1 / 41

5th International Workshop on Top Quark Physics September 16 - 21, 2012 Winchester, U.K.

Angular distributions at the Tevatron. Amnon Harel. 5th International Workshop on Top Quark Physics September 16 - 21, 2012 Winchester, U.K. Photo by Justin Eure. Day 4 of Top 2012. Previous talks already covered: Selections Estimations of sample composition Background modeling

hoang
Download Presentation

5th International Workshop on Top Quark Physics September 16 - 21, 2012 Winchester, U.K.

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. Angular distributions at the Tevatron Amnon Harel 5th International Workshop on Top Quark Physics September 16 - 21, 2012 Winchester, U.K. Photo by Justin Eure

  2. Day 4 of Top 2012 • Previous talks already covered: • Selections • Estimations of sample composition • Background modeling • Signal modeling • I mostly avoid these aspects of the analyses in this talk. • This is often in the spirit of the notes / papers which defer such details to the relevant cross-section measurements. Amnon Harel

  3. Overview ν 1. What is the W polarization? l W b - t t Decaying to leptons or hadrons 2. Is the top quark polarized? W b 3. Are the top pair spins correlated? Amnon Harel

  4. Measurements ofWhelicityin top decays • Combination with ≤5.4fb-1 – Phys. Rev. D 85, 071106 (2012) • Latest and greatest from each collaboration: • CDF lepton+jets, matrix element with 8.7fb-1 – CDF note 10855 • D0 with 5.4fb-1 - Phys. Rev. D 83, 032009 (2011) Amnon Harel

  5. Left-handed Right-handed Arbitrary Normalization Longitudinal SM cos* W helicity So far measurements support the SM prediction: f(tWb) = ~100% Breaking it down by W helicity states: Left handedλ=-1 SM: 30.3% Longitudinalλ=0 SM: 69.6% Right handedλ=1 SM: 0.1% SM uncertainties << Experimental uncertainties  can’t constrain SM parameters Firm SM prediction, in particular: tiny f+  looking for new physics Most relevant observable to distinguish between helicity states is* e (down type) q* t b W ne (up type) Amnon Harel

  6. W helicity µ+ l+ νµ ν b b W+ W+ t t - - t t - - W- W- b b q νe e- q’ Samples Lepton+jets(lepton=e/μ) Dilepton(lepton=e/μ) isolated, pT>20 GeV MET: l+jets: >20GeV Dileptons: discriminant input in ee/eμ btags 4 Jets 2 Jets ( ) Amnon Harel

  7. W helicity l+ ν b W+ t - t - W- b q q’ l+jets reconstruction Partons Observed objects Parton-level Particle-level Detector-level QCDSimulation Experimental resolutions & b-ID probabilities • fit parton energies to the measured objects, minimizing a χ2 • Constraints: mt1 = mt2= 172.5GeV; mW1 = mW2 = 80.4GeV • Do the fit for every combination of assigning a jet to a parton Amnon Harel

  8. W helicity Left-handed Fraction Right-handed Arbitrary Normalization Longitudinal cos* Fraction l+jets reconstruction gives… excellent cos* reconstruction! Parton level The hadronic W helps measure f0 Reconstructed Leptonic W Hadronic W e (down type) q* t b Can’t distinguish up and down type quarks AcceptancepTl & lwithin b jet W ne (up type) Amnon Harel

  9. W helicity µ+ νµ b W+ t - t - W- b νe e- Left-handed Fraction Right-handed Arbitrary Normalization Longitudinal cos* cos* Dilepton reconstruction • With two νs, reconstruction is harder. • “resolution sampling” • smear objects within their resolution • 500 times per event • for each b-jet & l combination and smearing, solve algebraically for cos* • use the 2 MET components + 4 mass constraints • 0-8 solutions • average all solutions Parton level Reconstructed Amnon Harel

  10. W helicity l+ ν b W+ t - t - W- b q q’ Without reconstruction Partons Observed objects Parton-level Particle-level Detector-level QCDSimulation Experimental resolutions & b-ID probabilities • Instead of reconstructing *, calculate a likelihood P(data | parameters) • Many observables per event  summarize • 2010s approach: machine learning? If so, fortunate this was tackled earlier… Amnon Harel

  11. W helicity Matrix element method • Calculate the likelihood using the SM (manual learning?) • with leading order calculations of the matrix elements • Likelihood accounts for all jet-parton assignments • makes use of hadronic side & of all jet kinematics, not just * •  20% than previous CDF technique (for same dataset) • Likelihood accounts for backgrounds • Pband Ps convolute the differential partonic cross-section (proportional to the matrix element) with the PDFs and the transfer functions • using W+jets matrix elements from VECBOS Amnon Harel

