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Search for New Phenomena in the CDF Top Quark Sample

Search for New Phenomena in the CDF Top Quark Sample. Kevin Lannon The Ohio State University For the CDF Collaboration. Why Look in Top Sample?. Quark Masses. t. b. c. s. d. u. GeV/c 2. Top only recently discovered Top turned 10 in 2005 Samples still relatively small

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Search for New Phenomena in the CDF Top Quark Sample

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  1. Search for New Phenomena in the CDF Top Quark Sample Kevin Lannon The Ohio State University For the CDF Collaboration

  2. Why Look in Top Sample? Quark Masses t b c s d u GeV/c2 • Top only recently discovered • Top turned 10 in 2005 • Samples still relatively small • Still plenty of “room” for unexpected phenomena • Top is really massive • Comparable to gold nucleus! • Yukawa coupling near unity • Special role in EWSB? • Many models include new physics coupling to top 5 orders of magnitude between quark masses! K. Lannon

  3. What Might We Find? • It’s not Standard Model top at all! • Charge not 2/3? [Phys.Rev.D59:091503,1999; Phys.Rev.D61:037301,2000] • Spin not 1/2? • It’s not only Standard Model top • Additional heavy particles decaying to high pt leptons, jets and missing energy (t ’) [Phys.Rev.D64:053004,2001; Phys.Rev.D65:053002,2002] • Heavy resonance decaying to tt [Phys.Lett.B266:419,1991] • tH+b • ttH production [Phys.Rev.D68:034022,2003] • Nothing but the Standard Model . . . . • Not as bad as it sounds • Test our abilities to calculate signal and background properties • Important at the LHC  top becomes background to other searches • Constrains models that put new physics in the top sample [hep-ph/0504221] K. Lannon

  4. The Tevatron and CDF • Tevatron accelerator • Highest energy accelerator in the world (Ecm = 1.96 TeV) • World record for hadron collider luminosity (Linst = 2.72E32 cm-2s-1) • Only accelerator currently making top quarks Muon Detectors Central Cal Plug Cal • CDF Detector • Trigger on high pT leptons, jets and missing ET • Silicon tracking chamber to reconstruct displaced vertices from b decays Silicon Tracker CentralTracker K. Lannon

  5. Tevatron Performance • Integrated luminosity at CDF • Total delivered: ~2.3 fb-1 • Total recorded: ~1.9 fb-1 (~ 17 Run I!) • So far for top analyses, used up to 1 fb-1 • More analyses with 1.0-1.2 fb-1 in progress for winter and spring • Doubling time: ~1 year • Future: ~4 fb-1 by 2007, ~8 fb-1 by 2009 Integrated Luminosity Peak Luminosity Today’s Presentation: 200 pb-1 ~ 1 fb-1 Analyzed by Summer K. Lannon

  6. Triggering on Top • Need high efficiency, low fake rate trigger for high pT leptons • Relies on track trigger (XFT) • Fake rate increases with occupancy • Occupancy increases with luminosity • 3x higher than original design because Tevatron didn’t reduce bunch spacing (392 ns  132 ns) Z  ee at low lum.9 add. Int./crossing Fake tracks can be made from segments of different real physical tracks. Instrumenting additional layers reduces fake rate. Efficiency stays high. fake Reduction factor ~ 4 Missing segments • Upgrade put into operation in October • Efficiency = 96% for high pT tracks • Fake track rejection factor = 5-7 Trigger  for muons without upgrade K. Lannon

  7. Top Quark Production at Tevatron (and LHC) • QCD pair production • NLO = 6.7 pb • First observed at Tevatron in 1995 ~85% ~15% 833 pb ~87% ~13% s-channel t-channel • EWK single-top production • s-channel: NLO = 0.9 pb • t-channel: NLO = 2.0 pb • Not observed yet 10.6 pb 247 pb Associated tW ??? • Other?: 62 pb K. Lannon

  8. Top Production Rates • Like finding a needle in a haystack . . . . Needle in haystack (approx.) • Efficient Trigger • ~90% for high pT leptons • Targeted event selection • Distinctive final state • Heavy top mass • Advanced analysis techniques • Artificial Neural Networks One top pair each 1010 inelastic collisions at s = 1.96 TeV K. Lannon

  9. SM Top Quark Decays • Particular analysis usually focuses on one or two channels • New physics can impact different channels in different ways • Comparisons between channels important in search for new physics BR(tWb) ~ 100% K. Lannon

  10. Top Signatures Electron or muon: pT > 20 GeV Jet: ET > 15-20 GeVcone = 0.4 Neutrino: Missing ET > 20-25 GeV b-jet: identified with secondary vertex tag Dilepton Lepton + Jets All Hadronic K. Lannon

