization • Anomalous couplings • Resonance Production b p l+ W+ n t Decay • Branching Ratios • Anomalous couplings • Rare / non-SM Decays t b q W- q p Top • W polarization • Mass, Spin, Charge W polarization and top spin correlation in ATLAS

  4. SM expectations and actual meas./limits b W W b spin =1/2 t spin =1/2 t t t V-A W+ spin=1 W b b Top Standard Model weak decay V-A couplingas for all fermions Longitudinal W+ Fraction F0 Left-handed W+ Fraction FL Right-handed W+ Fraction FR NLO 0.695 0.304 0.001 W polarization and top spin correlation in ATLAS

  5. W helicity Observable  angle between lepton in W rest frame and W direction in top rest frame W polarization is measured through angular distribution of charged lepton: F0=0.703 FL=0.297 FR=0.000 W polarization and top spin correlation in ATLAS

  6. Semileptonic tt event simulation Event simulation is performed using: • TopReX 4.05 or AcerMC 2.2 or AlpGen 1.33 : LO density matrix for production and decay of tt pairs including spin effects • Pythia 6.2 or Herwig 6.5 : hadronisation, fragmentation and decay • Tauola + Photos : tlepton decay and radiative corrections • ATLAS fast simulation + reconstruction • Default: Mtop=175 GeV, CTEQ5L structure function, ISR-FSR • b-tagging parametrization (pT,η): εb=60%, Ruds=100, Rc=10 • Jet cone algorithm: size ΔR=0.4 1 year of statistics (10 fb-1, 3.8 Mevents) simulated for each generator and each hadronization scheme W polarization and top spin correlation in ATLAS

  7. Event selection and reconstruction • pT and h cuts • 1 lepton pT> 20GeV (|η|<2.5) + PTmiss> 20GeV + 1 b-jet pT> 30GeV (|η|<2.5) • 2 non b-jets pT> 30GeV (|η|<2.5) + + 1 b-jet pT> 30GeV (|η|<2.5)  double b-tag • Event topology reconstruction • Quality cuts • lMwREC- Mwl < 20GeV and lMtREC- Mtl < 35 GeV l=e,μ LEPT. HAD. ε(sig)=4.5%, 115000 events per 10 fb-1 ~ easy event reconstruction, high statistics W polarization and top spin correlation in ATLAS

  8. Background Normalization and shape under control 1 LHC year of statistics (10 fb-1) simulated for each background, except * Poisson stat. rescaled by 63 * Stat. rescaled by 8 Non tt: 60 < S/B < 100 tt  +X: S/B=13 S/B~12; main background tt   + X W polarization and top spin correlation in ATLAS

  9. Measurement method (1) • Selection cuts and reconstruction distord the parton level distribution • use MC generator on an independent sample to parametrize these effects parton level, no ISR/FSR Correction function : Fit with 3rd order polynom in range [-0.9;0.9] Reconstruction and cuts apply weights event by event Use this unique weight parametrization everywhere in the following W polarization and top spin correlation in ATLAS

  10. Measurement method (2) TopReX, selection+ recons. + correction F0=0.699 ± 0.005 FL=0.299 ± 0.003 FR=0.002 ± 0.003 S+B, 10 fb-1 Fit results with 2 parameters (constraint F0+FL+FR=1) Compatible with parton level Robustness of the method assessed by varying number of bins, fit limits, correction polynom order, … W polarization and top spin correlation in ATLAS

  11. Systematic uncertainties MC level Reconstruction level • Parton generation • Hadronization scheme • Structure function • ISR, FSR • b-fragmentation • Input top mass • b-jet mis-calibration • Light jet mis-calibration • b-tagging efficiency (pT,η) Others • Pile-up • Background normalization Tevatron data can help to decrease these uncertainties W polarization and top spin correlation in ATLAS

