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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

Preliminary Results onFCNC Top Decays

António Onofre Nuno Castro João Carvalho Filipe Veloso

ATLAS Week

Top WG Meeting

23rd June 2004

1 introduction
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

2 mc generation and simulation
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

3 analyses
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

qz channel 2j 3l miss pre selection
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

qz channel 2j 3l miss selection cuts
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

qz channel 2j 3l miss mass reconstruction after the event selection
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

qz channel 2j 3l miss probability density functions

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

qz channel 3j 2l miss discriminant variables
qZ channel (3j+2l+miss) – Discriminant variables

Discriminant variable: ln( LS/ LB)

L=10 fb-1

Preliminary Results on FCNC Top Decays

q g channel 2j 1l 1 g miss pre selection
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

q g channel 2j 1l 1 g miss selection cuts
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

q g channel 2j 1l 1 g miss probability density functions
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

q g channel 2j 1l 1 g miss discriminant variables
qgchannel (2j+1l+1g+miss)– Discriminant variables

Discriminant variable: ln( LS/ LB)

L=10 fb-1

Preliminary Results on FCNC Top Decays

qg channel 3j 1l miss pre selection
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

qg channel 3j miss selection cuts
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

qg channel 3j 1l miss probability density functions
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

qg channel 3j 1l miss discriminant variables
qgchannel (3j+1l+miss)– Discriminant variables

Discriminant variable: ln( LS/ LB)

L=10 fb-1

Preliminary Results on FCNC Top Decays

4 results 5 discovery limits
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

4 results 95 cl expected limits absence of signal
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

5 summary and future perspectives
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