Study of Standard Model Backgrounds for SUSY search with 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 SUSY Event Topology is multijets + missing Et (+ leptons) multijets Missing Et η φ For SUSY discovery 1.High Pt Jets 2.Missing Et are important. ATLAS preliminary

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 Generated processσ +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

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. b c l ν Real missing Fake Missing Et Fake missing φ QCD 2 jets event Incomplete 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-pair Highest Pt of the additional jet ｇ ｇ ｇ ｔ ｔ ATLAS preliminary

7 Background Generation with ME Generation 1. High Pt partons are generated with ME(Alpgen). 2. Collinear and soft regions are covered with PS(Pythia) model. Pt ordering is applied. ME PS But, there is double counting problem. “Mangano Matching” is applied in order to remove double counts. t t Detail is Mangano’s HP

8  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 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. 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 mode Small branching ratio.But BG is expected to be small. lepton

10 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 (GeV) Count /400GeV/10fb-1 These figure show effective mass distribution after the standard event selection. ME study 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 (GeV) 0 lepton mode Event selection should be optimized depending on SUSY mass scale. ATLAS preliminary Count /400GeV/10fb-1

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

13 [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 Missing Et (GeV) ATLAS preliminary Count /10GeV/10fb-1

14 Fake missing Et 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 1.Dead material 2.Event topology Fake 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

16 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 Normalized entry 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 gaussian Double gaussian Linear scale QCD di-jet sample ∑Et=1500~1600GeV 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 eta Crack 20% worse All eta Crack ATLAS preliminary

19 [3-2] Topology dependence of Missing Et ν ν Z+3jets Z+6jets spherical and isotropic rectilinear 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 jet Fake missing Et I use 2 samples Z+3jet,Z+6jet

20 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) Normalized entry 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+6jets spherical and isotropic ATLAS preliminary

21 [4] Conclusion We estimated Background using ME generator. ME-PS matching is performed with Mangano method. The contribution of the background increase by factor lepton 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 m 1/2 (GeV) m 0 (GeV) ATLAS preliminary PS 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. m 1/2 (GeV)

23 Event display (SUSY) μ Missing Et jet EM Calorimeter Hadron Calorimeter Muon System