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CERN EP Seminar October 16, 2006

Measurement of the top quark B(t →Wb ) and electric charge with the D  detector Christophe Clément (CERN). CERN EP Seminar October 16, 2006. Top quark. t. Kinematics in -lepton data in Mark I Discovery of the Υ at FNAL GIM mechanism  b-quark in isodoublet!  t-quark must exist.

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CERN EP Seminar October 16, 2006

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  1. Measurement of the top quark B(t→Wb) and electric chargewith the D detectorChristophe Clément (CERN) CERN EP Seminar October 16, 2006

  2. Top quark t • Kinematics in -lepton data in Mark I • Discovery of the Υ at FNAL • GIM mechanism  b-quark in isodoublet! •  t-quark must exist • What Mass for top? [b/c/s = 4.5/1.5/0.5  Mtop = 15 GeV?] • Discovered at ~175 GeV (!) in 1995 by CDF and D0 • Is there anything else special about the top quark? • Does it have predicted properties, standard model quantum numbers? October 16, 2006 C.Clément CERN EP seminar 2/39

  3. Is there anything special about the top quark? • High mass  potentially important role for top quark • Yukawa coupling to Higgs: • New physics in EW symmetry breaking sector could be reflected in top quark properties. • Top decays in ~0.5×10-24 s, before hadronizes • Decays as a free quark: spin correlation, W-helicity in top decays... Heavy enough to decay to exotic particles (H+, W’…) Same experimental signature as some exotic particles, background to supersymmetry 171 GeV 0.98 October 16, 2006 C.Clément CERN EP seminar 3/39

  4. Tevatron p-pbar √s=1.96 TeV, L~1032cm-2s-1 Top at Tevatron Run I (√s=1.8 TeV L~1030cm-2s-1) 1995 CDF and D discover top quark Full Run I sample ~120pb-1 / experiment A few dozens of events (l + jets), dilepton channels ≤10 events Mass known ~3% Cross section ~25% B(t->Wb) ~30% Run II (√s=1.96 TeV L~1032cm-2s-1) Already >1 fb-1/experiment on tape Mass known to 1.2% (hep-ex/060832) Cross section ~10% -final states observed Precise tests of production and decay mechanisms Does it have SM quantum numbers? October 16, 2006 C.Clément CERN EP seminar 4/39

  5. Strong top quark production Strong production  pairs Electroweak production  single top • So far only observed in pairs produced via QCD • Cross section derived from pQCD (from mtop and √s) Phys. Rev. D68, 114014 (2003) σtt=6.8±0.4pb (theory) for p-pbar @ √s = 1.96 TeV LHC: p-p collisions @ √s = 14TeV dominated by gg processes σtt~ 833 pb (theory) October 16, 2006 C.Clément CERN EP seminar 5/39

  6. t-channel, ”tqb” σt~2.0 pb s-channel, ”tb” σs~0.9 pb Electroweak Top Production • Allows to measure directly |Vtb|, σs, σt∝|Vtb|2 • Experimentally challenging • because not too different from W+2 jet background • Use events with W→ev or W→v • Signature: 1 isolated lepton, MET, 2 or more jets • s-channel: 1,2 b-tagged jets t-channel: 1 b-tagged jet + 1 light jet • Major backgrounds: W+jets, , fake leptons • Not yet observed! October 16, 2006 C.Clément CERN EP seminar 6/39

  7. In SM top decays via V-A charged current ⇒ Mostly left handed b-quarks in the decay • 3 quark generations + direct measurements of Vub and Vcb predict Vtb,~1  B(t→Wb)~1 • SM predicts FCNC decays are tiny, t→Wq is dominant Top Quark Decay October 16, 2006 C.Clément CERN EP seminar 7/39

  8. All hadronic b t W q t W q q’ Lepton+jets q’ b b Dilepton t W q b t W l t q’ W l t W l v b v v b Top Pair Final States • How top quark was discovered! • Used for most measured top quark properties so far ”golden channel” e+jets & +jets ~32% 44% ee+e+ ~5% October 16, 2006 C.Clément CERN EP seminar 8/39

  9. Vtb unconstrained without 3x3 unitarity constraint B(t→Wb) • B(t→Wb)=1 usually assumed by CDF and D analyses • B(t→Wb) might deviate from unity: • Additional quark singlets or doublets • ”Pollution” of top sample by non-top process! • Non-SM processes in the production • Non-SM in the decay (H+,...) • Experimentally B(t→Wb) affects number of b-jets  need to experimentally discriminate b/w t→Wb and t→Wqlight October 16, 2006 C.Clément CERN EP seminar 9/39

