Study of Standard Model Backgrounds for SUSY search with ATLAS detector Takayuki Sasaki, University...
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Transcript of Study of Standard Model Backgrounds for SUSY search with ATLAS detector Takayuki Sasaki, University...
Study of Standard Model Backgrounds
for SUSY searchwith ATLAS detector
Takayuki Sasaki,
University of Tokyo
2
Outline of my talk
[1] Event topology of SUSY signal[2] Background estimation with Matrix Element[3] Studies of Missing Et
(1)Dead material (crack region)(2)Topology dependence
[4] Conclusion
3
are produced in pp collision. decay into and high pt
jets are emitted. They decay into .it makes
missing Et. Sometimes lepton is emitted .
[1] SUSY Event Topology
qg ~,~1
02~,~
01~
qg ~,~
SUSY Event Topology is multijets + missing Et (+ leptons)
multijets
Missing Et
η
φ
For SUSY discovery1. High Pt Jets2. Missing Etare important.
ATLAS preliminary
4
Background processes• 4 types of background a
re considered.• There are 2 types of Q
CD background.• QQjj (Q=b,c)
• It is heavy flavors and including real mis
sing Et.• multijets
• It is light flavors• There is no real missi
ng.Just fake missing Et
Generated process σ
+Njets (WW→lν l ν/ WW→lνqq)
~ 1nb
W+Njets(W→lν) ~10nb
Z+Njets(Z→νν/Z→ττ) ~ 1nb
QCD QQjj (Q=b,c) (4jets) ~10nb
QCD mlutijets (4 jets) ~ μb
SUSY (Scale~1TeV) ~1pb
tt
5
QCD BackgroundQCD background is categorized to 2 t
ypes QQjj (Q=b,c) : Real missing
• ν from semileptonic decay of b/c makes missing Et.
multijets (light flavor):Fake missing
• Limited resolution of energy measurement makes missing Et (See right figure)
• QCD has huge cross-section. So, fake missing Et should be treated carefully. This is important and difficult issue.
b
c l
ν
Real missing
Fake Missing Et
Fake missing
φQCD 2 jets eventIncomplete measurement of the energy makes “fake” missing Et.
η
φ
QCD mlutijets (4 jets) ~ μb
ATLAS preliminary
6
[2] BG estimation with ME
Parton Shower (PS) is used in the previous study. But PS has some problem.
For example, Jet is emitted in top pair-production.This figure shows Pt distribution of this jet.
Hard jet is not emitted in Parton Shower. PS is not good approximation at high Pt region. BG estimation using PS model is underestimated in high Pt region.
ME (Alpgen)PS (Pythia 6.2)
Pt(GeV)
Top-pairHighest Pt of the additional jet
g
g
g
t
t
ATLAS preliminary
7
Background Generation with ME
Generation1. High Pt partons are generat
ed with ME(Alpgen).
2. Collinear and soft regions are covered with PS(Pythia) model.
• Pt ordering is applied.
MEPS
But, there is double counting problem.“Mangano Matching” is applied in order to remove double counts.
t
t
Detail is Mangano’s HPhttp://mlm.home.cern.ch/mlm/
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parton generated with ME
○ jet activity
Mangano Matching
We applied Mangano method Jet should be matched to the parton generated with ME (R=0.7) except for th
e soft and collinear regions. For example, let's consider Z+4jets sample
Blue show perfect matching between ME parton and jet. Soft jet was emitted collinearly. →Matched (Accepted) One parton divided into 2 jets. (outside ME cone 0.7) → Not Matched
Such event should be covered with 5jet ME. (double counting)→this event is discarded.
ME
PS
Z→νν+4jets
One parton divided into 2 jets
Jet was emitted collinearly
Jet map in (η,φ) plain
ATLAS preliminary
9
Event selection
0 lepton mode No lepton At least 1 jet :Pt>100GeV At least 4 jets :Pt>50GeV Missing Et > 100GeV Transverse Sphericity >0.2
1 lepton mode 1 lepton (e,μ: Pt >10GeV) At least 1 jet :Pt>100GeV At least 4 jets :Pt>50GeV Missing Et > 100GeV Transverse Sphericity >0.2 Transverse mass between lepton and missing Et >100GeV
(In order to suppress W+Njets Background)
SUSY Event Topology is “multijets + missing (+leptons)”. There are two discovery channels.
0 lepton mode Large branching ratio. But Background is large.
1 lepton modeSmall branching ratio .But BG is expected to be small.
lepton
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Result : 0 lepton mode
BG increases by factor 2~5 than PS study.
BG slope is similar to signal.
QCD BG is the same order of the other background processes
PS study missEtPt (GeV)
Cou
nt /
400G
eV/1
0fb-
1
TeVqMgM 1~)~(~)~(These figure show effective mass distribution after the standard event selection.