  12. Systematics: Monte Carlo Generators • Generate 3 LHC year statistics in semileptonic channel with : • different Monte-Carlo generators • Pythia for hadronisation, fragmentation and decays (+Tauola, Photos) after selection + reconstruction + correction • Very good agreement TopRex-AlpGen • AcerMC harder pT spectrum induces a significant (5 σstat) variation W polarization and top spin correlation in ATLAS

  13. Systematics: Hadronization scheme • Generate 3 LHC year statistics in semileptonic channel with : • ACERMC 2.2 to generate partons • Pythia or Herwig for hadronization after selection + reconstruction + correction • Significant (5 σstat) impact of hadronization scheme W polarization and top spin correlation in ATLAS

  14. Systematics: b-jet calib. and top mass b-jet miscalibration expected behaviour : positive miscalibration  Mlb  cos   FR FL Top mass uncertainty expected behaviour W polarization and top spin correlation in ATLAS

  15. Systematics: summary TopReX + PYTHIA Alpgen + PYTHIA AcerMC + PYTHIA AcerMC + HERWIG CTEQ6L MRST 02 GRV 98 ISR FSR b-frag. εb=-0.0035 b-tag. efficiency 55% 65% b-jet. miscalib. -3% +3% light-jet. miscalib. -1% +1% Top mass 173 GeV 177 GeV 2.3 pile-up events Dominant contributions W polarization and top spin correlation in ATLAS

  16. Results W polarization with semileptonic tt events for S+B at 10fb-1 (±stat ±syst) • In 1 LHC year (10 fb-1), ATLAS can measure F0 with an accuracy ~3% and FR with a precision ~1.3% • Measurements largely dominated by systematic uncertainties • Tevatron expectations with 2 fb-1: δF0stat~0.09and δFRstat~0.03 W polarization and top spin correlation in ATLAS

  17. Sensitivity to tWb anomalous couplings (1) One of the main motivation for top physics:search for anomalous interactions • BR (tWb) ~ 100 % in SM • Many models beyond SM study the tWb vertex in a model independent approach, i.e. effective Lagrangian ) and 4 couplings (in SM LO using Determine sensitivity to W polarization measurement (insensitive to ) W polarization and top spin correlation in ATLAS

  18. Sensitivity to tWb anomalous couplings (2) ±1s ±1s ±1s Assume a variation of each coupling independently: Uncertainties on FR and F0 with 10 fb-1 Best sensitivity to (linear behavior) W polarization and top spin correlation in ATLAS

  19. tt spin correlation • Sensitivity to new physics :anomalous gtt interaction (chromomagnetic or/and chromoelectric dipole moment), new production mechanisms, discrete symmetry tests (CP), ... Production • Cross Section • Top Spin Polarization • Anomalous couplings • Resonance Production b p l+ W+ n t Decay • Branching Ratios • Anomalous couplings • Rare / non-SM Decays t b q W- q p Top quantum numbers • W polarization • Mass, Spin, Charge W polarization and top spin correlation in ATLAS

  20. Top spin • Top decays before hadronisation (t~3x10-25 s) due to high mass: «bare» quark • Top production and decay : perturbative QCD and NLO computation • No spin flip between production and decay … • … direct transmition to decay products • In top rest frame, polarisation effects (S) observed by measuring angular distributions of daughter particles: • qiangle between decay particle of the top and top spin quantization axis s • aidegree to which its direction is correlated with the top spin (spin analyzing power) * lej = least energetic jet in top rest frame W polarization and top spin correlation in ATLAS

  21. tt spin correlation A=0.33 A=-0.35 (in helicity basis) tt pairs not polarised (< 1% at NLO) but … LHC Correlations between spins of t and t s (a.u.) Tevatron Mass of tt system, Mtt (GeV) Applying a cut on Mtt (e.g. < 550 GeV) increases the asymmetry at LHC and reduces systematics coming from high pT tails W polarization and top spin correlation in ATLAS

