Transcript Search for resonances at the ATLAS (FullSim)

Search for tt resonances at the
ATLAS (FullSim)
V. Bednyakov, E. Khramov, N. Rusakovich, A. Tonoyan
Joint Institute for Nuclear Research, Dubna, Russia
E. Khramov
ATLAS-JINR Workshop, 22 Dec 2006
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Motivation
● Due to the short lifetime (τ ~ 10-24 s), there are no
SM particles decaying to top-quarks
● But there are several theories predicting the
existance of heavy resonances decaying into topantitop pair (Technicolor, Higgs etc.)
● New massive and electrically neutral gauge boson
(Z’) are a common feature of physics beyond the
SM
E. Khramov
ATLAS-JINR Workshop, 22 Dec 2006
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New heavy neutral gauge boson
• CDF excludes MZ'< 850 GeV/c2 at 95% CL
• Z' decay channel to top-antitop pair is considered
• The couplings of the Z' to quarks and leptons
correspond to the same couplings as that of the SM Z0
➔ υf = 0.387, αf = 1 vector and axial couplings of up type quarks to Z‘
• The analysing procedure to be described is applicable
to any tt resonance
E. Khramov
ATLAS-JINR Workshop, 22 Dec 2006
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Data samples
● Signal ( pp  Z '  tt )
– Private samples were generated with modified jobOptions file
DC3.Zprime.mumu_pythia_SSM1000.py
– Only decay channel was changed from mumu to ttbar
MZ`, GeV/c2
Γz`, GeV/c2
Number of
events
σ, fb
1000
30.62
19100
1.4
1500
45.53
11500
2.7·10-1
2000
62.42
10550
6.9·10-2
● Background ( pp  tt )
– NIKHEF Ntuples
csc11.005200.T1_McAtNlo_Jimmy.recon.AOD.v11004103._xxxxx.WouterNtuple.root
● 147K events, fully hadronic events were rejected
– Fast simulated samples without rejecting of fully hadronic events (420K)
– 4000K of fast simulated tt events (Pythia generator), all decay modes included
E. Khramov
ATLAS-JINR Workshop, 22 Dec 2006
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Tools and keys
Generation Pythia 6.323 (signal), Herwig 6.507
(background)
Simulation & Reconstruction 11.0.41 release
Top mass is assumed to be 175 GeV/c2
Pdf = CTEQ6
“Lepton+jets” channel is considered
ATLAS “standard” cuts for top physics are applied
E. Khramov
ATLAS-JINR Workshop, 22 Dec 2006
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Kinematic cuts
About 5% of background and about 14% of signal “lepton+jets”
events pass these criteria
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ATLAS-JINR Workshop, 22 Dec 2006
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pT of isolated leptons
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ET missing
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pT of light jets
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pT of b-jets
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ATLAS-JINR Workshop, 22 Dec 2006
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Mistagging of b-jets
When b-tagged jets were not indeed initiated by B-meson
Was studied with b-jets with pT>40 GeV/c and |η|<2.5
FastSim
Pythia bkg
MZ’=1 TeV/c2
M Z’ =1.5 TeV/c2
M Z’ =2 TeV/c2
1 mistag
2 mistags
Overall
N b-jets
145231
14650
174531
1526192
1388
102
1592
12546
839
68
975
8004
824
55
934
7478
Fraction
11.4%
12.7%
12.2%
12.5%
FullSim
Pythia bkg
1 mistag
2 mistags
Overall
N b-jets
7326
414
8154
94004
MZ’=1 TeV/c2
M Z’ =1.5 TeV/c2
M Z’ =2 TeV/c2
Fraction
11.5%
13.5%
18.5%
21%
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1502
116
1734
12876
1041
138
1317
7126
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927
164
1255
6018
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Efficiency of b-tagging
b-jets with pT>40 GeV/c and |η|<2.5
b-quark with pT>4 GeV/c, ΔR(match)=0.4
with pT>40
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ATLAS-JINR Workshop, 22 Dec 2006
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Overlapping of jets
E. Khramov
1.3%
2.5%
2.2%
5.6%
20%
29%
15.2%
37.4%
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Hadronic W boson
The combination of two light jets nearest to MWPDG  80.42 GeV c 2 is kept
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ATLAS-JINR Workshop, 22 Dec 2006
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Hadronic top
Events with m jj  MWPDG  20 GeV c 2 were rejected
jjb
The combination with the highest PT was accepted
Gen
Events with m jjb  M top
 40 GeV c 2 were rejected
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ATLAS-JINR Workshop, 22 Dec 2006
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Leptonic top (1/2)
● To reconstruct leptonic top it was assumed, that
all missing transverse energy in event is due to
miss

the neutrino ( ET  PT )
  2
2
2
● Solving the equation MW  ( E l  E )  ( pl  p )