  10. Missing transverse energy One high pT isolated lepton W t t b-jets? light jets? W light, c-jets Deriving B(t→Wb) experimentally.... • Select a top-enriched sample • e+jets and +jets channel • larges statistics, good S/B • # events with 0, 1 and ≥2 b-jets • B(t→Wb) • b-tagging efficiency • Jet identification efficiency • Probability to tag background October 16, 2006 C.Clément CERN EP seminar 10/39

  11. Lepton + jets sample composition • . • W+jets • Z+jets • WW, WZ, ZZ • single top • multijet True isolated lepton processes Ntrue fake isolated lepton processes Nfake • Fake isolated electron • Jets with leading /πo, convertions, γwith random tracks,... • Fake isolated  • inside jets from heavy flavor or in flight decays • Determine Ntrue , Nfake on a statistical basis • Two lepton ID criteria • loose ⊃ tight lepton • P(tight | loose) for fake and true leptons October 16, 2006 C.Clément CERN EP seminar 11/39

  12. Lepton + jets sample composition • . • W+jets • Z+jets • WW, WZ, ZZ • single top • multijet • Small • derived from MC • Lepton, jet efficiencies • calibrated on data • σfrom data or NLO • Nother Ntrue Nfake Nbefore tag= Ntt + NWj + Nfake + Nother Nn-tags = Pntt (B(t→Wb)) Nntt + PnWj Nnwj + N’nfake + Pnother Nnother n-tags = 0, 1, 2 Tagging probability Fit B(t→Wb), Ntt, Nwj to the Nn-tags October 16, 2006 C.Clément CERN EP seminar 12/39

  13. Lxy= How to identify b-quark jets... • Explicitely reconstruct displaced secondary vertices: Secondary Vertex Tagger • 1. Taggable jets are: • Calorimeter jets with pT>15 GeV, ||<2.5 • ΔR(calo jet, track jet)=0.5, • ≥ 2 tracks in ΔR=0.5, Δz<2cm • ≥ 1 hit in the innermost tracking detector, pT>0.5 GeV • ≥ 1 track with pT>1 GeV • Decouple b-tagging from experimental issues • 2. Tagged jets: • Are taggable jets • Contain a SV, χ2 • Lxy > n σLxy • Similar algorithm used by CDF Taggability, tagger independent B-tagging efficiency October 16, 2006 C.Clément CERN EP seminar 13/39

  14. b-tagging efficiency From di-jet data: extract b-tagging efficiency for muonic b-jets We need the b-tagging efficiency for ”all kinds of b-jets” b→, data b See for example Phys. Rev. D71, 052003 (2005) Pbtag(ET,) = b,MC ------------------b→, data Taggability  Ctaggability(b) b→,MC Transform semi-muonic b-tag efficiency into inclusive one October 16, 2006 C.Clément CERN EP seminar 14/39

  15. ttbar → l+jets Take into account ”contamination” by → ll Pntt versus B(t→Wb) • Probability to see n-tags in events (Pntt ) depends on the number of b-jets • Pn tt = R2 Pn tag(tt→bb) + 2 R(1-R)Pn tag(tt→bql) + (1-R)2Pn tag(tt→qlql) October 16, 2006 C.Clément CERN EP seminar 15/39

  16. Prediction 7pb & B(t→Wb)=1 • Fit B(t→Wb) and Ntt simultaneously to 12 bins: e/+ 3/≥4 jets, 0/1/≥2 tags • B(t→Wb) is constrained by relative 0/1/≥2 tags populations • Very poor S/B in 0-tag sample, Nttbar~√Nobs October 16, 2006 C.Clément CERN EP seminar 16/39

  17. R=1.0, σ=7pb R=0.5, σ=7pb Illustration... Single Tag l+jets Jet multiplicity Double Tag l+jets Jet multiplicity October 16, 2006 C.Clément CERN EP seminar 17/39

  18. The 0-tag sample Low B(t→Wb) Large σtt Cross Section • Ntt ~ √N(0-tag) • Without further information on the 0-tag events low B(t→Wb) + large Ntt (σtt) can still be consistent with data • Use topological properties of events in 0-tag sample for addititional constraint of Ntt in the 0-tag sample. R Preliminary result from summer 2004 (170pb-1) No 0-tag sample used October 16, 2006 C.Clément CERN EP seminar 18/39

