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Measuring properties of top quark; Is it really top of SM?

Measuring properties of top quark; Is it really top of SM?. Yen-Chu Chen Institute of Physics Academia Sinica. Content. Introduction Production of top quark Properties of top quark: Understanding the top quark Or is it really the top of the Standard Model?. The Tevatron. Chicago. CDF.

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Measuring properties of top quark; Is it really top of SM?

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  1. Measuring properties of top quark;Is it really top of SM? Yen-Chu Chen Institute of Physics Academia Sinica National Tsing Hus Univ.

  2. Content • Introduction • Production of top quark • Properties of top quark: • Understanding the top quark • Or is it really the top of the Standard Model? National Tsing Hus Univ.

  3. The Tevatron Chicago CDF D0 Tevatron Ring ( ~4 miles)! Main injector • pp collisions at √s = 1.96 TeV! • Already 4 fb-1 recorded! • Expect to collect • 6 fb-1 by the end of 2009! • ( 8 fb-1 by the end of 2010) National Tsing Hus Univ.

  4. Muon stations Calorimeter COT Silicon detector National Tsing Hus Univ.

  5. Top quark physics  q' l  q _ Charge asymmetry Production Cross section W  Br(t→Wb)/Br(t→Wq) b Production mechanism t  P P Top spin Life time, decay width Top charge   X b t Top mass W  FCNC W helicity _  q' l  q Rare decays, charged Higgs, etc. _ National Tsing Hus Univ.

  6. Sample separation • Di-lepton (DIL) channel: • both W decay to leptons, • in practice uses only e,μ. • Lepton+Jets (LJ) channel: • one W decays to leptons, • the other decays to quarks. • Hadronic channel: • both W decay to quarks. e+, μ+, u, c W + _ _ e, μ, d, s t b _ _ e –, μ–, u, c W̅ _ _ e, μ, d, s _ t _ b National Tsing Hus Univ.

  7. Production of top quark • Production mechanism • Production cross section • Forward backward asymmetry National Tsing Hus Univ.

  8. Production of top quark • In the SM: 6.7 pb (mt = 175 GeV/c2) • Single top production: q t q q t-channel 1.98 ± 0.21 pb Vtb s-channel 0.88 ± 0.07 pb q b Vtb t b National Tsing Hus Univ.

  9. Production mechanism • Di-lepton events; using data of 2 fb-1 • =azimuthal correlation of the two leptons National Tsing Hus Univ.

  10. Fit result (DIL, 2 fb-1): Fgg = 0.53 +0.36-0.38 (+0.35-0.37(stat) +0.07-0.08(sys)) • Previous result (L+J, 1 fb-1): Fgg = 0.07± 0.14(stat)± 0.07(syst) National Tsing Hus Univ.

  11. DIL, 2.8 fb-1 Production cross section  = 6.67 ± 0.77(stat) ± 0.43(sys) ±0.39(lum) pb  = 7.81 ± 0.92(stat) ± 0.68(sys) ± 0.45(lum) pb National Tsing Hus Univ.

  12. National Tsing Hus Univ.

  13. National Tsing Hus Univ.

  14. Single top production National Tsing Hus Univ.

  15. Forward backward asymmetry • Afb = 0.17 ± (0.07)stat ± (0.04)syst • To be compared with theory: Afb = 0.04 ± 0.01 National Tsing Hus Univ.

  16. Properties of top quark • Lift time • Expected: ~10-25 sec • Decay width • Decay mode • Mass and its implication • Expected: none, a free parameter in the SM • W helicity in top decay • Expected: 70% LH, 30% Lo, ~0 RH • Charge • Expected: +2/3 National Tsing Hus Univ.