  11. Top Event Yields • To give an idea of CDF sample sizes . . . . • Based on top cross section of 6.7 pb • Background and signal numbers based on event yields from current analyses, scaled by luminosity • Assume no changes in event selection, efficiency, etc. • L+J: ~2k signal events with 4 fb-1 (signal:background ~ 1 : 5) • L+J (b-tag): ~1k signal events with 4 fb-1 (signal:background ~ 2:1) K. Lannon

  12. Searching for New Physics • Precision study of top properties • Non-SM behavior from top quark • Evidence of something other than top in sample • Direct search for new phenomena in top sample • Resonant production • Non-SM decays • New particles with “top-like” signature • New particles produced in association with top Vtb K. Lannon

  13. Top Properties Working Group • Studying all properties of top quark (except mass) • ~ 150 faculty, postdocs, students • ~15 papers (so far) • ~50 active analyses Vtb K. Lannon

  14. Precision Study: Cross Section • Cross section • Measured in different final states • New physics can affect different final states differently • Different techniques used in same final state • Results combined at end for most precise answer • tt production calculated to NLO • Central value: 6.7 pb — 6.8 pb • Uncertainties: 5.8pb — 7.4 pb • For mtop = 175 GeV/c2 • Combined result: • 7.3  0.9 pb NTop = Nobs- Nbackground, or from fit K. Lannon

  15. Two Best Measurements • Both in Lepton + Jets Channel • Vertex Tag (weight = 0.50, pull = + 0.88) • Uses b-tagging to increase ratio of signal to background • Counting experiment • Count W+jets events with a b-tag • Subtract expected background • Excess attributed to top • Kinematic Artificial Neural Net (weight = 0.32, pull = -1.14) • Uses kinematic variables to separate signal from background • Combines several variables in a neural network to increase sensitivity • Fit for the number of top events • Does not use b-tagging (lower signal to background ratio) K. Lannon

  16. B-Tagging • b-tagging: Identifying jets containing a b quark • Take advantage of long b lifetime • Look at precision tracking information for tracks within jet • Reconstruct secondary vertices displaced from primary • Efficiency • Per jet • 40% for b jet • 9% for c jet • 0.5% for light jet • Per event (tt ) • 60% for single tag • 15% for double tag K. Lannon

  17. Sample Composition Control region Signal region Backgrounds that produce W + jets signature Agreement between data and background checks accuracy of background estimate Excess of data over background attributed to top production Number of events with an identified W +  1 jets 695 pb-1 K. Lannon

  18. One Tag + HT Cut 8.2 ± 0.6 (stat.) ± 1.0 (sys.) pb Two tags, no HT Cut Cross check 8.8 +1.2-1.1 (stat.) +2.0-1.3 (sys.) pb Lepton + Jets Vertex Tag Result HT = scalar sum of lepton, jet, and missing ET K. Lannon

  19. Using Kinematics to Identify Top • Look for central, spherical events with large transverse energy • Signal: PYTHIA tt monte carlo • Background: ALPGEN + HERWIG W + 3p monte carlo • Normalized to unit area • HT scalar sum of lepton, jet, and missing ET • Aplanarity uses lepton, jet and missing ET • Max jet  uses 3 highest ETjets; all others use 5 highest K. Lannon

  20. Evaluate expected fit fractional error using MC-based pseudo experiments Single variable fits: fit signal fraction using distributions of a single kinematic variable Plotted Points: median fit fractional error Error bars: 68% interval Statistical Sensitivity K. Lannon

  21. Structure Composed of nodes modeled after neurons in nervous system Organized into layers Input layer: initialized by input variables Hidden layer: takes information from each input node and passes to output layer Output node: new discriminating variable with range [0,1] Training Neural net output determined by exposure to training data Iteratively adjust parameters to minimize error: Training accomplished through JETNET program(Peterson et al. CERN-TH/7135-94) Multivariate Approach: Neural Nets 7 kinematic variables  7 input nodes Output node—range [0,1]—signal = 1 1 hidden layer, 7 hidden nodes Information flow K. Lannon

  22. Evaluate expected fit fractional error using MC-based pseudo experiments Single variable fits: fit signal fraction using distributions of a single kinematic variable NN: fit NN output of data to NN templates Plotted Points: median fit fractional error Error bars: 68% interval NN Fit performs significantly better than single variable fits Statistical Sensitivity K. Lannon

  23. Basic Approach Train NN to distinguish tt signal from backgrounds PYTHIA tt MC as signal model ALPGEN + HERWIG W + 3p MC as background model Use this NN to make templates for fitting the data Use same signal model as above Also extract QCD multijet template from data Supplement electroweak template with contributions from other processes: WW,WZ, Z + jets, single top Fit templates to NN distribution from data Binned maximum likelihood fit Three component fit Signal and electroweak float QCD constrained to value estimated using isolation vs missing ETmethod Using NN to Fit Data K. Lannon

