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Helicity of W bosons in Top Quark Decays at CDF

Helicity of W bosons in Top Quark Decays at CDF. Outline: Introduction Motivation Overview of W helicity studies 1D measurement of W helicity fractions with 955pb -1 of data 2D measurement of W helicity fractions with 955pb-1 of data Summary. Shulamit Moed, University of Geneva.

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Helicity of W bosons in Top Quark Decays at CDF

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  1. Helicity of W bosons in Top Quark Decays at CDF • Outline: • Introduction • Motivation • Overview of W helicity studies • 1D measurement of W helicity fractions with 955pb-1 of data • 2D measurement of W helicity fractions with 955pb-1 of data • Summary Shulamit Moed, University of Geneva

  2. Why is top quark interesting? • Youngest member of the SM family • Striking characteristic: HUGE mass , at EWSB scale (Yukawa coupling ~1) - what does this tell us? • Unique opportunity to probe bare quark properties (spin? charge?) • Top special relation to the Higgs boson • Is top the gateway to new physics? Branching ratios Rare decays Non-SM decays Decay kinematics W helicity Anomalous coupling |Vtb| Production cross section Resonance production Production kinematics Spin polarization Top charge Top spin Top lifetime Top mass

  3. The Tevatron Chicago  Booster CDF DØ Tevatron p source Main Injector p-pbar collisions with 1.96TeV center-of-mass energy. Until LHC turns on - the only place to study top quark W helicity result with 955 pb-1

  4. Top production at the Tevatron g g ~1 top event every 10 BILLION inelastic collisions Top pair production Main mechanism for top physics at Tevatron ~85% ~15% For top mass = 175 GeV @ √s= 1.96 TeV : CDF combined (theory) estimated bkg Single top production selected events • Not yet observed • different final states than pair production • Larger background geometric and kinematic acceptance

  5. t-tbar Final States lepton+jets 15% all-jets 44% dilepton 5% lepton+jets 15% Vtb~1; Mtop>MW+Mb: Decays to real W BR(tWb) ~ 100% • Top decays before hadronizing • ~10-25 s (due to large mass) Final states are classified by the decay of the W’s BR(Wl) = 1/3 BR(Wqq) = 2/3 In all cases, the final state has 2 b quarks Lepton+jets Di-lepton All hadronic e jets e jets

  6. Lepton+Jets Channel • Best compromise  higher statistics than dilepton, less background than all-hadronic • Purity of the l+jet channels is not that high (S:B ~1:3) • Increase S:B by using b-tagging • Fully reconstruct the event Detected objects for full event reconstruction: 4 energetic jets 1 isolated charged lepton Missing Et for the neutrino Do not know with certainty the correct assignment between parents and decay products ??

  7. The CDF detector y q f z x • Muon system • CMP ; CMU |η|<0.6 • CMX 0.6<|η|<1.0 calorimeters Up to |η|<3.6 pseudorapidity Tracking system: • Silicon detector -> b tagging • COT : central outer tracker Eff. for charged particle tracks: • ~100% for |η|<1.0 • ~40% for |η|≈ 2.0 Excellent lepton ID: ~80% eff. for central electrons ~90% eff. for high Pt muons

  8. W helicity in top quark decays SM top decays via the weak interaction  V-A coupling like all other fermions: b spin=1/2 t spin=1/2 Helicity: W spin=1 The longitudinal fraction: • This measurement: • Test of the SM, non-zero V+A? • EWSB – prediction of high longitudinal W fraction

  9. What do we measure ? 1/2 V+A is suppressed Left-handed longitudinal Right-handed SM prediction of helicity fractions (assuming Mt=175GeV): • longitudinal f0 = 0.7 • left-handed f- = 0.3 • right-handed f+ = 0 We fully reconstruct the event using:

  10. Sensitive Variables Lepton transverse momentum Lepton-b invariant mass complexity M2lb = 1/2 (M2t – M2W)(1 + cos θ*) cos(θ*) …

  11. Other W Helicity Measurements Previously at CDF D0 09/2006 370 pb-1 cos(θ*) method RunI (Mlb)2 : Integrated luminosity= Early RunII:

  12. Analysis Overview • lepton+jets selection • fully reconstruct the leptonic top decay. • calculate cos(θ*) • construct templates for left-handed, right handed and longitudinal W’s and background • fit helicity fractions using unbinned likelihood fitter. • correct for acceptance effects. • estimate systematic uncertainties.