  17. Life time ct < 52.5 m @ 95% CL National Tsing Hus Univ.

  18. Decay width top < 12.7 GeV @ 95% CL National Tsing Hus Univ.

  19. Decay mode • Br(t→Wb)/Br(t->Wq) = 1.12 +0.21–0.19(stat) +0.17-0.13 (sys) (160 pb-1) • Br(t→Zq) < 3.7% (1.9 fb-1) • Br(t→Zc) < 13% (1.9 fb-1) • Br(t→gc) < 12% (1.9 fb-1) • Br(t→c) < 18% (1.9 fb-1) National Tsing Hus Univ.

  20. Mass of the top quark Tevatron Top mass, 2007 Mar. • Reaching below 1% uncertainty! • CDF and D0 are working together on the common systematic issues to reduce uncertainty. National Tsing Hus Univ.

  21. MTM method (CDF, 2.7 fb-1) • log Lsig(mt, JES) = Σi[log Li(mt, JES)] - nbg log Lavg(mt, JES | background) National Tsing Hus Univ.

  22. mt = 172.2 ± 1.0 (stat.) ± 0.9 (JES) GeV/c2 National Tsing Hus Univ.

  23. National Tsing Hus Univ.

  24. W helicity in top decay • (1/Γ) (dΓ/dcosθ*) = f- (3/8) (1- cosθ*)2 + f0 (3/4) ( 1 - cosθ* 2 ) + f+ (3/8) ( 1 + cosθ* )2 , • θ* is the decay angle of lepton in the W rest frame with respect to the W direction in the top rest frame • f0 = 0.703, f-=0.297, f+=3.410-4 National Tsing Hus Univ.

  25. National Tsing Hus Univ.

  26. f0 = 0.65 ± 0.19 ± 0.03 • f+ = -0.03 ± 0.07 ± 0.03 National Tsing Hus Univ.

  27. National Tsing Hus Univ.

  28. Combination in CDFfor W helicity studies • f0 = 0.62 ± 0.11 assuming f_+ = 0.0 (the SM value) f+ = -0.04 ± 0.05 assuming f_0 = 0.7 (the SM value) • f0 = 0.66 ± 0.16 and f+ = -0.03 ± 0.07, correlation coeff = -0.82 with no assumption National Tsing Hus Univ.

  29. Charge of the top quark • The particle we have been called “top” for more than ten years could be a top like quark from an exotic model. • D. Chang, and E. Ma, Phys. Rev. D58, 097301 (1998). • D. Chang, W.-F. Chang, and E. Ma, Phys. Rev. D59, 091503 (1999); • The only way to tell is the charge of the top quark, all the other properties are the same: • SM : +2/3, XM : -4/3 National Tsing Hus Univ.

  30. To get result of top chargein DIL samples • Study top charge based on • jet charge calculation and • two kinds of pairing methods: MlbsMaxand KIN • Using single b tagged samples to reduce the background. • Taking the part of not b tagged samples as control region. (Higher contamination, lower purity) National Tsing Hus Univ.

  31. Using jet charge to identify b flavor- common in both LJ and DIL - Pjet , α = 0.5 pi Qi Additional tracks from the particles interacting with the detectors could cause bias, since they are mostly protons and ! => pt > 1.5 GeV/c,|d0| < 0.15 cm National Tsing Hus Univ.

  32. b quark charge from jet • Assume jet charge symmetry: • If > 0 → ; < 0 → • Purity of identifying jet charge correctly: • PYTHIA, Top mass 175 GeV/c2 (ttop75) : • = 0.601 ± 0.007; = 0.614 ± 0.007 (DIL) National Tsing Hus Univ.

  33. Jet charge from di-jet ( trigger) data • Calculate of the away jet: • Nos : number of opposite charge sign jets with respect to the lepton • Nss : number of same sign jets • Found b fraction in jets. • There are also c decay, b mixing and background. National Tsing Hus Univ.

  34. Purity in di-jet data analysis is consistent with jet charge from Top MC. • Take purity from MC study and apply the scale factor (SF) to take care of the difference between data and MC. • SF = 1.01 ± 0.01 ± 0.03 • No difference seen between jet charge from different muon charge sign samples! National Tsing Hus Univ.

