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Searches for New Physics in the Top Quark Samples at CDF

Searches for New Physics in the Top Quark Samples at CDF. Luca Scodellaro Instituto de Fisica de Cantabria For the CDF Collaboration. HADRON07 XII International Conference on Hadron Spectroscopy October 8 th -13 th , 2007, Frascati (Italy). Motivation s. Top quark is really massive

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Searches for New Physics in the Top Quark Samples at CDF

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  1. Searches for New Physics in the Top Quark Samples at CDF Luca Scodellaro Instituto de Fisica de Cantabria For the CDF Collaboration HADRON07 XII International Conference on Hadron Spectroscopy October 8th-13th, 2007, Frascati (Italy)

  2. Motivations • Top quark is really massive • Lot of top properties still to be tested • Samples getting larger and well understood • Yukawa coupling to Higgs near unity • Special role in EWSB? • Sensitivity to physics beyond SM? • Top charge, width, couplings • Production mechanism • Direct searches for new phenomena

  3. The Tevatron and CDF Integrated Luminosity at CDF • Proton-antiproton collider -s = 1.96 TeV -Record luminosity 2.9x1032cm-2s-1 - Aim to collect 6-8 fb-1 by 2009 • Only top factory in the world Muon chambers • Tracking system - charged particles and b-tagging • EM and HAD calorimeters - electrons and jets • Muon chambers Calorimeters Tracking system

  4. Top Production at the Tevatron - - • Pair production via strong interaction dominant sNLO (pp  tt)  6.7± 0.8 pb for mt = 175 GeV/c2 Cacciari et al. JHEP 0404:068 (2004) Kidonakis & Vogt PRD 68 114014 (2003) • Decay via electroweak interaction BR(tWb) ≈100% • Observed final states given by W boson decay } Dilepton: tt  lνlνbb (5%) Lepton+jets: tt  lvqqbb (30%) Hadronic: tt  qqqqbb(45%) l=electron or muon

  5. Top Quark Samples at CDF • Dilepton sample • Lepton+jets sample • About 1-1.7 fb-1 analyzed • Cross section measured • Two high-PT isolated leptons • Two high-ET jets • Large missing transverse energy • One high-PT isolated lepton • ≥ 4 high-ET jets (≥1 b-tagged) • Large missing transverse energy • Good agreement with SM prediction • Sample composition well understood

  6. q’ - q b W- - t - p p t b W+ ν What Can We Test? • top production mechanism - • top properties - - - • (ggtt)/(pptt) • forward-backwardasymmetry • top mass • top charge • top width • new particles • top couplings - • tt resonances • massive t’ • heavy W’ • V+A weak coupling • Flavor Changing Neutral Current l+

  7. Top Quark Properties

  8. New!!! Top Quark Mass • Challenging measurement -final state reconstruction - jet energy measurement - neutrinos undetected • Best world measurement lepton+jets channel: mt = 172.7 ± 2.1 GeV/c2 • Tevatron combination March’07 mt = 170.9 ± 1.8 GeV/c2

  9. Constraints on Higgs Boson Mass • Top mass (together with W) mass measurements allow to constrain Higgs boson mass through radiative corrections • Electroweak fit to SM Higgs boson mass using latest Tevatron combination: MH=76+33-24 GeV/c2 MH<144 GeV/c2 @ 95% CL

  10. Top Charge: +2/3 or -4/3? • Suggestions for a fourth generation quark with Q=-4/3 D. Chang et al.PRD 59 091503(99) • Top charge reconstructed from decay product tWb • Observed charge distribution tested vs Q=+2/3 and Q=-4/3 hypotheses • W charge from leptonic decay W±l±v • b flavor from charged tracks information in jet • data consistent with a SM top hypotesis • exotic quark excluded at 87% CL

  11. Top Quark Width • Top lifetime in SM extremely short (τt~ 4x10-25s) • Top mass spectrum sensitive to top width: templetes fit to extract t • Compare tfit to results from pseudo-experiments • t < 12.7 GeV @ 95% CL • τt>5.2x10-26 s @ 95% CL

  12. Top Production Mechanism

  13. - - - σ(gg→tt)/σ(pp →tt) - • Fraction of tt events from gluon fusion provides - test perturbative QCD - sensitivity to new top production and decay • First idea: radiation from gluons tends to carry low P fraction ~85% ~15% • Low-PT track multiplicitymeasured in no-gluon and gluon-rich samples • Fitting data distribution to <NTRK> templates - - - (ggtt)/(pptt) = 0.07 ± 0.16

  14. - - - σ(gg→tt)/σ(pp →tt) • Second idea: top-antitop spin correlation - like-sign in gluon fusion production -unlike-sign in qq annihilation • Neural Network approach: - 2 kinematic variables describing the production in tt frame - 6 angles between decay products → spin correlation • Fitting data to NN output shapes - - - - - (ggtt)/(pptt)<0.33 @ 68% CL

  15. Forward-Backward Asymmetry • Production diagrams interference Nt (cos ) – Nt (cos ) Nt (cos ) + Nt (cos ) • 4-6% asymmetry expected at NLO • Use ΔY*Ql in lepton+jets sample: AΔY*QNLO~4-7% - ____________________ Afb = - Afb = 28± 13(stat) ± 5(syst) %

