preliminary results on fcnc top decays
DESCRIPTION
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
Preliminary Results onFCNC Top Decays
António Onofre Nuno Castro João Carvalho Filipe Veloso
ATLAS WeekTop WG Meeting23rd June 2004
Top WG meeting, 23-06-2004 Preliminary Results on FCNC Top Decays2
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
Top WG meeting, 23-06-2004 Preliminary Results on FCNC Top Decays3
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)
Top WG meeting, 23-06-2004 Preliminary Results on FCNC Top Decays4
3. Analyses
• 3 top decay channels being studied:
t→qZ(2jets+3l+missing)
t→q(2jets+1l+1+missing)
t→qg(3jets+1l+missing)
Top WG meeting, 23-06-2004 Preliminary Results on FCNC Top Decays5
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)
Top WG meeting, 23-06-2004 Preliminary Results on FCNC Top Decays6
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
Top WG meeting, 23-06-2004 Preliminary Results on FCNC Top Decays7
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
Top WG meeting, 23-06-2004 Preliminary Results on FCNC Top Decays8
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)
Top WG meeting, 23-06-2004 Preliminary Results on FCNC Top Decays9
qZ channel (3j+2l+miss) – Discriminant variables
•Discriminant variable: ln( LS/ LB )
L=10 fb-1
Top WG meeting, 23-06-2004 Preliminary Results on FCNC Top Decays10
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
Top WG meeting, 23-06-2004 Preliminary Results on FCNC Top Decays11
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:
Top WG meeting, 23-06-2004 Preliminary Results on FCNC Top Decays12
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
Top WG meeting, 23-06-2004 Preliminary Results on FCNC Top Decays13
q channel (2j+1l+1+miss) – Discriminant variables
•Discriminant variable: ln( LS/ LB )
L=10 fb-1
Top WG meeting, 23-06-2004 Preliminary Results on FCNC Top Decays14
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
Top WG meeting, 23-06-2004 Preliminary Results on FCNC Top Decays15
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:
Top WG meeting, 23-06-2004 Preliminary Results on FCNC Top Decays16
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
Top WG meeting, 23-06-2004 Preliminary Results on FCNC Top Decays17
qg channel (3j+1l+miss) – Discriminant variables
•Discriminant variable: ln( LS/ LB )
L=10 fb-1
Top WG meeting, 23-06-2004 Preliminary Results on FCNC Top Decays18
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:
Top WG meeting, 23-06-2004 Preliminary Results on FCNC Top Decays19
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 ---
Top WG meeting, 23-06-2004 Preliminary Results on FCNC Top Decays20
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