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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 Turned ~10 years old last year 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 • Turned ~10 years old last year • 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? • 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 • It’s not Standard Model top at all! • Spin not 1/2? • Charge not 2/3? • Not likely . . . . • It’s not only Standard Model top • Additional heavy particles decaying to high pt leptons, jets and missing energy (t ’) • Heavy resonance decaying to tt • tH+b • ttH production 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 = 2.29E32 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: ~2.0 fb-1 • Total recorded: ~1.6 fb-1 (~ 15 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 fall and winter • Doubling time: ~1 year • Future: ~2 fb-1 by 2006, ~4 fb-1 by 2007, ~8 fb-1 by 2009 Peak Luminosity Integrated Luminosity at CDF Today’s Presentation: 200 pb-1 ~ 1 fb-1 K. Lannon

  6. Triggering on Top • Need high efficiency, low fake rate trigger for high pT leptons • Relies on track trigger XFT • Tevatron bunch spacing • Original plan: switch from 396 ns to 132 ns at high lum • Now: staying at 396 ns • 3x more interactions per crossing! • ~ 9 interaction per crossing at highest luminosity 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 passed final review last week • Efficiency = 97% 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 • Advanced analysis techniques • Artificial Neural Networks • b-tagging 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) • Consistency • Overall combination has 2 of 4.96 for 5 degrees of freedom • Only Vertex Tag and ANN: 2 of 2.91 for 1 degrees of freedom • 7% probability of getting worse agreement • Not statistically significant yet, but keep watching! 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 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

  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 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. 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

  31. 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

  32. Current Results • Current limits are getting close to SM expectation • Apriori projections (ignoring systematics) • 2.4  excess with 1 fb-1 • 3  excess around 1.5 fb-1 • 1 fb-1in progress • Stay tuned! Result with 695 pb-1 Statistical errors only Based on SM single-topcross section K. Lannon

  33. 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

  34. 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

  35. Summary • This is an exciting time to be at the Tevatron • 1 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 • 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

  36. The Future: Top at LHC • “Top physics will be easy at the LHC” • Top cross section increases by factor of ~ 100 • Background cross sections increase by factor of ~10 • Top used for LHC detector calibrations • Searches for new physics • Mtt distribution • Associated Higgs production: ttH • Above assumes top quark holds no “surprises” at the LHC • Precise understanding of top properties and mass will speed the process • If there’s more than top in the top sample . . . . • Test and validate ability to model backgrounds K. Lannon

  37. Extra Slides 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. 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

  43. 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

  44. 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

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