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SUSY Discovery Potential in Trilepton Final States at ATLAS: the Strive for Lepton Purity

This study explores the potential for discovering SUSY in trilepton final states at ATLAS and focuses on achieving high lepton purity. It discusses the preselection criteria, lepton isolation, and the measurement of the isolated lepton rate from b-jets. The study also analyzes the fake isolated leptons from b-jets in ttbar events and identifies relevant backgrounds.

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SUSY Discovery Potential in Trilepton Final States at ATLAS: the Strive for Lepton Purity

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  1. SUSY Discovery Potentialin Trilepton Final States at ATLAS:the Strive for Lepton Purity Oleg Brandt – in collaboration with ATLAS CSC 7 / 5 supervisors: Pawel Brückman Alan Barr Tony Weidberg

  2. Today‘s menu • Short overview of lepton preselection • Lepton isolation: • Track isolation • Calorimeter isolation • OSSF lepton selection: • First thoughts on measuring the isolated lepton rate from b-jets • Conclusion • Outlook SUSY discovery potential in trilepton final states: Technical Aspects

  3. Leading lepton Pt for SU2 SUSY Signal 10 fb-1 10 fb-1 Need high lepton purity! SUSY discovery potential in trilepton final states: Technical Aspects

  4. Preselection outlined • Preselection (ATLAS SUSY WG cuts in blue): • Muons: • MuID • | η | < 2.5 • Calorimeter isolation in R = 0.2 cone: < 10 GeV • bestMatch(), isCombinedMuon() • chi²/NDF for track < 5, track match: < 20 • Jet isolation: no jets in R = 0.4 cone (against b) • Isolation w/r/t each other in: R = 0.1 (against J/Psi & Y) • Pt > 10 GeV • Electrons: • | η | < 2.5 • (isEM() & 0x3FF) == 0 • reconstructed by eGamma algorithm • Isolation w/r/t each other in: R = 0.1 • Pt > 15 GeV SUSY discovery potential in trilepton final states: Technical Aspects

  5. Track isolation: after 3rd lepton requirement  e Define track isolation as: (inspired by H->4l) where normalised to 1 normalised to 1 SUSY discovery potential in trilepton final states: Technical Aspects

  6. Calorimeter isolation:after 3rd lepton requirement  „Conventional“ ATLAS calorimeter isolation from AOD, rel. 12.0.7 normalised to 1 e normalised to 1 Additional discrimination power? SUSY discovery potential in trilepton final states: Technical Aspects

  7. Track vs. Calorimeter isolation • Calo isolation potentially easier to understand • Use the discrimination variable first understood!  • not 100% correlated :) • for tight track isolation calo isolation not efficient :( full MC stat. e full MC stat. SUSY discovery potential in trilepton final states: Technical Aspects

  8. Study fake isolated leptons from b-jets in ttbar events • Study isolated lepton fake rate from b-jets: • Need a reasonably pure sample of b-jets: • bb • tt • bb: • difficult due to high QCD backgrounds, e.g. W+j • not realistic • tt: • Several handles to tag ttbar events without using one of the b-jets • Use semileptonic tt channel • Orthogonal sample • approx. 10x more statistics - - - - - SUSY discovery potential in trilepton final states: Technical Aspects

  9. Study fake isolated leptons from b-jets in ttbar events: Ansatz opposite sign SUSY discovery potential in trilepton final states: Technical Aspects

  10. Study fake isolated leptons from b-jets in ttbar events: Ansatz Not feasible due to dileptonic ttbar decays! SUSY discovery potential in trilepton final states: Technical Aspects

  11. Study fake isolated leptons from b-jets in ttbar events: Ansatz same sign! SUSY discovery potential in trilepton final states: Technical Aspects

  12. Study fake isolated leptons from b-jets in ttbar events: Ansatz full top decay chain same sign! Tags dijet mass = m_W SUSY discovery potential in trilepton final states: Technical Aspects

  13. Study fake isolated leptons from b-jets in ttbar events • Possible backgrounds (probably in order of importance): • Dileptonic tt • lepton charge mismeasurement • Single top • associated W+t production: • hadronic W: contribution to the desired „signal“ sample • leptonic W: with additional contribution to „background“ • most of other single top (t+bb ...): • contribution to the „signal“ • Z + QCD • lepton charge mismeasurement + additional jets to mimic tt • bb + jets • semileptonic b-decays with oscillaton / consecutive c-decays • W + QCD • 1 true lepton and one from QCD + additional jets • WZ, ZZ • leptons from W / Z + additional jets • ... Ideas? - - - - SUSY discovery potential in trilepton final states: Technical Aspects

  14. Conclusion Studied lepton isolation: • Calorimeter isolation • Probably easier to understand • Track isolation • More powerful • Correlation • Resulting table of statistical significancies for SUx: 10 fb-1 SUSY discovery potential in trilepton final states: Technical Aspects

