Background estimation for OS and SS dilepton mode

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Background estimation for OS and SS dilepton mode

• T. Suzuki, Y. Tomishima, Y. Kataoka, S. Asai
• U. Tokyo
• 2007/10/10

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About Dilepton
opposite sign
same sign
Missing ET(GeV)
Missing ET(GeV)
Dilepton mode Opposite sign and Same sign are
both discoverable.
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Samples / Conditions

• CSC V12 samples are used
• Alpgen BG samples
• tt-gtbblnqq 55305533
• tt-gtbblnln 55355538
• tt-gtbbqqqq 55415543
• W-gtenu 52235226
• W-gtmunu 82038205
• W-gttaunu 82088211
• Z-gtee 51615165
• Z-gtmumu 81098111
• Z-gttautau 81208123
• Z-gttauatau(mEt) 81148117
• Z-gtnunu 51245126
• QCD 50615064
• bb 80828083
• SUSY samples
• SU1 005401
• SU3 005403

• Electron selection
• isEM()0x3ff
• no Softe
• pT gt 10 or 20GeV, ? lt 2.5
• Isolation cone 0.2
• with ET sum lt 10GeV
• Muon selection
• Combined track(MuID)
• track match ?2 lt 20
• track fit ?2/ndof lt 5
• pT gt 10 or 20GeV, ? lt 2.5
• Isolation cone 0.2
• with ET sum lt 10GeV
• Jet
• Cone 0.4
• pT gt 20GeV, ? lt 5
• Missing ET
• MET_Final

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Applied selection

• Dilepton selection
• mET gt max(100GeV, Meff 0.2)
• 1st leading jet pT gt 100GeV
• 2nd 4th leading jet pT gt 50GeV
• ST gt 0.2
• 2 lepton with pT gt10GeV

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AboutOpposite Sign
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About dilepton mode (opposite sign)

• SU3 point (bulk point)
• m0 100GeV, m1/2 300GeV

Missing ET(GeV)
The main BG of Dilepton mode(OS) is tt?lvlv
(75). The second most important BG is tt?lvqq
(20). This is the contribution of semi-leptonic
decay of bottom quark. Dilepton mode BG main
component is top(95). So we will need a top
enhanced sample to estimate BG.
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Control Sample selection
control sample
tt?bblvqq is the main component of the Control
sample (not tt?bblvlv).
MT(GeV)
top enhanced region but W contamination exist
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Control Sample (top and W)

• Dilepton BG
• tt?bbl?l? (75)
• tt?bbl?qq (25)
• Control Sample
• tt?bbl?qq (65)
• W (30)

control sample W and top is normalized at
mEt 100GeV
CS W
Dilepton BG
top component of the control sample has almost
the same distribution as true BG but little
softer and W component is much harder.
CS top
Missing ET (GeV)
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Control Sample VS true BG
But by mixing top (60) and W (30) of the
control sample, we observe a good agreement,
because W components is not so high (only 30).
So the discrepancy becomes small.
Missing ET(GeV)
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With this method some other distribution of the
dilepton mode can be estimated explicitly.
Effective Mass(GeV)
Leading jet PT(GeV)
Leading lepton PT(GeV)
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Systematic error (Fast Simulation v11)

• Original sample parameters
• renormalization scale pt
• minimal PT of parton 40GeV
• minimal separation of parton
• ?R 0.7
• M LO

We observe that the discrepancy of this method
comes from W process. And we estimate that
systematic error of this method is about 50.
(for lepton PT gt 20GeV)
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AboutSame Sign
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About dilepton mode (Same sign)

• SU3 point (bulk point)
• m0 100GeV, m1/2 300GeV

Missing ET(GeV)
The main BG of dilepton (SS) is tt?lvqq. At
high missing ET region the SUSY signal are BG
free.
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The applied method
Lepton Pt Function
lepton
leading jet
apply X to 1 jet
estimated semi-leptonic decay
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BG Estimation
1st leading jet replaced
We have recalculate the missing ET as follow
mEt mEt replaced jet pt (1 - X) and the
effective mass, transverse sphericity were
recalculated. We required that the replaced
lepton has a PT larger than 20GeV.
It is an overestimation in high missing ET
region. We should do futher investigation on
which jet should be choosen.
Missing ET(GeV)
not well reproduced still investigating.
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Conclusion

• About Opposite sign dilepton mode
• The main component of dilepton BG is tt?lvlv.
• The dilepton BG can be estimated with Control
  sample.
• This method is sensitive to the cross section of
  W process and SUSY. The systematic error
  of this method is about 50.
• We must check the systematic error with ATLFAST
  v12.
• About Same sign dilepton mode
• The main component of dilepton BG is tt?lvqq.
• Futher investigation must be done about lepton pt
  function applying method

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BACK UP
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Opposite Sign
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FULL vs FAST
There is a big underestimation in tt?lvqq and
W?lv since mEt distribution is not well
reproduced in the current ATLFAST. So we need a
correction on mEt distribution.
20
if SUSY exist
The estimated BG is contaminated by SUSY and
there will be an overestimation, but true
BGSUSY will be larger. SUSY can be discovered
by observing this excess.
Missing ET(GeV)
21
The same overestimation is observed in other
distributions. Except for leading lepton PT
distribution (average of lepton PT for dilepton
mode) where the distribution becomes softer with
the presence of SUSY.
Effective Mass(GeV)
Leading lepton pt(GeV)
Leading jet pt(GeV)
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if SUSY exist

• with SU1
• m0 70GeV, m1/2 350GeV

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The other distribution with SU1. Like the SU3
case, the estimated BG is overestimated, but the
true BGSUSY has a larger excess.
Effective Mass(GeV)
Leading lepton pt(GeV)
Leading jet pt(GeV)
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Systematic error (Fast Simulation v11)
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Same Sign
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1st leading jet replaced
The other distribution are also overestimated.
(for 1st leading jet replaced)
Effective Mass(GeV)
Leading jet pt(GeV)
Leading lepton pt(GeV)
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2nd leading jet replaced
Effective Mass(GeV)
Missing Et(GeV)
Leading jet pt(GeV)
Leading lepton pt(GeV)