  22. tt spin correlation observables Measurement of Angular distribution of decay particles tt spin correlation (i.e. ‘spin analyzer’) on each side (t, t) Tevatron • An optimal basis (i.e. A(qq)=1 at LO) exists = « off-diagonal » basis: A~0.8 NLO • First measurement by D0 (run I, 125 pb-1) : A>-0.25 at 68% C.L. LHC • No optimal basis, asymmetry in helicity basis A=0.33 • Smaller QCD corrections (a few %), theoretical uncertainties under control angle bwnspin analyzersdirection in the t(t) rest frame W polarization and top spin correlation in ATLAS

  23. tt spin correlation measurement • Event selection and reconstruction exactly the same as for W polarization • except, add cut on tt invariant mass to enhance spin correlation: Mtt< 550 GeV • Measure C and D with simple unbiased estimators • Selection cuts distord the parton level distribution • Unique correction function, apply weights event by event • Redo the complete study of systematic uncertainties (e =75%) C = -9 < cosq1cos q2 > D = -3 < cosΦ > W polarization and top spin correlation in ATLAS

  24. Systematics: summary TopReX + PYTHIA Alpgen + PYTHIA AcerMC + PYTHIA AcerMC + HERWIG CTEQ6L MRST 02 GRV 98 ISR FSR b-frag. εb=-0.0035 b-tag. efficiency 55% 65% b-jet. miscalib. -3% +3% light-jet. miscalib. -1% +1% Top mass 173 GeV 177 GeV Signal / Bcknd -10% +10% 2.3 pile-up events Dominant contributions W polarization and top spin correlation in ATLAS

  25. Results Spin correlation with semileptonic tt events for S+B at 10fb-1 (±stat ±syst) (Mttcut included) • In 1 LHC year (10 fb-1), ATLAS can measure spin correlation ~15% • D observable can be measured more precisely than C • Measurements largely dominated by systematic uncertainties • Tevatron expectations with 2 fb-1: δCstat/C~40% • Need input from theoreticians to derive sensitivity to anomalous gtt couplings W polarization and top spin correlation in ATLAS

  26. Conclusions - Perspectives • LHC will produce a high top statistics during the first year(10 fb-1) precise measurements with tt semilep. events(clean signature, high stat, high S/B) • Measurements dominated by systematics, complete study performed • In 1 LHC year (10 fb-1), ATLAS can measure W polarization in top decay precisely ~1-3% and spin correlation ~15% (> 5s sensitivityonSM) • ~2-4 times better than Tevatron statistical expectations with 2 fb-1 • Search for anomalous couplings in a model independent approach • 2 ATLAS notes written:ATL-PHYS-PUB-2005-001andATL-COM-PHYS-2005-015 • Combine with dileptonic tt results (analysis done by Prague) publication W polarization and top spin correlation in ATLAS

  27. SPARE SLIDES W polarization and top spin correlation in ATLAS

  28. Spare: Monte Carlo Generators W polarization and top spin correlation in ATLAS

  29. Spare : pT comparison between MC TopReX AcerMC AlpGen Other jet lej Kinematical cuts (pT and h cuts) applied top lepton • TopRex~AlpGen • AcerMC harder pT (top) spectrum compared to TopReX, AlpGen W polarization and top spin correlation in ATLAS

  30. Spare: b-tagging b-jets Light jets • Complete revisiting of ATLAS potential with latest inner detector designs and refined simulations • Improvement of algorithms: weight jets by combining signed impact parameter (2D+3D) and secondary vertices reconstruction (mass, number of vertices, …) Light jet rejection vs b-tagging efficiency b and light jet 2D weight 2D 2D+1D 3D+SVX eb=60% R=65,90,150 W polarization and top spin correlation in ATLAS

  31. Spare : Event reconstruction • Jet calibration, b-tagging • Select the 2 non-b jets with Mjj closest to MW • Select the b jet with Mjjb closest to Mt • PTmiss for PTνand Pzν by constraining Mlν to Mw • Select the b jet closest to the lepton • Select the ν with Mlνb closest to Mt W polarization and top spin correlation in ATLAS