  16. Searches for Non-SM Top Couplings

  17. +1 +1/2 -1/2 W 0 t +1/2 b W +1/2 t W b t -1/2 W b +1 +1/2 Right-Handed Weak Coupling • V+A coupling signal of new physics - Left-right symmetry, PRL 38,1252(1977) - Mirror particles, J.Phys.G 26,99 (2000) - Beautiful mirrors, PRD 65,53002,(2002) • V-A character of top decay tested by W helicity W0 longitudinal fraction F0 = 0.7 W- left-handed fraction F- = 0.3 W+ right-handed fraction F+ = 0.0 Suppressed in SM

  18. W Helicity in Top Decay • Two variables sensitive to W helicity in top decays • cos* = angle between lepton and top in W rest frame: - Left-handed ~ (1- cos*)2 - Longitudinal ~ (1- cos2*) - Right-handed ~ (1+cos*)2 • Mass of the charged lepton- b quark system: - Mlb2  (mt2 - mW2) cos * _ 3 8 3 _ 4 _ 3 8 _ 1 2

  19. W Helicity Results • Different fitting techniques in cos* analysis • Binned fit to theoretical shapes: - f0 = 0.38 ± 0.22 (stat) ± 0.07 (syst) f+= 0.38 ± 0.22 (stat) ± 0.07 (syst) - f+ < 0.12 @ 95% CL(f0 = 0.7fixed) • Unbinned fit to Monte Carlo shapes: -f0 = 0.61 ± 0.20 (stat) ± 0.03 (syst) f+= -0.02 ± 0.08 (stat) ± 0.03 (syst) -f+ < 0.07 @ 95% CL(f0 = 0.7fixed) • Using Mlb to measure fraction of V+A: - f+< 0.09 @ 95% CL(f0 = 0.7 fixed)

  20. FCNC: t  Zq • In SM BR(t  Zq) = O(10–14) • BSM scenarios allow BR up to O(10–2) - J.A.Aguilar-Saavedra, Acta Phys.Polon.B35, 2695-2710 • LEP limit BR(t  Zq)<13.7% • Counting experiment tt  Zq Wb - Z  l+l- - W  qq’ - 2 of kinematic fit to top mass best discriminator • No excess observed BR(t  Zq) < 10.6% @ 95% CL - -

  21. Searches for New Particles

  22. - tt Resonances - • Several models suggest tt resonant production: • Fit to reconstructed tt mass Model: narrow Z’ (Z’~1.2% MZ’) no interference with s-channel tt • No deviation from SM observed • Extended gauge theories - A.Leike, Phys.Rep.317,143(1999) • KK states of gluons or Z - B.Lillie et al.,arXiv:hep-ph/0701166 - R.Rizzo,Phys.Rev.D61,055005(2000) • Axigluons and Topcolor - L.Sehgal et al.,Phys.Lett.B200(1988) - C.Hill et al.,Phys.Rev.D49,4454(1994) - - - σ(Z’→tt) < 0.7 pb @ 95% CL for MZ’ > 700 GeV/c2

  23. Search for Massive t’ → Wb • 4th generation of heavy fermions compatible with EWK data • 2-dim likelihood fit to data • N=2 SUSY models: H.J.He et al., arXiv:hep-ph/0102144 • “Beautiful mirrors” model: C.Wagner et al., arXiv:hep-ph/0109097 • HT: sum of transverse energy of all objects in final state • Mreco: Wb reconstructed mass • No evidence for t’ observed • Set 95% CL upper limit on (pp→t’t’) x BR(t’→Wb)2 - - • mt’>256 GeV/c2 @ 95% CL

  24. Heavy W’ Production • Search for W-like heavy boson decaying to a top-bottom pair • Single top production signature: • No evidence for W’ boson in reconstructed Mwbb mass spectrum • Limits on W’ production and its couplings to fermions • Extra dimensions: PRD 74, 75008(2006) • Little Higgs: arXiv:hep-ph/051212 • Topcolor: PL B385, 304(1996) • Charged lepton • Large missing energy • Two jets σ∙BR(W’→tb)<0.4 pb @ 95% CL for MW’ > 700 GeV/c2

  25. Conclusions - • The CDF top quark samples in 1-1.7 fb-1 of pp collision data at the Tevatron have been established and well understood • Lot of precision measurements and first results in searches for new physics have been achieved • Beginning to have sensitivity to unexpected top quark properties and new phenomena in our samples

  26. Backup Slides

  27. b Quark Tagging • Tagging the b quark improves parton-jet assignments: lepton+jets: 1 tag → 6 combinations, 2 tags → 2 combinations • Requiring a b tagged jet also reduces background contamination • B hadrons are long lived Jet tagging efficiency → 42% False tag rate → 0.5%

  28. Best Top Mass Measurement • Matrix Element Method with in situ jet calibration • Build event likelihood as a function of mt and JES • Sum over events and background subtraction logLtot(mt,JES)= ( logL(yi|mt,JES) – fbg(yi)logL(bg|mt,JES) ) • Top mass extracted by maximizing profile likelihood Lprof (mt) = max Ltot(mt, j) Integration over phase space Normalization factor Matrix element Sum over parton jet assignament b-tag weight Transfer function connecting partons to jets PDFs Acceptance Σi jєJES

  29. Kinematic Fit • Lowest 2 parton-jet assignamnt used in analyses • Correct combination in 70% of the cases Fluctuate momenta according to their resolution - t and t have the same massMt Constrain on W masses

  30. Search for t → H+b - • SMMS predicts large BR(t→H+b) (>10%) for small and large β • Four different channels considered: H+ → τν, cs, t*b, W+h0 • Compare predictions to data to set limits on MH - tan(β) plane • Scan all combinations of H+ BR’s to set model independent limits on BR(t→H+b) - - +

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