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

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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

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[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

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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

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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

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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

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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

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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

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[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

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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

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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

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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)

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Event display (SUSY)

μ

Missing Et

jet

jet

EM CalorimeterHadron Calorimeter

Muon System