1 / 43

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.

liko
Download Presentation

Helicity of W bosons in Top Quark Decays at CDF

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related