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Higgs Boson Search with L3 at LEP II

This seminar outlines the theoretical aspects of Higgs Boson search, the LEP II Collider and L3 Detector, and various analysis and search strategies used by L3 to search for the Higgs Boson. Conclusions and perspectives are also discussed.

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Higgs Boson Search with L3 at LEP II

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  1. Higgs Boson Search with L3 at LEP II Steve Muanza, IPN Lyon (France) • Outline • Theoretical Aspects • LEP II Collider and L3 Detector • Search for SM Higgs Boson in L3: • Production Mechanisms and Cross Section • Decay Modes and Final State Topologies • SM Backgrounds • Four jet channel • “Missing Energy” channel • Leptonic channels • L3 Channels Combination • L3 Limit • Combination with other LEP Experiments • Conclusions and Perspectives BNL Seminar, 2 May 2002

  2. Theoretical Aspects Higgs Mechanism in SM Mass Generation • Gauge Bosons: • Charged Fermions: • Higgs Boson: BNL Seminar, 2 May 2002

  3. Theoretical Aspects Theoretical Indirect Constraints on Higgs Boson Mass Triviality: of the coupling Vacuum stability: BNL Seminar, 2 May 2002

  4. Theoretical Aspects Experimental Indirect Constraints on Higgs Boson Mass BNL Seminar, 2 May 2002

  5. LEP II Collider Located at CERN (R=4.2 km) 4 Experiments: ALEPH, DELPHI, L3, OPAL • LIL: 600 MeV • EPA • PS: 3.5 GeV • SPS: 22 GeV • LEP BNL Seminar, 2 May 2002

  6. LEP II Collider Collided e+e- LEP I (1989-1995) LEP II (1996-2000) L3 Integrated Luminosity BNL Seminar, 2 May 2002

  7. L3 Detector TEVATRON BNL Seminar, 2 May 2002

  8. L3 Detector Particle/Object ID BNL Seminar, 2 May 2002

  9. Analyses: Higgs Boson Production Production Mechanisms Signal Cross Section BNL Seminar, 2 May 2002

  10. Analyses: Higgs Boson Decays Higgs Boson Branching Fractions Z Boson Branching Fractions B-tagging is crucial BNL Seminar, 2 May 2002

  11. Analyses: Higgs Boson Decays Signal Topologies (82%) BNL Seminar, 2 May 2002

  12. Analyses: Higgs Boson Decays BNL Seminar, 2 May 2002

  13. Analyses: SM Backgrounds Main Backgrounds BNL Seminar, 2 May 2002

  14. Analyses: Search Strategy Loose Preselection (Topology) Tight Preselection Multivariate Selection (NN or Likelihood) Mass Discriminant Final Discriminant Main variable: B-tagging Constrained Kinematic Fits (5C or 4C) Signal Estimator Confidence Levels BNL Seminar, 2 May 2002

  15. Analyses: B-Tagging L3 Implementation • NN combination of: • Non-isolated muon tagging, • Non-isolated electron tagging, • Decay length (high IP tracks), • Explicit 2ndary vertex reco.,… BNL Seminar, 2 May 2002

  16. Analyses: Higgs Boson Mass Reconstruction • Use kinematic fits • Aim: Improve the mass resolution • Method: do the HZ kinematic hypothesis • Take advantage of: • Momentum conservation (3C) • Energy conservation (1C) • Mass constraint (1C) BNL Seminar, 2 May 2002

  17. Analyses: 4j Channel BNL Seminar, 2 May 2002

  18. Analyses: 4j Channel BNL Seminar, 2 May 2002

  19. Analyses: 4j Channel BNL Seminar, 2 May 2002

  20. Analyses: 2j+Emiss Channel BNL Seminar, 2 May 2002

  21. Analyses: 2j+Emiss Channel BNL Seminar, 2 May 2002

  22. Analyses: 2j+Emiss Channel BNL Seminar, 2 May 2002

  23. Analyses: 2j+Emiss Channel BNL Seminar, 2 May 2002

  24. Analyses: 2j+Emiss Channel BNL Seminar, 2 May 2002

  25. L3 Detector BNL Seminar, 2 May 2002

  26. Analyses: 2j+Emiss Channel Checking the EGAP response BNL Seminar, 2 May 2002

  27. Analyses: 2j+Emiss Channel Checking the EGAP response BNL Seminar, 2 May 2002

  28. Analyses: ¨Lepton¨ Channel BNL Seminar, 2 May 2002

  29. Analyses: ¨Lepton¨ Channel BNL Seminar, 2 May 2002

  30. Analyses: Limits Derivation Statistical Estimator BNL Seminar, 2 May 2002

  31. Analyses: L3 Search Results BNL Seminar, 2 May 2002

  32. Analyses: L3 Search Results BNL Seminar, 2 May 2002

  33. Analyses: Limits Derivation Confidence Levels Systematics uncertainties are accounted for by varying (gaussian distr.) S and B in the MC trials CLs(1-CLB) : measures resp. the incompatibility between the observed events and the S(B) hypothesis (resp. exclusion / discovery CL) Exclusion: CLS= CLS+B / CLB CLS < 5% (S hypothesis excluded at 95% CL) Discovery: 1-CLB < 2.7x10-3 (3s: signal ¨hint¨) 1-CLB < 5.7x10-7 (5s: actual signal discovery) BNL Seminar, 2 May 2002

  34. Analyses: L3 Search Results BNL Seminar, 2 May 2002

  35. Analyses: L3 Search Results Systematic Uncertainties LEP DECM=30 MeV Lumi: 1% Analyses Topological cuts: O(1%) B-tagging: 3-5% MC Stat.: 2% (S) 8% (B) Theory Ds/s=O(1%) • Overall systematics: • Signal: 4% • Background: 8-10% • + qq background (populates the high mass region for 4j and 2j+Emiss channels) • Uncertainty on Nexp(background): 6-15% • Uncertainty on Nexp(signal): 3-6% • (Both for mH close or beyond HZ kinematic limit) BNL Seminar, 2 May 2002

  36. Analyses: LEP Search Results BNL Seminar, 2 May 2002

  37. Analyses: LEP Search Results Most Significant Candidates BNL Seminar, 2 May 2002

  38. Analyses: LEP Search Results BNL Seminar, 2 May 2002

  39. Analyses: LEP Search Results BNL Seminar, 2 May 2002

  40. Analyses: LEP Search Results BNL Seminar, 2 May 2002

  41. Analyses: LEP Search Results BNL Seminar, 2 May 2002

  42. Analyses: LEP Search Results BNL Seminar, 2 May 2002

  43. Analyses: LEP Search Results BNL Seminar, 2 May 2002

  44. Conclusions • L3: No signal observed (despite the interesting candidate) • Limit: • LEP: Interesting excess observed 2.1s • Compatible with a background fluctuation • ¨Compatible¨ with a 115 GeV signal • The case is still open!!! • Limit: BNL Seminar, 2 May 2002

  45. Prospects at TEVATRON Run II BNL Seminar, 2 May 2002

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