  13. Event Reconstruction • Selection main features: • only one isolated lepton with PT >20 GeV • at least 4 jets with ET > 15 GeV and |η|<2.0 (JETCLU with ΔR=0.4) • missing ET > 20 GeV • at least one jet is tagged with a secondary vertex tagging • veto on electrons from photon conversion • veto on events tagged by cosmic ray tagger • scalar sum of transverse energies of all reconstructed objects (Ht) > 200 GeV Reconstructed objects – 4 jets + 1 lepton 24 permutations of possible combinations , which one we choose?

  14. b-jet Tagging • Expect t  W b b jet tagging is a very important tool. - Every ttbar event contains 2 b-jets - Less than 20% of the dominant background (W+jets) contains Heavy Flavor (b/c quarks) • B decay signature: displaced vertex • Long life time c ~ 450 m: travels Lxy~3mm before decaying CDF Event: Close-up View of Layer 00 Silicon Detector Require at least 1 jet tagged with the secondary vertex tagging algorithm. b-tag b-tag 1.2 cm Reduce permutations from 24 to 12! MET

  15. Jet Energy Scale Corrections applied to estimate the original parton energies from the observed jet energy in the calorimeter • Jets are corrected for: • η dependence correction – homogenous calorimeter response. • subtraction of energy due to pile-up of multiple interactions in the same bunch crossing. • correction for non-linearity and energy loss in the uninstrumented regions of the detectors. • Underlying event energy that falls inside the jet cone. • Jet energy radiating out of the jet cone. • Top specific corrections – flavor and topology of ttbar events.

  16. Kinematical Fit PT resolutions 2.5 GeV 1.5 GeV MW , Mt = pole masses • Provides constraints on W mass, t mass= anti-top mass etc. • Fit lowest Χ2 used to select the most likely combination Χ2 efficiency ≈ 33%

  17. Selected Data Sample Use lepton+jets selection with at least one b-tagged jet and Ht>200GeV cut (to reduce QCD background) for events with njets >= 4: Data 220 events (89% signal fraction) Total background 22.8 events Scaled to 955pb-1 Background composition Process bkg events fraction fraction Mistag 9±1.35 39.5% 4.1% W+h.f. 6.4±1.85 28% 2.9% Single top 0.54±0.17 2.4% 0.25% Diboson 1.36±0.07 6% 0.61% QCD 5.5±1.08 24.1% 2.5%

  18. Monte Carlo Samples • Used HERWIG based samples with top mass of 175 GeV. • In these samples one the leptonic W is forced to a specific helicity (longitudinal, left handed or right handed). • The hadronically decaying W decays according to SM. particle-level reconstructed

  19. Parameterizations • Fit to 3rd order polynomial times exponential. • Background model is a mix of Wbbpp, W4p and diboson sample. left-handed right-handed longitudinal background • Comparison of signal and background fits used for likelihood fitter parameterizations.

  20. The Likelihood Fitter to extract helicity fractions: Gaussian bkg constraint Poisson probability for number of observed events shape information longitudinal right-handed left-handed • Use unbinned maximum likelihood fit. • Pseudo experiments generated from ideal functions look fine. • Tested ‘real’ pseudo experiments with arbitrary f0, f-, f+. (from templates) • Tested ‘real’ pseudo experiments SM ttbar sample. (pythia) • Fit residual and pull width look fine. Extract two results by fitting for: F0 while F+=0 F+ while F0 is fixed to the SM value @Mt=175GeV

  21. The likelihood Fitter – Linearity Checks • Linearity checks and sensitivity with <S+B>=220 events: • All fits are consistent with “no bias”. Expected stat. uncertainty for f0 :δf0 = 0.12and for f+: δf+ = 0.058

  22. The Likelihood Fitter – SM example Means and pulls for realistic pseudo experiments constructed with SM values: f0 = 0.7 F0 = 0.708 f+ = 0 F+ = 0.006 Right-handed, f0 fixed Longitudinal, f+ fixed σ = 0.990.023 σ = 10.023

  23. Acceptance In the fitter, all templates are normalized to 1, but – our acceptance for WL≠ W0≠ WR Recall: Cuts on lepton PT and isolation  left (right) acceptance is smaller (larger) relative to the longitudinal W’s. Applying an acceptance correction:

  24. Acceptance Correction αi = accptance for helicity i F0=measured fraction ; f0= true fraction for the right-handed fraction: Correction for f+ is very small (~0.01)  not applied. Instead – assign a 1% systematic.