  32. Spare: W helicity Observable  W polarization is measured through angular distribution of charged lepton:  : angle between lepton in W rest frame and W direction in top rest frame Left-handed W+ Longitudinal W+ Right-handed W+ l+ (+1/2) l+ (+1/2) (-1/2) W W W W direction of flight (-1/2) (-1/2) l+ (+1/2) l+ opposite to W (~ π) l+ transverse to W (~ π/2) l+ same as W (~0) W polarization and top spin correlation in ATLAS

  33. Spare: Related observables • cos  have to reconstruct the whole event topology • In the Mb=0 approx. : • Mlb2 (lepton-b invariant mass): simpler • assume Mt=175 GeV in the formula • can even look at the lepton PT Tevatron studies LHC dilep. events • not necessary to reconstruct the whole event topology • high systematics due to jet energy scale and top mass We tested both observables for The first one gives a final total systematics ~2 times lower ATLAS semilep. events W polarization and top spin correlation in ATLAS

  34. Spare: Measurement of aW, AFB Information which can be derived from the W polarization: 1. Spin analyzing power of the W (aW) in the polarized top decay with qW angle between W and top spin polarization (12% accuracy) 2. Forward Backward Assymetry (AFB), related to the angle between the charged lepton and the b-jet in W rest-frame (9% accuracy) W polarization and top spin correlation in ATLAS

  35. Spare: dileptonic results, W polarization  V. Simak and K. Smolek W polarization with dileptonic tt events for S+B at 10fb-1 (±stat ±syst) • Same precision as semileptonic channel for F0 • ~2-3 times worse for FLandFR than for semileptonic channel W polarization and top spin correlation in ATLAS

  36. Spare: Beyond SM - V+A component The correction used is extracted from a V-A hypothesis (70% FO - 30% FL) Correction changes in case of V+A component bias expected Iterative process W polarization and top spin correlation in ATLAS

  37. Spare: Sensitivity to tWb anomalous couplings stat+syst --- stat (10 fb-1) 10 fb-1 Best sensitivity to (linear behavior) W polarization and top spin correlation in ATLAS

  38. Spare: Sensitivity to tWb anomalous couplings Comparison with other expectations (2s limit): *1: 2 fb-1, assuming a 10% systematic uncertainty *2:100 fb-1, assuming a 5% systematic uncertainty Preliminary studies Sensitivity 2-4 times better than Tev. expectations with 2 fb-1 and competitive with single top at LHC (100 fb-1,high luminosity) W polarization and top spin correlation in ATLAS

  39. Spare: off-diagonal basis Helicity (A=-0.35) Beamline (A=0.78) Off-diag (A=~0.8) Tevatron: « off-diagonal » = optimal basis (i.e. A(qq)=1 at LO) • top velocity • q*top scattering angle wrt pp ZMF • near treshold: « off-diagonal »  beamline basis • Far above treshold: « off-diagonal »  helicity basis W polarization and top spin correlation in ATLAS

  40. Spare: Top spin correlation observable Two spin correlation observablesCandD(hep-ph/0403035) : C and D unbiased estimators Standard Model C=0 (No spin correlation) D=0 (No spin correlation) W polarization and top spin correlation in ATLAS

  41. Spare: Measurement method Spin correlation: use a Non Correlated MC generator (PYTHIA) to parametrize reconstruction + cuts effects : C=0 Reconstruction and cuts apply weights event by event W polarization and top spin correlation in ATLAS

  42. Spare: dileptonic results, spin correlation  V. Simak and K. Smolek Spin correlation with dileptonic tt events for S+B at 10fb-1 (±stat ±syst) (Mttcut included) • ~ same precision as semileptonic channel • Large dilution due to reconstruction effects W polarization and top spin correlation in ATLAS

  43. Systematics: Monte Carlo Generators and hadronization scheme • Compare results obtained with: • different Monte-Carlo generators including spin correlation • Pythia or Herwig for hadronisation, fragmentation and decays after selection + reconstruction + correction • Very good agreement • Better agreement than for W polarization W polarization and top spin correlation in ATLAS

  44. Systematics: jet calibration and top mass W polarization and top spin correlation in ATLAS

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