relativly PZ we obtain two solutions or no real
solution
Gen
 Case of two solutions: the m lb nearest to M top
was chosen
 Case of no real solution: the event was rejected
FastSim
FullSim
4000K
NIKHEF
M(Z’)=1 TeV/c2
M(Z’)=1.5 TeV/c2
M(Z’)=2 TeV/c2
~77%
-
~79%
~51%
~79%
~47%
~77%
~47%
~71%
~48%
Gen
Events with mlb  M top
 40 GeV c 2 were rejected
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Leptonic top (2/2)
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ATLAS-JINR Workshop, 22 Dec 2006
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tt system inv. mass (1/2)
Before top mass cuts!
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tt system inv. mass (2/2)
After top mass cuts!
17 events
10 events
79 events
47 events
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ATLAS-JINR Workshop, 22 Dec 2006
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PT of had. & lept. tops
The idea is based on the assumption, that transverse momentum of top quark from
resonance decay is higher than in direct production
Leptonic top
E. Khramov
MZ’=1 TeV/c2
ATLAS-JINR Workshop, 22 Dec 2006
Hadronic top
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PT of had. & lept. tops
NO top mass cuts!
Leptonic top
MZ’=1.5
E. Khramov
TeV/c2
ATLAS-JINR Workshop, 22 Dec 2006
Hadronic top
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PT of had. & lept. tops
NO top mass cuts!
Leptonic top
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MZ’=2 TeV/c2
ATLAS-JINR Workshop, 22 Dec 2006
Hadronic top
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top
cut
T
Choosing P
value (1/2)
• εsignal=Ns1 / Ns_all; εbkg=Nbkg1 / Nbkg_all
• The
top
T
P
value was chosen if :
 εsignal / εbkg=max, should be maximal
 N s1 / N bkg1  max , confidence level
 Ns1 > 10, minimal number of signal events for
evidence threshold
Where: Ns1 and Nbkg1 -number of signal and background events after after
Ns_all and Nbkg_all -number of signal and background events before
E. Khramov
ATLAS-JINR Workshop, 22 Dec 2006
PTtop cut
PTtop cut
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top
cut
T
Choosing P
value (1/2)
Was done using AtlFast Pythia background (4 mln) and FullSim events
M(Z’)=1.0 TeV/c2
M(Z’)=1.5 TeV/c2
M(Z’)=2.0 TeV/c2
Lept. top
Lept. top
Lept. top
Had. top
3.91
2.5
1.86
1.8
2.47
pT cut, GeV/c
370
330
390
420
500
1.44
1.3
1.24
1.14
2
N s1 / N bkg1
εsignal
0.48
0.6
0.71
0.61
0.63
εbkg
0.12
0.24
0.38
0.34
0.26
Ns1 (Ns_all)
31 (65)
39 (65)
22 (31)
19 (31)
12 (19)
Nbkg1 (Ns_all) 461 (3777) 911 (3777) 313 (821) 277 (821) 36 (141)
2.4
550
2
0.58
0.24
11 (19)
34 (141)
Had. Top
Had. top
εsignal / εbkg
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Fast simulation
Signal + background (1/2)
N signal
N bkg
5
MZ’=1 TeV/c2
Full simulation
N signal
N bkg
 3.3
MZ’=1.5 TeV/c2
Signal x 3.0
Signal x 2.0
N signal
N bkg
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 2.75
ATLAS-JINR Workshop, 22 Dec 2006
N signal
N bkg
 2.96
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Signal + background (2/2)
Fast simulation
MZ’=2 TeV/c2
Full simulation
Signal x 3.0
N signal
N bkg
E. Khramov
N signal
 3.1
N bkg
ATLAS-JINR Workshop, 22 Dec 2006
 3.32
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Conclusion
● Was considered:
– 4 millions of background events (Pythia, AtlFast)
– ~20 k events of Z’ with mass 1.0 TeV/c2
– ~10 k events of Z’ with mass 1.5 TeV/c2
– ~10 k events of Z’ with mass 2.0 TeV/c2
● Obtained results coresponding to 5 fb-1 (half year
at low luminosity):
– ~4 pb is necessary for 1.0 TeV/c2 giving 2.75σ C.L.
– ~3 pb is necessary for 1.5 TeV/c2 giving 2.96σ C.L.
– ~3 pb is necessary for 2.0 TeV/c2 giving 3.32σ C.L.
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ATLAS-JINR Workshop, 22 Dec 2006
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Backup slides
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● Electron selection:
– isEM()%16==0
– hasTrack()
– author = 1 (high pT electron)
– emWeight/(emWeight+pionWeight)>0.9
– 1 electron with pT>20 GeV/c2 and |η|<2.5
– Isolation: ET (ΔR<0.2) < 10 GeV/c2 around electron
● Muon selection:
– hasCombinedMuonTrackParticle()
– author = 1 (high pT muon)
– Χ2/ndof < 30
– 1 muon with pT>20 GeV/c2 and |η|<2.5
– Isolation: ET (ΔR<0.2) < 5 GeV/c2
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ATLAS-JINR Workshop, 22 Dec 2006
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● Jets selection:
– Cone with ΔR size of 0.4
– ≥ 2 jets with pT > 40 GeV/c2 and |η|<2.5
● b-jets selection:
– Cone with ΔR size of 0.4
– ≥ 2 b-jets with pT > 40 GeV/c2 and |η|<2.5
– lhSig > 0.9
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