LHC???ATLAS??????t??????????????????????????????

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LHC???ATLAS??????t??????????????????????????????
Introduction Motivation Tau identification Require
ment for Rejection conclusion
????(?????), ????,????, ????,???, ???
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Introduction(1) LHC

• LHC _at_ CERN

2007????? ??-???????? ???????? 14 TeV

• ??
• Higgs ?????
• ?????????? ???
• ???????

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Introduction(2) ATLAS
LAr HEC
Tile HD

• ATLAS???

LAr FCal

• ?????
• Pixel ???????????
• SCT ????????????????
• TRT ?????????????????

LAr EM
Muon System

• ???????
• LAr EM ????????????? (Pb)
• Tile HD ???????????????
• LAr HEC???????????(Cu)
• LAr FCal ?????????? (Pb)
• Muon ???
• MDT?????????????
• TGC,RPC ,CSC??????????

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Introduction(3) Higgs search
Production process

• VBF process

??

• gluon fusion ??????cross section
• High Pt ?Forward jet ??????
• Jet ????rapidity gap ????

Decay mode
H-gttau tau H-gtgg H-gtWW
Next talk and next next talk
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• H-gttt-gthnlnn

Signal selection
Trigger electron Ptgt25GeV,
muon Ptgt20GeV Forward Jet At least 2 Jet
Ptgt20GeV,(Leading 40GeV)
(h1h2lt0,h1-h2gt4.4,Mjjgt700GeV) Transverse Mass
lepton and missing Et lt30GeV
Mass reconstruction
t ?Pt?????mass ?????????n ?????t ?????????
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• Signal Significance

VBF H-gttau tau ??????5s ????? ??Higgs
?????significance ???
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Motivation (TauID???)
Invariant mass distribution of VBF H?tautau
(MH120GeV) with fast simulation
VBF H?tau tau is promising channel for Higgs
discovery
But as we can see the statistics of the signal is
limited.
BG
Signal
Pt distribution of tau jet (MH125GeV)
Our Pt threshold of tau jet is 40GeV But many
signals are in the lower Pt region.
Low Pt Tau jet identification and reconstruction
are very important to increase statistics.
? We have developed tau identification optimized
low Pt region
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Tau identification

• Tau candidates are selected based on track
  (finding Isolate 1 track or 3 tracks)
• We also use cells information .

Tau 1prong
Tau 3prong
QCD Jet
We also required isolation there is no more
Ptgt2GeV track in DRlt0.4.
Find Pt gt5 GeV tracks
only 1 track
3tracks in DRlt0.1 (Find two more Ptgt2GeV tracks)
Find a seed cell associated with this track, and
make Cluster (The seed is defined as a cell with
larger than 6s of Noise near the track)
reconstruction
identification
Narrowness cut is applied for identification (
I will talk in later slides)
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• Energy deposit on EM calorimeter

ATLAS?EM calorimeter ??presampler ??3??sampling
?????? Sampling 2??main component
??DhxDf0.025x0.025??segment ?????
phi
eta
We use cell based narrowness definition not DR.
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• Cell-based narrowness

Tile Sampling 0 LArHEC Sampling 0
LArEM Sampling 2
If Seed Cell is not on LArEM
Seed Cell
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Narrowness distribution after 1prong/3prong
selection
We can enhance the signal against the BG by
setting narrowness threshold. For example
0.7/0.6(1prong/3prong)
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• Efficiency Rejection

We plotted efficiency/rejection as the function
of visible Tau Pt.
For di-jet sample
(20GeV-140GeV)
1pr0.7 3pr0.6
1prong
3prong
0.7 0.6
Tagging Efficiency
1/R
Visible tau Pt GeV
Jet Pt GeV
50?efficiency ??????Rejection??????full
simulation ?????????????????
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Requirement for Rejection

• QCD Background

• W3jets Wjjj -gt ltaujj

Jet ?tau ???????? TauID ?rejection???????????

• 4jets jjjj -gt ltaujj

Jet ?tau ?????????????Jet ?lepton?????? 2??suppres
sion ???s????
-gt ??talk
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W3jets background
Sample 4.9M event (3.48 fb-1)
Rejection factor R70 ????????signal selection
cut ???????Higgs Mass window (110-135GeV) ??
2event ???-gt 0.57 fb too low stat.
BG??????????Rejection factor ?0.1??????cross
section ?0.1????????????? 50GeV-220GeV
?background?flat????????Mass window ???0.27fb???
Rx0.1
?
s in M.W. Event_at_30fb-1
signal 0.42 fb 12.6 event
Wjjj 0.27 fb 8.1 event
M(tautau) GeV
Wjjj?serious ?BG?????????? R????3-5??????????
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Conclusion

• VBF process ?????tau ??????????Higgs
  ???????????????low Pt?tau?optimize???track base
  ?tauID???????????????
• Efficiency ??????50????????full
  simulation??????Rejection??????????????????
• ???QCD background???????W3jets??4jets?cross
  section ????????????????????????serious
  ?background??????????
• ????QCD background ?suppress ?????Rejection
  factor ?????3-5???????????

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Back Up
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Signal selection

1. electron Ptgt25GeV,muon Ptgt20GeV
2. Tau jet with Ptgt40GeV DR gt0.7(from lepton)
3. Forward Jet At least 2 Jet , Ptgt20GeV(Leading
   40GeV)
4. Forward Jet h1h2lt0,h1-h2gt4.4
5. h10.7lt hl,t lth2-0.7
6. cosflh lt 0.9 for missing Et
7. Transverse mass between lepton and missing energy
   less than 30GeV
8. Missing Et gt 30GeV
9. Invariant mass of the tagged forward jets larger
   than 700GeV
10. Central Jet Veto There is no Ptgt20GeV jet in
    h10.7lt h lth2-0.7

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As a full simulation problem
Track multiplicity in Jet
Rec Track (iPad)
QCDJ3(70-140GeV) Sample
10 becomes 1pr/3pr candidate
Mean 6.3
of Tracks in 1 Jet
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Znjets background
Mtautau distribution
Fas Simulation Znjets (n2,3,4) ALPGEN
Z2j Z3j Z4j signal
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Background
0.42fb
Signal

• Major QCD background

• W3jets Wjjj -gt ltaujj -gt depend on mainly
  TauID
• bbjj bbjj -gt l tau jj
• bbjj -gt l j tau j
• jjjj jjjj -gt l tau jj
• WWjj WWjj -gt tau tau jj -gt cross
  section is small
• ttbar WWbb -gttau tau jj

0.27fb
last talk of this session
?? fb
0.00fb
0.04fb
Fake of W-gtlepton(tau) is also suppress with W
transverse Mass cut

• Z-gttt background

0.19fb

• Znjets Zjj -gt tau(l)tau(h)jj

Final state is the same as the signal So we can
only use central jet veto for suppress the BG