  12. W helicity Matrix element method • The ME-based likelihood does not model everything: • Missing diagrams, beyond LO • Events without a correct assignment • Effect of other backgrounds •  In general, we expect the fitted fractions to be biased • Calibrate using ensemble tests • Separate calibrations for 1D and 2D fits • Example of 1D calibration • Found linear dependences •  simplifies interpretation  Amnon Harel

  13. W helicity Combination results Results of 2D fit: with SM Results of 1D fits: with SM Amnon Harel

  14. W helicity New CDF results For maximal confusion: x and y-axis are flipped… Results of 2D fit: 58 stat. syst. Results of 1D fits: with SM with SM Amnon Harel

  15. W helicity Putting them together For maximal confusion: x and y-axis are flipped… Results in good agreement with the SM Amnon Harel

  16. Checks fortop polarizationin top pair production Amnon Harel

  17. Analyzing top spin Top lifetime(~3·10-25s) Spin flip time scale(~3·10-21s) ν l W ? b - t t Decaying to leptons or hadrons W b Production @ Tevatron != production @ LHC – best measure both No measurements of the polarization from the Tevatron. But there is some relevant public data… Amnon Harel

  18. Analyzing top spin Angle between polarization axis and decay product (l) in the top’s rest frame Polarization vector Analyzing strength, κl=1 ν l e.g. for longitudinal polarization W b - ? t t Decaying to leptons or hadrons W b Amnon Harel

  19. Top spin Longitudinal polarization? arXiv:1207.0364 (2012) Z’ model with Z-like coupling to quarks Dilepton • Reconstruction in dilepton channel uses the neutrino-weighting method. • Sum possible neutrino ηs, each one: • weighted by compatibility with MET • Statistical sample for jet resolutions, etc. • solved analytically using known mt l+jets From CP: PDF(cosθ+)+PDF(cosθ-) Hints at SM-like, unpolarized top pair production Amnon Harel

  20. Top spin Related plot From CDF note 10719, which presents this as a check of data modeling. My apologies for reinterpreting… Dilepton From CP: PDF(cosθ+)+PDF(-cosθ-) from lepton±’s direction in t± frame, to p±’sin the frame (i.e. the beam axis) Hints at SM-like, unpolarized top pair production Amnon Harel

  21. Measurements ofspin correlationin top pair production • CDF note 10719 – dilepton (2011) • D0: Phys. Rev. Lett. 108, 032004 (2012) • Combined with Phys. Reb. Lett. 107, 032001 (2011) • CDF note 10211 – l+jets (2010) Amnon Harel

  22. Analyzing spin correlations Analyzing strength: αl=1, αb= -0.41,αd=0.97 Correlation strength (in production)(@NLO QCD) ν • , or as above: l “u” b W b - Beam axis “d” t t W ? Again: Production @ Tevatron != production @ LHC – measure both Amnon Harel

  23. Spin correlations µ+ νµ b W+ t - t - W- b νe e- Dileptonreconstruction • With two νs, reconstruction is harder. • Assuming “true” lepton and jet 4-vectors and a “true” MET, we have • 6 unknowns (νs momenta) • 6 constraints (4 masses, “true” MET) • Algebraic solution • Kinematic fit: • Fit for b-jet energies, MEX, & MEY and best jet-parton assignment • Minimize a likelihood with • χ2 terms for each parameter • A-priori probability densities for , , and • As simulated with Pythia Amnon Harel

  24. Spin correlations Template construction Beams basis • Signal simulated with Pythia, i.e., without spin correlations. • Spin correlations modeled by reweighting of: • See also backup slide Templates fit to 2D polynomials (up to order 4, constrained by P & CP): • Templates fit for correlation strengths -1, -0.8, -0.6, … 1 • Templates for and for • Also derived for backgrounds • dibosons w. Pythia, Z+jets w. Alpgen, fakes from data Amnon Harel

  25. Spin correlations Extreme templates Amnon Harel

  26. Spin correlations Fit to data • Maximal likelihood fit • Simultaneous fitfor and for . • Using the expected sample composition. • Calibration checked with ensemble tests: no calibration needed Measured correlation strength (“C” in previous slide): Amnon Harel

