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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