Preliminary Results on FCNC Top Decays

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


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)


3. Analyses

• 3 top decay channels being studied:

t→qZ(2jets+3l+missing)

t→q(2jets+1l+1+missing)

t→qg(3jets+1l+missing)


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)


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


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


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)


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

•Discriminant variable: ln( LS/ LB )

L=10 fb-1


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


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)


tt 1250.0

t 26.7

Total 1340.4

%4.2signal

%90

%32)(

lepton

lWBR

Signal:


q channel (2j+1l+1+miss) – Probability Density Functions

For each event:




• ln( LS/ LB )

L=10 fb-1


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


L=10 fb-1


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


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)


tt 12412.5

t 293.7

Total 12742.4

%18.0signal

%90

%32)(

lepton

lWBR

Signal:


qg channel (3j+1l+miss) – Probability Density Functions

For each event:




• ln( LS/ LB )

L=10 fb-1


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


L=10 fb-1


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:


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 ---


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

Preliminary Results on FCNC Top Decays

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