preliminary results on fcnc top decays
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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 - PowerPoint PPT PresentationTRANSCRIPT
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Preliminary Results onFCNC Top Decays
António Onofre Nuno Castro João Carvalho Filipe Veloso
ATLAS WeekTop WG Meeting23rd June 2004
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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
FCNC decay BR in SM
t → qZ ~10-12
t → q ~10-12
t → qg ~10-10
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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)
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3. Analyses
• 3 top decay channels being studied:
t→qZ(2jets+3l+missing)
t→q(2jets+1l+1+missing)
t→qg(3jets+1l+missing)
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qZ channel (2j+3l+miss) – pre-selection
Pre-selection:• ≥3 lepton (2.5)
• ptlepton 1>20 GeV/c
L=10 fb-1
L=10 fb-1
signal(arbitrary normalization)
background(normalized to 10 fb-1)
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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
• 2.5
backgrounds Nevents (10 fb-1)
Z+jets 39.4WW,ZZ,WZ 4.2
tt 702.3
Total 745.8
33 %)90(%90
%10)(
%32)(
leptonslepton
llZBR
lWBR
Signal: %7.1signal
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qZ channel (2j+3l+miss) mass reconstruction after the event selection
invariant mass
lilj (best Z) + non-b jet
invariant mass lilj
(mass closest to mZ)invariant mass (l,,b-jet)
Ptmissing assigned to
Pzmissing evaluated from mW
L=10 fb-1
L=10 fb-1
L=10 fb-1
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qZ channel (2j+3l+miss) – Probability Density Functions
For each event:
• LS: signal likelihood
• LB: background likelihood
Discriminant variable:
• ln( LS/ LB )
L=10 fb-1
PDFs
kktotali nnL
#
1
)/(ni
k =1
k =2
Lowest m(li,lj)
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qZ channel (3j+2l+miss) – Discriminant variables
•Discriminant variable: ln( LS/ LB )
L=10 fb-1
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q channel (2j+1l+1+miss) – pre-selection
Pre-selection:• ≥ 1 lepton (2.5)
• ptlepton 1> 20 GeV/c
• ≥ 1 photon (2.5)
• ptphoton 1> 50 GeV/c
L=10 fb-1
L=10 fb-1 L=10 fb-1
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q channel (2j+1l+1+miss) – selection cuts
Event selection:• ptmissing >20 GeV/c 2 leptons • at least 2 jets:
• ptjet> 20 GeV/c • 2.5• jet 1 or 2 is a b-jet
Backgrounds Nevents (10 fb-1)
Z+jets 49.9WW,ZZ,WZ 13.9
tt 1250.0
t 26.7
Total 1340.4
%4.2signal
%90
%32)(
lepton
lWBR
Signal:
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q channel (2j+1l+1+miss) – Probability Density Functions
For each event:
• LS: signal likelihood
• LB: background likelihood
Discriminant variable:
• ln( LS/ LB )
L=10 fb-1
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q channel (2j+1l+1+miss) – Discriminant variables
•Discriminant variable: ln( LS/ LB )
L=10 fb-1
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qg channel (3j+1l+miss) – pre-selection
Pre-selection:• 1 lepton (2.5)
• ptlepton 1> 20 GeV/c
• ≥ 3 jets
• no photons
L=10 fb-1
L=10 fb-1 L=10 fb-1
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qg channel (3j+miss) – selection cuts
Event selection:
• ptmissing >20 GeV/c• total energy > 300 GeV• all jets with 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
Backgrounds Nevents (10 fb-1)
Z+jets 33.2WW,ZZ,WZ 2.5
tt 12412.5
t 293.7
Total 12742.4
%18.0signal
%90
%32)(
lepton
lWBR
Signal:
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qg channel (3j+1l+miss) – Probability Density Functions
For each event:
• LS: signal likelihood
• LB: background likelihood
Discriminant variable:
• ln( LS/ LB )
L=10 fb-1
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qg channel (3j+1l+miss) – Discriminant variables
•Discriminant variable: ln( LS/ LB )
L=10 fb-1
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4. Results: 5 discovery limits
• Assuming a 5 signal significance (5 S/√B):
ttppL
BBR
2
5
tt cross-section
(multiplied by the appropriate
branching ratios)
channel present analysis limits
(after LS/LB cut)
previous limits(TDR, ATL-COM-PHYS-99-039 and ATL-PHYS-2001-007)
qZ 5 x 10-4 (cut=6.0) 2 x 10-4
q 3 x 10-5 (cut=2.6) 1 x 10-4
qg 2 x 10-3 (cut=1.8) 7 x 10-3 (95% CL)
Results for L=100 fb-1:
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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
channel present analysis
(L=10 fb-1)
CDF
qZ BR < 1 x 10-3 BR < 3.3 x 10-1
q BR < 1 x 10-4 BR < 3.3 x 10-2
qg BR < 5 x 10-3 ---
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5. Summary and Future Perspectives
Preliminary results on the BR for the qZ, q 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