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Study of Standard Model Backgrounds for SUSY search with ATLAS detector

Study of Standard Model Backgrounds for SUSY search with ATLAS detector. Takayuki Sasaki, University of Tokyo. 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)

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Study of Standard Model Backgrounds for SUSY search with ATLAS detector

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  1. Study of Standard Model Backgrounds for SUSY searchwith ATLASdetector 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. [1] SUSY Event Topology • SUSY Event Topology is multijets + missing Et(+ leptons) • are produced in pp collision. • decay into and high pt jets are emitted. • They decay into .it makes missing Et. • Sometimes lepton is emitted . ATLAS preliminary Missing Et multijets η • For SUSY discovery • High Pt Jets • Missing Et • are important. φ

  4. Background processes • 4 types of background are considered. • There are 2 types of QCD background. • QQjj (Q=b,c) • It is heavy flavors and including real missing Et. • multijets • It is light flavors • There is no real missing.Just fake missing Et

  5. QCD Background QCD background is categorized to 2 types • 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. ATLAS preliminary c l ν Fake MissingEt b Real missing η φ Fake missing φ QCD 2 jets event Incomplete measurement of the energy makes “fake” missing Et.

  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. ATLAS preliminary ｇ Top-pair Highest Pt of the additional jet ｇ ｔ ｇ ｔ ME (Alpgen) PS (Pythia 6.2) Pt(GeV)

  7. Generation High Pt partons are generated with ME(Alpgen). Collinear and soft regions are covered with PS(Pythia) model. Pt ordering is applied. BackgroundGeneration with ME ME PS t t • But, there is double counting problem. • “Mangano Matching” is applied in order to remove double counts. Detail is Mangano’s HP http://mlm.home.cern.ch/mlm/

  8. Mangano Matching Z→νν+4jets ATLAS preliminary • We applied Mangano method • Jet should be matched to the parton generated with ME (R=0.7) except for the 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. PS • parton generated with ME ○ jet activity Jet was emitted collinearly ME One parton divided into 2 jets Jet map in (η,φ) plain

  9. Event selection SUSY Event Topology is “multijets + missing (+leptons)”. There are two discovery channels. • 0 lepton mode Large branching ratio. But Background is large. • 1 lepton mode Small branching ratio .But BG is expected to be small. • 0 lepton mode • No lepton • At least 1 jet :Pt>100GeV • At least 4 jets :Pt>50GeV • MissingEt＞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 • MissingEt＞100GeV • Transverse Sphericity >0.2 • Transverse mass between lepton and missing Et >100GeV (In order to suppress W+Njets Background) lepton

  10. Result : 0 lepton mode Effective mass is sum of leading 4jet scalar Pt and missing Et ME study ATLAS preliminary • 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 These figure show effective mass distribution after the standard event selection. Count /400GeV/10fb-1 (GeV) PS study In order to control QCD BG, It is important to study fake missing Et.

  11. The other SUSY scale 0 lepton mode ATLAS preliminary ATLAS preliminary Count /400GeV/10fb-1 Count /400GeV/10fb-1 (GeV) (GeV) Event selection should be optimized depending on SUSY mass scale.

  12. Result : 1 lepton mode • Clear excess can be observed. • BG is dominant. • Z→nn, QCD background can be suppressed • 1 lepton mode has better discovery potential than 0 lepton mode. ATLAS preliminary top BG Count /400GeV/10fb-1 (GeV)

  13. [3] Missing Et Clear excess can be observed ( Missing Et >800GeV) Missing Et is very important for SUSY search ATLAS preliminary 0 lepton mode Missing Et distribution Count /10GeV/10fb-1 Missing Et (GeV)

  14. Fake missing Et ATLAS preliminary • Missing Et should correspond to • But, limited resolution of jet also makes missing Et. • This is fake missing Et • It is very danger BG. • Since QCD cross section is huge. • Fake missing depends on • Dead material • Event topology Fake missing Et distribution QCD 4jets sample (Full Simulation) Fake Missing Et(GeV) Long tail appears close to 1TeV

  15. [3-1] Dead material in ATLAS detector 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

  16. Missing Et resolution in crack region ATLAS preliminary QCD di-jet sample We estimate effect of the limited energy resolution with full simulation. This figure shows the fake missing Ex distribution. ∑Et=1500~1600GeV All eta Crack Normalized entry Fake missing Ex (GeV) ( Ex is x-axis component of Et) Missing Et resolution is worse in the crack sample.

  17. Double Gaussian fit ATLAS preliminary QCD di-jet sample ∑Et=1500~1600GeV • 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. Linear scale Single gaussian Double gaussian ATLAS preliminary Log scale

  18. Missing Ex Resolution (double gaussian fit) narrow sigma (GeV) broad sigma (GeV) ATLAS preliminary These Figures show result of double gaussian fitting. Left figure shows narrow component. And, right figure show broad component . We need more study. 40 ATLAS preliminary 40 All eta Crack 20% worse All eta Crack 20 20 10 10 0 2000 1000 0 2000 1000 0 ∑Et (GeV) ∑Et (GeV)

  19. [3-2] Topology dependence of Missing Et Resolution of jet energy is limited. Miss-measurement of each jet energy make fake missing. Truth jet Measuredjet Fake missing Et Fake missing Et distribution depends on Jet multiplicity. I use 2 samples Z+3jet,Z+6jet ν ν Z+3jets Z+6jets spherical and isotropic rectilinear

  20. Fake missing Ex distribution ATLAS preliminary ν Tail shape is clearly different. Z+3jets has larger tail than 6jets. Z+3jets (Znn) Z+6jets (Znn) Z+3jets rectilinear Normalized entry ν Z+6jets spherical and isotropic Fake missing Ex (GeV) 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.

  21. [4] Conclusion For SUSY search ,Jet and missing Et are important. • We estimated Background using ME generator. • ME-PS matching is performed with Mangano method. • The contribution of the background increase by factor 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 Detail study in realistic condition is in progress

  22. Discovery potential m1/2(GeV) PS Study ATLAS preliminary ME Study 1 lepton mode m1/2(GeV) m0(GeV) 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.

  23. Event display (SUSY) EM Calorimeter Hadron Calorimeter Muon System jet MissingEt jet μ

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