  19. Likelihood discriminant in 0-tag sample l + 4 jets before tagging 1. Sphericity S = 3(λ2+λ3)/2 , λ’s smallest eigenvalues of momentum tensor M (ttbar S~1) 2. K’Tmin K’Tmin = Rminjj/EWT with EWT = ElT + MET 3. Centrality C=HT/H , HT is scalar sum of jets ET and H is the sum of the jet energies. H’T2 H’T2 = HT2/Hz, HT2 : pT sum of all jets but leading jet, Hz is the scalar sum of all jets |Ez| plus |Ez| of the neutrino (W-assumption) October 16, 2006 C.Clément CERN EP seminar 19/39

  20. Likelihood discriminant in l+4jets 0-tag sample Data and prediction for 7pb and R=1 Nevents Data Likelihood discriminant output October 16, 2006 C.Clément CERN EP seminar 20/39

  21. Systematics on template shapes Some systematic uncertainties can affect the template shapes... • Systematics on template • shapes on ttbar→ l+jet • JES • JetID • Jet energy resolution • W-modeling (W+jets) • Taggability • Tagging probabilities for b, c • and light jets October 16, 2006 C.Clément CERN EP seminar 21/39

  22. ≥4 jets 3 jets ≥4 jets, 0 tag Results (230 pb-1) October 16, 2006 C.Clément CERN EP seminar 22/39

  23. Confidence contour plots in R, Ntt No 0-tag B(t→Wb) B(t→Wb) Phys. Lett. B 639 (2006) B(t→Wb) = 1.03+0.19-0.17 October 16, 2006 C.Clément CERN EP seminar 23/39

  24. Lower limit on B(t→Wb) and |Vtb| • Prior π(B(t→Wb))=0 outside [0,1] • Monte Carlo integration over 191 nuisance parameters associated to systematic errors • Provides a 2D p.d.f. for B(t→Wb) and Ntt • Limit on |Vtb| can be derived using |Vtb|=√B(t→Wb) (SM) 68% CL : B(t→Wb)>0.78 |Vtb|>0.88 95% CL : B(t→Wb)>0.61 |Vtb|>0.78 October 16, 2006 C.Clément CERN EP seminar 24/39

  25. OR ? Mtop~270GeV +2/3 -1/3 -4/3 t Q1 mixing b Q4 M(Q4)~175GeV Top Quark Charge Lift ambiguity present in all top analyses! t→W+b or ”t”→W-b Test exotic models... Phys.Rev. D65 (2002) 053002 October 16, 2006 C.Clément CERN EP seminar 25/39

  26. OR ? kinematic fit Qtop + = Ingredients What is the expected shape of Qtop for • SM top • ”top” with 4e/3 charge? October 16, 2006 C.Clément CERN EP seminar 26/39

  27. Analysis strategy Double tagged events • Discriminate between |Qtop| = 2e/3 and |Q”top”|=4e/3 • Two |Qtop| per event • Use the pure sample -- lepton+4≥jets events with 2 SVT • Compute the jet charge of the 2 b-tagged jets • Associate the b-jets to correct W boson (charged lepton) • Combine the 2 jet charges and the lepton charge to derive the 2 |Qtop| • Compare the observed |Qtop| with expected SM and exotic distributions S/B~10 October 16, 2006 C.Clément CERN EP seminar 27/39

  28. Algorithm: Jet Charge Algorithm pTi,qi Compute jet charge only for b-tagged jets • (2 per events) Jet charge = Optimizaton on MC gives a=0.6 Sum over tracks with • pT>0.5GeV, • ΔR(track, jet) <0.5 of the jet axis Derive expected shape of Qjet from data with minimal input from simulation October 16, 2006 C.Clément CERN EP seminar 28/39

  29. Charge of highest pT hadron Charge of b-quark Why does it work? The charge of the quark is correlated with the charge of the highest pT hadron resulting of the hadronization Simple study carried out with Pythia: Generated QCD 2→2 process, pT>15GeV And look at hight pT b quarks produced in the process. Usually large number of hadrons Produced, most quite modest pT Thís is then smeared by detector effects... The original b-quark not always In highest pT hadron October 16, 2006 C.Clément CERN EP seminar 29/39