ME study
MissEt|Pt|tsLeading4je
EffM
Effective mass is sum of leading 4jet scalar Pt and missing Et
In order to control QCD BG,It is important to study fake missing Et.
ATLAS preliminary
11
The other SUSY scale
missEtPt (GeV) missEtPt (GeV)
TeVqMgM 7.0~)~(~)~(TeVqMTeVgM 5.1~)~(,7.1~)~(
0 lepton mode
Event selection should be optimized depending on SUSY mass scale.
ATLAS preliminary ATLAS preliminary
Cou
nt /
400G
eV/1
0fb-
1
Cou
nt /
400G
eV/1
0fb-
1
12
Result : 1 lepton mode
Clear excess can be observed.
BG is dominant. Z→, QCD backgroun
d can be suppressed 1 lepton mode has bett
er discovery potential than 0 lepton mode.
missEtPt (GeV)
Cou
nt /
400G
eV/1
0fb-
1
TeVqMgM 1~)~(~)~(
tt
ATLAS preliminary
top BG
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[3] Missing Et
Clear excess can be observed ( Missing Et >800GeV)
Missing Et is very important for SUSY search
0 lepton mode Missing Et distribution
TeVqMgM 1~)~(~)~(
Missing Et (GeV)
ATLAS preliminary
Cou
nt /
10G
eV/1
0fb-
1
14
Fake missing Et
Missing Et should correspond toBut, limited resolution of jet also
makes missing Et.
This is fake missing EtIt is very danger BG.Since QCD cross section is hug
e.
Fake missing depends on1. Dead material2. Event topology
,~01Fake missing Et distribution
Fake Missing Et(GeV)
QCD 4jets sample (Full Simulation)
Long tail appears close to 1TeV
ATLAS preliminary
15
[3-1] Dead material in ATLAS detector
|η|=1.4~1.6 is boundary of Barrel and Endcap. There are a lot of dead materials in front of calorimeter (cables, services…)Energy loss is expected to be large in these dead materials. Then energy measurement becomes worse.
We define “crack region” as |η|=1.4~1.6
Dead material
ATLAS detector
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We estimate effect of the limited energy resolution with full simulation.
This figure shows the fake missing Ex distribution.
Missing Et resolution in crack region
All eta
Crack
Missing Et resolution is worse in the crack sample.
Fake missing Ex (GeV)
QCD di-jet sample
( Ex is x-axis component of Et)
∑Et=1500~1600GeV
Nor
mal
ized
ent
ry
ATLAS preliminary
17
Double Gaussian fit
These figures show fake missing Ex distributions with single gaussian fit and double gaussian fit.
Missing Ex resolution is not single gaussian.
Single gaussian fitting fail to fit in both central region and tail.
Log scale
Single gaussianDouble gaussian
Linear scale
QCD di-jet sample∑Et=1500~1600GeV
ATLAS preliminary
ATLAS preliminary
18
Missing Ex Resolution (double gaussian fit)
These Figures show result of double gaussian fitting. Left figure shows narrow component. And, right figure show broad component . We need more study.
broad sigma (GeV)
∑Et (GeV)
narrow sigma (GeV)
∑Et (GeV)
All etaCrack 20% worse
All etaCrack
10000
20
10
40
20000
10000
20
40
2000
10
ATLAS preliminaryATLAS preliminary
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[3-2] Topology dependence of Missing Et
ν ν
Z+3jets Z+6jetsspherical and isotropicrectilinear
Fake missing Et distribution depends on Jet multiplicity.
Resolution of jet energy is limited.Miss-measurement of each jet energy make fake missing.Truth jet
Measured jetFake missing Et
I use 2 samples Z+3jet,Z+6jet
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Fake missing Ex distribution
Tail shape is clearly different. Z+3jets has larger tail than 6jets.
Z+3jets (Z)Z+6jets (Z)
Fake missing Ex (GeV)
Nor
mal
ized
ent
ry
Fake missing Et comes from limited resolution of each jet.Miss-measurement cancel out in 6jet sample,
since 6 jets sample is more spherical and isotropic.
ν
Z+3jets
rectilinear
ν
Z+6jetsspherical and isotropic
ATLAS preliminary
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[4] Conclusion
We estimated Background using ME generator. ME-PS matching is performed with Mangano metho
d. The contribution of the background increase by facto
r 2-5. 1lepton mode is important.
Missing Et tail and resolution is very important for QCD Background Resolution become worse in crack region. Topology dependence
For SUSY search ,Jet and missing Et are important.
Detail study in realistic condition is in progress
22
Discovery potential
1 lepton mode
m1/
2(G
eV)
m0(GeV)
ATLAS preliminaryPS Study
ME Study
These figures show Discovery potential for an integrated luminosity of 10fb-1 after Cut Optimization.Discovery potential of ME study is same as PS study.We can find 2TeV scale SUSY for 10fb-1.
m1/
2(G
eV)