  35. Pairing of lepton and b in DIL • MlbsMax : • Calculate the invariant mass of lepton + jet paired • For wrong pairing this value tends to go larger than the correct pair. • Reject the pairing having the largest Mlb2 of the four possible pairs. • Make cut on the maximum Mlb2 to increase the purity. • KIN : • Take the top mass as input and resolve the kinematic equation set. National Tsing Hus Univ.

  36. Pairing using MlbsMax Efficiency • correct pairing • wrong pairing Mlbs max cut Purity Mlbs max cut National Tsing Hus Univ.

  37. Efficiency and purity of MlbsMax • Pythia MC • Based on ttop75, top mass 175 GeV/c2 • Mlb2 max cut at 21K • Efficiency = 0.395 ± 0.004 • Purity = 0.948 ± 0.002 National Tsing Hus Univ.

  38. Pairing using KIN • Assuming top mass to resolve the equation set of energy momentum conservation. • Details described in: • CDF note 8638, page 51. • Presentation at top property meeting, 2008/08/22, Yen-Chu Chen National Tsing Hus Univ.

  39. Brief review of KIN pairing • For given lepton and jet momenta, try to resolve the equation set via Newton’s method. • Multiple solutions could be found • Pick the one having smallermtt2 • Two possible way of pairing • Pick the one having higher reconstruction probability • Probability of miss-reconstruction is higher in cases similar to RH W helicity than the cases similar to LH W helicity. • The nature is mostly LH (30%) and LO (70%). National Tsing Hus Univ.

  40. Efficiency and purity of KIN National Tsing Hus Univ.

  41. MlbsMax vs. KIN Choose MlbsMax as nominal and KIN for cross check! National Tsing Hus Univ.

  42. Study of top charge, MC only • Basic formula to get final top charge : • Final form: Ptq = fs * Ps + fnb * Pnb + fLJ * PLJ + fbk * Pbk The Four components: • Signal: true (l, b) pair from top decay • None b: the jet used in pair is not b • LJ faking DIL: L+J in HEP but identified as DIL • General DIL background, • FAKE, DY, WW, WZ, ZZ National Tsing Hus Univ.

  43. Event/pair estimation National Tsing Hus Univ.

  44. Efficiencies of jet charge cal. • Jet charge calculation requires good tracks. The efficiencies of applying jet charge calculation are studied based on MC for top signal and various background samples respectively. • The jets being b tagged or not do make difference in term of efficiency. National Tsing Hus Univ.

  45. Estimation of (l, b) pair of top(b tagged, jet q applied) National Tsing Hus Univ.

  46. MlbsMax + jet chargenot b tagged samples These samples are dominated by backgrounds. • Using jets that are b matched. • None b fraction = 0.61 • Assumed values Ps(sig+LJ) = 0.570 ± 0.006 Pbkg (non-b + bkg) = 0.502 ± 0.012 National Tsing Hus Univ.

  47. MlbsMax + jet chargeb tagged samples • Using jets that are b matched. • None b fraction = 0.38 • Assumed values Ps(sig+LJ) = 0.586 ± 0.008 Pbkg (non-b + bkg) = 0.55 ± 0.07 National Tsing Hus Univ.

  48. Systematic uncertainties of MlbsMaxon b tagged samples Ptq (Comb) = 0.584 ± 0.004 (stat) +0.008-0.010(sys) National Tsing Hus Univ.

  49. KIN + jet chargenot b tagged samples • Using jets that are b matched. • None b fraction = 0.59 • Assumed values Ps(sig+LJ) = 0.546 ± 0.004 Pbkg (non-b + bkg) = 0.501 ± 0.009 National Tsing Hus Univ.

  50. KIN + jet chargeb tagged samples • Using jets that are b matched. • None b fraction = 0.37 • Assumed values Ps(sig+LJ) = 0.545 ± 0.003 Pbkg (non-b + bkg) = 0.543 ± 0.066 National Tsing Hus Univ.

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