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Experimental aspects of top quark physics Lecture #3

Experimental aspects of top quark physics Lecture #3. Regina Demina University of Rochester Topical Seminar on Frontier of Particle Physics Beijing, China 08/15/05. Outline. Top charge W-helicity in top decays ttbar resonances Single top Control questions. Top charge.

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Experimental aspects of top quark physics Lecture #3

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  1. Experimental aspects of top quark physics Lecture #3 Regina Demina University of Rochester Topical Seminar on Frontier of Particle Physics Beijing, China 08/15/05

  2. Outline • Top charge • W-helicity in top decays • ttbar resonances • Single top • Control questions Regina Demina, Lecture #3

  3. Top charge • Electric charge is as fundamental quantum number as mass • SM leaves little options for the predicted value of top-quark charge (+2/3e) • Some beyond SM models suggest that the discovered “top quark” has actually a charge of -4/3e, while true top quark resides at ~270GeV and thus have eluded discovery • Chang,Chang,Ma PhysRevD59,091503(1999) • From the experimental point of view this measurement is rather challenging, so we could not resist Regina Demina, Lecture #3

  4. Top charge (the method) Main question: is how to determine the charge of the b-quark Or rather how to distinguish b-quark from anti-b Jet-charge algorithm • Rely on charge conservation • Qt=QW+Qb • Q(top)= - Q(antitop) • Q (leptonic W)=Q (lepton) • Q (hadronic W)= - Q (leptonic W) • Use double-tagged sample • Very good S/B • 2 b-jets/event • fewer permutations • B-jet to W is assigned using kinematic fitting procedure (HITFIT), which utilizes constraints due to • top mass, W-mass • total energy-momentum conservation (zero pT of ttbar system) • Use only the best solution from HITFIT Regina Demina, Lecture #3

  5. Jet charge algorithm • Total charge of the tracks within jet cone DR is correlated with the charge of the initial quark (b) • Track with pT>0.5 GeV/c and |ztrk-zPV|<0.1cm, charge qi • Jet charge is defined as Regina Demina, Lecture #3

  6. Jet charge algorithm • Optimize discriminating power: • ab mean, Vb- variance • Optimal values of a=0.6 and DR=0.5 were found using ttbar MC Regina Demina, Lecture #3

  7. Jet charge calibration on data • Use double-tagged muon+jet data – very clean bbar • Reconstruct jet charge on probe jet • Use correlations with muon’s charge for calibration Regina Demina, Lecture #3

  8. Jet charge • Performance on data must be corrected for • B-mixing (does not depend on pTRel) • Sequential decays of b-hadrons (depends on pTRel) • Contamination of ccbar (depends on pTRel) No public result yet… Regina Demina, Lecture #3

  9. W helicity in top decays In SM F-=0.30, F0 =0.70, F+ =0 CDF RunI result: F+<0.18(95%CL) Regina Demina, Lecture #3

  10. W helicity F+ <0.24 (90% CL) with tag F+ <0.24 (90% CL) LepPt: lep+jets + dileptons F0=0.27+0.35-0.24 F0 < 0.88 (95%cl) F0=0.89+0.30-0.34 (stat) _-0.17 (syst) F0>0.25 (95%CL) Regina Demina, Lecture #3

  11. ttbar resonances in l+jets with b-tag • Check ttbar invariant mass for possible resonance production DØ RunII Preliminary, 363pb-1 sNNLO(tt)=6.77±0.42 • Events are kinematically constrained • mT=175GeV/c2 • Leptonic and hadronic W masses Regina Demina, Lecture #3

  12. ttbar resonances in l+jets with b-tag • Limit M(Z’)>680 GeV/c2 with G=1.2%MZ’ at 95%CL DØ RunII Preliminary, 363pb-1 * *R. Harris, C. Hill, S. Parke hep-ph/9911288 Run I limit 560 GeV/c2 Run II limit 680 GeV/c2 Regina Demina, Lecture #3

  13. Kinematics in l+jets sample DØ RunII Preliminary, 363pb-1 Regina Demina, Lecture #3

  14. Single (weak) top production production production Vtb s-channel 0.88+0.07-0.06pb (NLO, mt=175GeV,sqrt(s)=1.96TeV) t-channel 1.98+0.23-0.18pb t-channel sensitive to new top couplings(FCNC…) s-channel is sensitive to extra particles and dimensions decay high pt lepton high missEt two bjets (+1 light q) Regina Demina, Lecture #3

  15. Singletop search s-channel t-channel The signal is kinematically between W+jets and ttbar hep-ex/0505063 Just submitted PLB paper with most stringent limits to date Preliminary results for this summer, even better: σs< 5.0pb σt< 4.4pb Regina Demina, Lecture #3

  16. New Physics, New Particles? Higgs boson: h0 H0 A0 H± CMS is looking for new physics in particular the Higgs boson, supersymmetry and … One of the promising reactions: Httbar We watch for interesting events: ~1 / 400,000 or ~ 100 per second Example of ttbar collision Regina Demina, Lecture #3

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