  24. Lepton + Jets Kinematic ANN Result K. Lannon

  25. Kinematics of b-Tagged Events • Looks like top! K. Lannon

  26. Systematic Uncertainties • Main Systematic Uncertainties uncorrelated • Lepton + Jets Vertex Tag • b-tagging efficiency: 6.5% • Background estimation: 3.4% • Kinematic ANN • Background shape modeling: 10.2% • Jet Energy Scale: 8.3% • For both results, uncertainty dominated by systematics • Both are working to reduce for 1.2 fb-1 publications K. Lannon

  27. Search for t H+b Phys.Rev.Lett. 96 (2006) 042003 • Compare top yield in four different channels • Measurements consistent with SM • Consider correlated effect of tH+b decays on four channels • Exclude when changes make expectation inconsistent with data • Limits for 6 sets of MSSM parameters and less model-specific scenarios Varying model parameters changes: BR(tH+b) BR(H+) BR(H+cs) BR(H+t*b) BR(H+W+h0) BR(H+W+A0) Shown here: Variations as a function of tan particular set of MSSM parameters K. Lannon

  28. MSSM Limits • Calculate BR(tH+b) and H+ BR’s as a function of MH+ and tan() • Use 6 different MSSM “benchmarks” • Results for “Benchmark #1” shown below K. Lannon

  29. “Tauonic Higgs” Model Assume BR(H+) = 1 i.e. MSSM with high tan() “Worst” Limit Find arbitrary combination of H+ BR’s that give least stringent limit Less Model Dependent Limit K. Lannon

  30. t’ Production • Consider possible contribution to “top” sample from heavier particles with “top-like” signature (t’) • Examples • 4th chiral generation consistent with precision EWK data [Phys. Rev. D64, 053004 (2001)] • “Beautiful Mirrors” Model: additional generation of quarks that mix with 3rd generation [Phys. Rev. D65, 053002 (2002)] • Consider decay of t’Wq • Happens when mt’ < mb’ + mW • Precision EWK data suggests mass splitting between b’and t’ small • Search for by fitting HT vs Mreco • HT = sum of transverse momenta of all objects in event • Mreco = Wq invariant mass reconstructed with a 2 fitter (same technique used in top mass reconstruction) K. Lannon

  31. t’ Search Results • No evidence for t’ observed • Set 95% confidence level limits on t’BR(t’Wq)2 • Exclude mt’ < 258 GeV for BR(t’Wq) = 100% • Interesting behavior in high mass tails K. Lannon

  32. Summary Top Quark Lifetime (~10-24s in SM) Result: c < 52.5 m at 95% confidence level Consistent with detector resolution. Resonant top production: No evidence seen Exclude Leptophobic Z’ with Mz’ < 725 GeV/c2 Top Mass measured to 2.4 GeV/c2 (1.4%) uncertainty! W Helicity in Top Decay: SM: F0 = 0.7, F- = 0.3, F+ = 0.0 Result: F0 = 0.610.13, F+ < 0.09 • Even More results on the public webpage http://www-cdf.fnal.gov/physics/new/top/top.html • No deviations from Standard Model so far • Many results statistically limited • More results with 1-1.2 fb-1 coming soon • Results for ~2fb-1 by this summer There are many more CDF results than I could show here. • Many new and updated analyses in progress • Improved cross section measurements • Single-top • Top charge • Flavor changing neutral currents • Direct search for tH+b http://www-cdf.fnal.gov/physics/new/top/top.html K. Lannon

  33. The Future: Top at LHC Looks a little like B physics at the Tevatron • “Top physics will be easy at the LHC” • Top cross section increases by factor of ~ 100 • Background cross sections increase by factor of ~10 • Probe for new Physics • Mtt distribution • Associated Higgs production: ttH • Even used for LHC detector calibrations • High precision results from Tevatron important • Discover new physics • ~ 1-2 GeV/c2 precision on mass • Production and decay well understood precision physics K. Lannon

  34. Extra Slides K. Lannon

  35. Top Cross Section vs Mass K. Lannon

  36. Search for Resonant Production • Motivation • Some models predict particles decaying to top pairs • Should be visible as resonance in tt invariant mass spectrum • Example model: Topcolor assisted technicolor • Extension to technicolor that includes new strong dynamics • Couples primarily to 3rd generation • Includes new massive gauge bosons: topgluons and Z’ K. Lannon