  15. Outlook • Proceed with writing up CSC 7 / CSC 5 notes! • Study triggers for : • look at new trigger menu version in 13.0.30 • Identify optimal handles to adjust trigger rate • Study isolated lepton fake rate from b-jets: • Identify relevant backgrounds • First steps towards a way to measure this rate in data ... • Do more studies: • background • systematics SUSY discovery potential in trilepton final states: Technical Aspects

  16. Backup & further discussion Backup slides following SUSY discovery potential in trilepton final states: Technical Aspects

  17. Lepton trigger study • First running will be at • Focus on the „Worst Case“ scenario • the most critical SUSY analysis • cannot trigger on jets • triggering on MEt dodgy • need lepton triggers • rather low-Pt leptons • Lepton triggers for start-up: • efficient • unprescaled • failsafe: • no „risky“ cuts • rate adjustment • simple to understand SUSY discovery potential in trilepton final states: Technical Aspects

  18. Lepton triggers analysed: 10,2 fb-1 Full SU2 selection with jet veto SUSY discovery potential in trilepton final states: Technical Aspects

  19. Lepton triggers: Summary SU2 „ 0“ SU2 inclusive SU2 inclusive jet veto NO jet trigger fired all of SU2 to be updated with updated selection SUSY discovery potential in trilepton final states: Technical Aspects

  20. SUSY SU2 in Trilepton Channel • MSSM: 105 paramers • SUSY spontaneously broken -> SUGRA • Mass unification at GUT scale -> mSUGRA • ATLAS mSUGRA benchmark points: • SU2: very hard to detect • SU3: „generic“ • SU4: light SUSY • „flat“ search performance in multilepton final states SUSY discovery potential in trilepton final states: Technical Aspects

  21. SUSY SU2 in Trilepton Channel • SU2 (Focus point region): • Very few sfermions produced • charginos / neutralinos „light“ • Look for signal in the „gold-plated“ trilepton channel • 2/3 signal from decay chains starting with gluinos • 6/7 due to higher Pt scale • „Worst case“ scenario: • gluino mass exempt from unification at GUT scale • m_gluino ~ m_squark, not ~850 GeV • ONLY EW associated  0 production • ONLY multileptonic signal for discovery SUSY discovery potential in trilepton final states: Technical Aspects

  22. Technical aspects of the analysis • Signal and background MC used: • csc11 and mc12 full simulation • detector geometry ATLAS-CSC-01-02-00 • AOD‘s in Athena 12.0.6 SUSY discovery potential in trilepton final states: Technical Aspects

  23. Comparison: selection of the OSSF pair • All CSC 7 analyses require Opposite Sign Same Flavour (OSSF) lepton pair • Different treatment when eee or µµµ present: • take pair with • take pair with • take pair with • All CSC7: Take 3rd lepton with highest Pt SUSY discovery potential in trilepton final states: Technical Aspects

  24. Comparison: selection of the OSSF pair • All CSC 7 analyses require Opposite Sign Same Flavour (OSSF) lepton pair • Different treatment when eee or µµµ present: • take pair with • take pair with • take pair with • All CSC7: Take 3rd lepton with highest Pt • full tables: http://www-pnp.physics.ox.ac.uk/~obrandt/TrileptonAnalysis/DifferentOSSF.pdf • The preferred way to go: 3) 1) 2) 3) 10 fb-1 after full selection, SUSY discovery potential in trilepton final states: Technical Aspects

  25. Cut on MEt and dilepton mass 10 fb-1 SUSY discovery potential in trilepton final states: Technical Aspects

  26. „Worst Case“ scenario 10 fb-1 SUSY discovery potential in trilepton final states: Technical Aspects

  27. Same for SU3 10 fb-1 SUSY discovery potential in trilepton final states: Technical Aspects

  28. Same for SU4 10 fb-1 SUSY discovery potential in trilepton final states: Technical Aspects

  29. Cut Flow Table SUSY discovery potential in trilepton final states: Technical Aspects

  30. Selection outlined • „Worst case“ scenario: • eliminate decay chains starting with gluino pairs • 2/3 of signal Xsec, enhanced to 6/7 due to higher Pt scale • Jet veto: • No jet with Et > 30 GeV in eta < 2.4 • No jet with b-jet LH > 0.3 • Jet overlap removal crucial: • take only jets with eta < 2.4 • Consider overlap with Electrons / Photons defined as: • (isEM() & 0xFB) == 0 OR (isEM() & 0xF7) == 0 • this means no track based cuts • no identification on 1st / 2nd LAr sampling) • Overlap distance: R = 0.2 SUSY discovery potential in trilepton final states: Technical Aspects

  31. Dilepton mass vs. missing Et No normalisation here SUSY discovery potential in trilepton final states: Technical Aspects

  32. 3rd lepton requirement + track-isolation (SU2) 10 fb-1 OSSF dilepton pair 10 fb-1 OSSF + 3rd lepton, track isolation SUSY discovery potential in trilepton final states: Technical Aspects

  33. Backup & further discussion SUSY discovery potential in trilepton final states: Technical Aspects

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