  25. Systematic Uncertainties We use realistic pseudo experiments to estimate systematic uncertainties while keeping the fit unchanged. Source Bkg model JES Signal model PDF ISR/FSR MC statistics Instantaneous luminosity Lepton energy scale Acceptance correction Total syst. δf0 ±0.038 ±0.013 ±0.020 ±0.009 ±0.010 ±0.020 ±0.007 ±0.001 ±0.001 ±0.053 δf+ ±0.017 ±0.010 ±0.010 ±0.006 ±0.005 ±0.010 ±0.002 ±0.002 ±0.001 ±0.027 Expected stat. uncertainty 0.12 0.06

  26. Results – Data Fit Fitting the data: F0 = 0.65 (measured) f0 = 0.60 ± 0.12 ± 0.06, (corrected) f+ = 0 fixed f+ = -0.06 ± 0.06 ± 0.03, f0 fixed to SM value @Mt=175 GeV Fit for longitudinal fraction Fit for right-handed fraction

  27. Results - Likelihood Curves For right-handed fraction For longitudinal fraction

  28. Results – Setting Upper Limit on f+ • Bayesian method for setting a limit@95% C.L: • Model systematic uncertainties as a gaussian with =0, σ= 0.027 . - Have verified f+ systematic independent of f+ • Convolute with likelihood - as expected the effect is small, dominated by statistics. W systematics w/o systematics f+<0.11@95% C.L

  29. Expected Statistical Uncertainty • Assuming no improvements, stat~syst with 4fb-1.

  30. 2D Fit – First Simultaneous f0, f+ measurement ! • With the increasing luminosity: • Interest in a model independent measurement • V+A coupling bounded by CLEO bsγ data at a level that cannot be reached even at the LHC. • No assumption on helicity fractions while fitting, probe any deviation from SM (super-symmetry, dynamical electroweak symmetry breaking models, Extra dimensions ….) Same data, same reconstruction, same templates etc. • fit for f0 and f+ simultaneously, rather than: • Fixing f+ to 0 (=SM) and fitting for f0 • Fixing f0 to 0.7 (=SM) and fitting for f+ ---> Less precision, but a more general result when fixing one fit parameter to its SM value (1D fit), the correction is either simple (f0) or negligible (f+)

  31. Uncertainties for Simultaneous Fit Systematics Compared with 1D fit – 0.053 for f0 0.027 for f+ Expected sensitivity from 1000 SM p.e: 0.25 statistical Compared with 1D fit – 0.12 for f0 0.06 for f+ 0.10

  32. 2D Fit Results

  33. Limit on f+? • Form probability surface • Find contour of constant probability that captures 95% of the volume under the surface • No systematics in likelihood shape. but for 2D fit: stat  syst = stat

  34. Summary • First simultaneous measurement of right-handed and longitudinal W helicity fractions! • Improvement of CDF 1D results of longitudinal and right handed W fractions. • Our knowledge of t-W-b vertex is still statistically limited. • CDF now factor of 2 better than previous measurements. • However still factor of 2 above current systematics - This is worth doing as a 4 fb-1 analysis on CDF. • Measurement consistent with SM predictions – top decay is of V-A nature. • Current status - working towards a publication . Results: 1D fit f0 = 0.6 ± 0.12(stat.) ± 0.06(sys.), f+ = 0 fixed f+ = -0.06 ± 0.06(stat.) ± 0.03(sys.), f0 fixed to SM value @175GeV f+ < 0.11 @ 95% C.L (including systematics) 2D fit

  35. Back up slides

  36. Top Mass Dependence • Top mass is not constrained in this analysis. • Fit to a linear function yields a correction of 0.5% for a 1σ variation of the top mass (3 GeV).

  37. Systematic Uncertainties – Background Background shape systematic: • Assume 100% W4p or 100% Wbb2p • Add 25% special QCD sample Vary q2 for W sample Special QCD sample Multi-jet trigger 0.8<em<0.95 Ntracks>3 reminder - estimated ~5 QCD events out of 220

  38. Background Dominated Samples Comparison of 0-tag sample and bkg model We have a reasonable background model

  39. Systematic Uncertainties – MC Statistics Statistical uncertainty of the parameterizations is not propagated through the analysis  systematic uncertainty: • Re-fitting templates 1000 times, Poisson fluctuate the bins around central value. • Draw pseudo-experiments from the different fits. • Take difference in RMS of fitted values as a systematic : δf0=0.02 δf+=0.01

  40. Systematic Uncertainties - PDF • difference between MRST72 and CTEQ5L. • difference between MRST75 and MRST72. • variation of the 20 CTEQ6M eigenvectors.

  41. 2D Pull Distributions

  42. 955pb-1 – Data/MC Comparison

  43. 955pb-1 – Data/MC Comparison

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