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Preliminary Results on FCNC Top Decays

Preliminary Results on FCNC Top Decays. António Onofre Nuno Castro João Carvalho Filipe Veloso. ATLAS Week Top WG Meeting 23rd June 2004. 1. Introduction. FCNC t -quark decays: GIM suppressed in the SM

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Preliminary Results on FCNC Top Decays

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  1. Preliminary Results onFCNC Top Decays António Onofre Nuno Castro João Carvalho Filipe Veloso ATLAS Week Top WG Meeting 23rd June 2004

  2. 1. Introduction • FCNC t-quark decays: • GIM suppressed in the SM • Significant enhancements of FCNC BR in some SM extensions(Higgs doublets, Supersymmetry, exotic fermions, …) • Any observation of FCNC top decays would be a signature of new physics Preliminary Results on FCNC Top Decays

  3. 2. MC Generation and Simulation • Generation: background: • PYTHIA 6.206: • QCD (bb), W+jets, Z+jets, WW, ZZ, ZW • TopReX 4.05: • tt production, single t production signal: • TopReX 4.05 (t→qZ, t→q, t→qg) • ATLAS simulation • ATLFAST 2.53 • ATLFASTB: (b-tag)=33% (rejection factor of 1381 against light quarks) Preliminary Results on FCNC Top Decays

  4. 3. Analyses 3 top decay channels being studied: t→q (2jets+1l+1+missing) t→qZ (2jets+3l+missing) t→qg (3jets+1l+missing) Preliminary Results on FCNC Top Decays

  5. qZ channel (2j+3l+miss) – pre-selection • Pre-selection: • ≥3 lepton (|h|<2.5) • ptlepton 1>20 GeV/c L=10 fb-1 background (normalized to 10 fb-1) signal (arbitrary normalization) L=10 fb-1 Preliminary Results on FCNC Top Decays

  6. qZ channel (2j+3l+miss) – selection cuts • Event selection: • ptlepton 2 >10 GeV/c • ptlepton 3 >10 GeV/c • ptmissing >20 GeV/c • at least 2 jets: • 1 b-jet • ptjet> 20 GeV/c • |h|<2.5 Signal: Preliminary Results on FCNC Top Decays

  7. qZ channel (2j+3l+miss) mass reconstruction after the event selection L=10 fb-1 invariant mass lilj (mass closest to mZ) invariant mass (l,n,b-jet) Ptmissing assigned to n Pzmissing evaluated from mW L=10 fb-1 invariant mass lilj (best Z) + non-b jet L=10 fb-1 Preliminary Results on FCNC Top Decays

  8. Lowest m(li,lj) qZ channel (2j+3l+miss) – Probability Density Functions ni k =1 For each event: • LS: signal likelihood • LB: background likelihood Discriminant variable: • ln( LS/ LB ) L=10 fb-1 k =2 Preliminary Results on FCNC Top Decays

  9. qZ channel (3j+2l+miss) – Discriminant variables Discriminant variable: ln( LS/ LB) L=10 fb-1 Preliminary Results on FCNC Top Decays

  10. qgchannel (2j+1l+1g+miss) – pre-selection • Pre-selection: • ≥ 1 lepton (|h|<2.5) • ptlepton 1> 20 GeV/c • ≥ 1 photon (|h|<2.5) • ptphoton 1> 50 GeV/c L=10 fb-1 L=10 fb-1 L=10 fb-1 Preliminary Results on FCNC Top Decays

  11. qgchannel (2j+1l+1g+miss) – selection cuts • Event selection: • ptmissing >20 GeV/c •  2 leptons • at least 2 jets: • ptjet> 20 GeV/c • |h| < 2.5 • jet 1 or 2 is a b-jet Signal: Preliminary Results on FCNC Top Decays

  12. qgchannel (2j+1l+1g+miss)– Probability Density Functions L=10 fb-1 For each event: • LS: signal likelihood • LB: background likelihood Discriminant variable: • ln( LS/ LB ) Preliminary Results on FCNC Top Decays

  13. qgchannel (2j+1l+1g+miss)– Discriminant variables Discriminant variable: ln( LS/ LB) L=10 fb-1 Preliminary Results on FCNC Top Decays

  14. qg channel (3j+1l+miss) – pre-selection • Pre-selection: • 1 lepton (|h|<2.5) • ptlepton 1> 20 GeV/c • ≥ 3 jets • no photons L=10 fb-1 L=10 fb-1 L=10 fb-1 Preliminary Results on FCNC Top Decays

  15. qgchannel (3j+miss) – selection cuts • Event selection: • ptmissing >20 GeV/c • total energy > 300 GeV • all jets with |h|<2.5 • 1 b-jet (has to be jet 1, 2 or 3) • jet 1, 2 and 3: pt > 75 GeV/c • 100 GeV/c2 < m(q,g) < 200GeV/c2 • q,g are the two most energetic non-b jets Signal: Preliminary Results on FCNC Top Decays

  16. qgchannel (3j+1l+miss)– Probability Density Functions L=10 fb-1 For each event: • LS: signal likelihood • LB: background likelihood Discriminant variable: • ln( LS/ LB ) Preliminary Results on FCNC Top Decays

  17. qgchannel (3j+1l+miss)– Discriminant variables Discriminant variable: ln( LS/ LB) L=10 fb-1 Preliminary Results on FCNC Top Decays

  18. 4.Results: 5 discovery limits • Assuming a 5 signal significance (5 S/√B): tt cross-section (multiplied by the appropriate branching ratios) Results for L=100 fb-1: Preliminary Results on FCNC Top Decays

  19. 4.Results:95% CL expected limits (absence of signal) • In the absence of signal, 95% CL expected limits on BR can be derived: • “Modified frequentist likelihood method” was used Preliminary Results on FCNC Top Decays

  20. 5.Summary and Future Perspectives • Preliminary results on the BR for the qZ, qg and qg decay channels were obtained • Results compatible with those reported in TDR, ATL-COM-PHYS-99-039 and ATL-PHYS-2001-007 • Further studies to be done: use all available event information, selection criteria stability, correlations, photon detection efficiency, jet/photons rejection, … • Collaboration with theoreticians • ATLAS note in the near future Preliminary Results on FCNC Top Decays

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