  30. Tight di-jet sample Jet Charge Performance in Data • Tag and probe methodin ”pure” events In reality: • Is it pure ? ? flavor excitation? g→ ? • B→ • B →D →  • →  • B→ light hadrons → • Charge misidentification _ bb >3.0 Ideal case: sign of q = sign of qb _ bb _ cc _ bb ´ _ Bo→ Bo Charge flipping processes October 16, 2006 C.Clément CERN EP seminar 30/39

  31. Discriminant Power Data Calibration Corrections MC truth on tag side Tag and probe Method: data Tag and probe Method: Z→bb October 16, 2006 C.Clément CERN EP seminar 31/39

  32. - Is the triple tag sample pure ”bb”? • The fraction of c-jets in the triple tag sample is determined by pTrel fit of the order of a few percents, • Flavor excitation/ splitting? >3.0 2 b-jets back to back dominate Phys. Rev. D 65, 094006 (2002) October 16, 2006 C.Clément CERN EP seminar 32/39

  33. Tight di-jet sample - p.d.f.’s Qb, Qb, Qc, Qc from data... - - - P+ (Qjet) = (1-xc) (1-xflip)Pb (Qjet)+ (1-xc)xflip Pb + xc Pc _ cc Fraction of charge flipping processes 30±1% from MC, Cross checked on data Fraction of derived from pTrel spectrum of  (1+2-1%) p.d.f of Qjet in probe jet + Similar equation for P- (Qjet) 4 Unknown p.d.f’s Pb, Pb, Pc, Pc - - October 16, 2006 C.Clément CERN EP seminar 33/39

  34. - - P+ (Qjet) = 0.69 Pb (Qjet) + 0.30 Pb + 0.01 Pc P- (Qjet) = 0.30 Pb (Qjet) + 0.69 Pb + 0.01 Pc P´+(Qjet) = 0.567 Pb (Qjet) + 0.243 Pb + 0.19 Pc P´- (Qjet) = 0.243 Pb (Qjet) + 0.243 Pb + 0.19 Pc - - - - - p.d.f.’s Qb, Qb, Qc, Qcfrom data... - Tight di-jet sample loose di-jet sample • Correct for different • t→Wb and bb kinematics October 16, 2006 C.Clément CERN EP seminar 34/39

  35. qB qB qb qb SM Top Charge Observables • We need an observable and an expectation for the ”2e/3” and ”4e/3” scenarios • Consider only lepton+jets channel (e/µ + 4 jets) double-tagged events • Two top quarks in the event  measure the charge ”twice” • Use kinematic fit to assign b-jets to correct W-bosons in MC • qb and qB are taken from the data derived jet charge templates qb = b lept. side qB = b hadr. side October 16, 2006 C.Clément CERN EP seminar 35/39

  36. Exotic Top Charge Observables • qb and qB are taken from the data derived jet charge templates • The exotic scenario is obtained by permuting the charge of the SVT tagged jets qB = b hadr. side qb = b lept. side qB qB qb qb October 16, 2006 C.Clément CERN EP seminar 36/39

  37. Admixture of Q4 and t-quark - not excluded by - fQ4 <0.52 at 68% C.L. σtt Result on ~0.4 fb-1 of D data • 21 l + jets double tagged events • 16 events with converged kinematic fit  32 measured top charges • Perform likelihood ratio test b/w 4e/3 and 2e/3 hypothesis Exclude 4e/3 at 92%C.L. (91 % expected) hep-ex/060844 October 16, 2006 C.Clément CERN EP seminar 37/39

  38. Systematic uncertainties Jet charge specific Kinematic fit October 16, 2006 C.Clément CERN EP seminar 38/39

  39. Conclusions • Acknowledge collaborators: J. Strandberg and P. Hansson • Large data sets from CDF and D allow new fundamental measurements of the top quark properties • Development of analysis techniques: often analyses outperform expectation • First ”measurement of the top electric charge”... October 16, 2006 C.Clément CERN EP seminar 39/39

  40. Backup slides

  41. Top pair final states

  42. Ensemble tests Fitted versus true B(t→Wb) Coverage for B(t→Wb)

  43. Expected statistical errors Statistical error on R vs R Statistical error on σ vs σ

  44.   b-tagging efficiency from data   ”n-sample”: 1 jet with a   Not -tagged n -tagged n Not -tagged, SVT tagged nSVT -tagged, SVT tagged n ,SVT ”p-sample”: 2 b-2-b jets 1 jet with a    -tagged, SVT tagged p,SVT -tagged p Not -tagged, SVT tagged pSVT Not -tagged p

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