  27. Spin correlations Matrix element method Used both indilepton channel (shown in Top11), and in l+jet channel (preliminary version shown in Top11). Again, efficiently summarizes all event kinematics. But here, following Melnikov & Schulze (PLB 700, 17 (2011), they are summarized into a discriminant: • is the probability assuming SM signal (including spin correlations) • is the probability assuming signal without spin correlations • Calculating with LO matrix-elements taken from Mahlon and Parke. • l+jets: 5D integration, 4 jet-parton assignments (u-jet distinct from d-jet) • Dilepton: 4-6D integration (but weakly constrained), 2 assignments Amnon Harel

  28. Spin correlations Fit to data Fit forf the fraction of events with spin correlations Templates generated with MC@NLO Dilepton l+jets • 4 l+jets subchannels: • 4 or ≥5 jets • Mjj ≈MW • Only the best one shown Amnon Harel

  29. Spin correlations Results Dilepton: l+jets: Combined: “No spin correlation” excluded at 3σ, i.e., evidence Amnon Harel

  30. Summary • The Tevatron experiments measured • W helicity fractions in top pair decays • To better than 10% • Spin correlations in top pair production • Evidence for spin correlation from D0 • Not all results included in this talk • Many measurements still using ~half of the Tevatron data • No full measurements, but some indications from Tevatron data that top quarks in top pair production are not 100% polarized… Good agreement with the standard model Amnon Harel

  31. Back up slides Amnon Harel

  32. Claimer / Disclaimer Claimer: Following long discussion with the authors, or at least, the current experts in the collaboration, a few definitions & descriptions in this talk knowingly do not follow the corresponding documentation. Can discuss as needed after the talk… Disclaimer: But as always, I might have introduced silly mistakes, which should not reflect on the excellent work reported here. Amnon Harel

  33. W helicity µ+ νµ b W+ t - t - W- b νe e- eμ sample Isolated μ, pT>15 GeV, |η|<2.0 2 Jets (pT>20 GeV, |η|<2.5)maybe a couple of b-tags… Discriminant construction and fit procedures similar to those in l+jets Events Isolated e, pT>15 GeV, |η|<1.1 / 1.5<|η|<2.5 • A strong experimental signature • no MET requirements • looser lepton ID requirements cut value Amnon Harel Discriminant

  34. Spin correlation Signal modeling • Signal without spin correlations taken from Pythia. • Signal with spin correlations is modeled by reweighting the Pythia events by , where the angles and C are in the beam basis. • Based on the LO matrix element of the decay • Exact for a pure state that is a spin eigenstate for this quantization axis • For mixed states, C=1-2p<1 • “top-tbarspin state is approximately in the eigen-state in beamline basis and weighting method works fairly fine.” Amnon Harel

  35. Spin correlations µ+ l+ νµ ν b b W+ W+ t t - - t t - - W- W- b b q νe e- q’ Samples Lepton+jets(lepton=e/μ) Dilepton(lepton=e/μ) isolated, pT>20 GeV MET: l+jets: >20GeV Dileptons: discriminant input in ee/eμ btags 4 Jets 2 Jets Beam Basis Matrix element Helicity basis Templates Beam basis Templates Amnon Harel

  36. Spin correlations CDF systematics • Leading sources: • Up to 0.21 from higher order terms, evaluated with MC@NLO • Up to 0.19 from ISR/FSR simulation • Up to 0.17 from sample composition & instrumental fakes sample • Up to 0.13 from color reconnection Amnon Harel

  37. Spin correlations D0 systematics Amnon Harel

  38. Matrix method 4-vectors of quarks, lepton, ν Diff. x-sec. Proportional to ME PDFs • Transfer functions • QCD modeling • Detector modeling Jet-quark assignments Observable as a function of the 4-vectors Amnon Harel

  39. Phys. Rev. D 83, 032009 (2011) CDF Note 10333 Older W helicity results Lepton+jets and dilepton channels Dilepton channels SM Physically allowed region 68% 95% Corrections are small For maximal confusion: x and y-axis are flipped… Longitudinal: f0 = 0.67± 0.10 f0 = 0.78± 0.20 Right handed: f+ = 0.02 ± 0.05 f+ = -0.12 ± 0.10 Amnon Harel

  40. Experimental Apparatus Fermilab Tevatron ColliderRun II 2002-2011 The detectors CDF Tevatron MainInjector DØ • The collisions • Ec.m.= 1.96TeV General purpose detectors Top physics relies on tracking, calorimetry and muon detectors. Amnon Harel

  41. subject T T Amnon Harel

More Related