  37. Search for Resonant Production • Look for generic, spin 1 resonance (X0) decaying to top pairs • Assume X0 = 1.2%MX0 • Test masses between 450 GeV and 900 GeV in 50 GeV increments • Results • No evidence for resonance • Set 95% confidence level limit for X0 at each mass • Exclude leptophobic Z’ with Mz’ < 725 GeV K. Lannon

  38. W Helicity in Top Decay W+ Right-Handed fraction F+ 0 -1/2 +1 +1/2 W +1/2 +1/2 b W W t t t b b W -1/2 +1 +1/2 • Helicity of W determined by V-A structure of EWK interaction • 70% longitudinal • 30% left-handed • Right handed forbidden V-A Forbidden W-Left-Handed fractionF- W0 Longitudinal fraction F0 K. Lannon

  39. W Helicity in Top Decay • Can be tested by measuring W helicity angle: * • *= angle of the lepton relative to negative the direction of the top in the W rest frame. • Can also use Mlb2  0.5(mt2-mW2)cos * K. Lannon

  40. W Helicity Results • Two CDF results with 955 pb-1 • Use different kinematic fitters to reconstruct tt system: cos* • Very consistent measurements of F0 and limits on F+ • F0 = 0.61 0.12(stat)  0.04 (syst) and F+ < 0.11 at 95% C.L. • F0 =0.59  0.12(stat) +0.07-0.06(syst) and F+ < 0.10 at 95% C.L. • One measurement with 750 pb-1 • Uses Mlb and measures fraction of V+A • FV+A < 0.29 at 95% C.L. • Assuming F0 = 0.7 F+ < 0.09 at 95% C.L. K. Lannon

  41. Top Quark Lifetime • Measure impact parameter of lepton from Lepton + Jets top decay • Evidence of displaced top suggests • Production via decay of long-lived particle • New long-lived particle in top sample • Anomalous top lifetime Templates for SM processes Result: c < 52.5 m at 95% confidence level K. Lannon

  42. Sample Composition Number of events with an identified W +  1 jets Event count before applying b-tagging W+light flavor: From pretag using mistag matrix W+heavy flavor: From pretag using MC for HF fraction and b-tagging eff. Difference between observed and predicted background attributed to top Single Top and Diboson: Estimated using theoretical cross section Non-W QCD: Estimated from MET and lepton isolation side-bands K. Lannon

  43. The Search for Single Top • Standard Model • Rate  |Vtb|2 • Spin polarization probes V-A structure • Background for other searches (Higgs) • Beyond the Standard Model • Sensitive to a 4th generation • Flavor changing neutral currents • Additional heavy charged bosons • W’ or H+ • New physics can affect s-channel and t-channel differently Tait, Yuan PRD63, 014018(2001) K. Lannon

  44. Signal and Backgrounds Backgrounds Other EWK Single-top Signature tt e or : pT > 20 GeV : MET> 20 GeV Multi-jet QCD W + Heavy Flavor W + Light Flavor (Mistags) 2 jets: ET > 15 GeV,  1 b-tag Must use multivariate, kinematic techniques to separate signal from background Total Background: 64696 events Expected Single-Top: 28  3 events Signal / Background ~ 1/20 K. Lannon

  45. Multivariate Discriminants • Improve signal discrimination by combining several variables into a multivariate discriminant • Neural Network and multivariate likelihood function both used • Variables: ℓb and dijet invariant masses, HT, Q, angles, jet ET and , W-boson , kinematic fitter quantities, NN b-tag output ZOOM K. Lannon

  46. Single Top Multivariate Likelihood Result • Best fit result for s- and t-channel separately • s-channel: • t-channel: • 95% CL upper limit on combined s- + t-channel: K. Lannon

  47. Combined search: s-channel + t-channel combined in SM ratio Best fit: 95% CL Limit: Separate search s- and t-channel vary separately Best Fit: t-channel: s-channel: 95% CL Limit: t-channel: s-channel: Single Top Neural Network Result K. Lannon

  48. Single Top Matrix Element Result Best fit result: K. Lannon

  49. Summary • This is an exciting time to be at the Tevatron • 1.2 fb-1 sample currently in hand and being analyzed • Top sample has grown from ~30 events in Run I to ~ several hundred • Larger samples coming soon (almost 2 fb-1) by summer • Analysis techniques becoming increasingly mature and sophisticated • Look forward to  1 fb-1 publications this winter • No evidence for new physics in top sample so far • Have many more top measurements than covered in this talk (see CDF public results webpage) • Increasing precision continues to test consistency of measurements in different channels • Many new analyses on their way (as well as updates of current results) • Improved cross section measurements • Single-top • Top charge • Flavor changing neutral currents • Direct search for tH+b K. Lannon

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