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Search for the Higgs Boson Decay Mode - ATLAS Experiment

Discover the search for the Higgs boson in the decay mode by Remi ZAIDAN from The University of Iowa on behalf of The ATLAS Experiment. Focus on the observation of a new Higgs-like particle, its mass, decay into vector bosons, and the yet unobserved direct decay into fermions. Explore motivations, Higgs production mechanisms, signal channels, jet calibration, b-tagging, background estimations, and exclusion limits. Results include exclusions expressed in terms of cross-sections relative to the standard model prediction.

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Search for the Higgs Boson Decay Mode - ATLAS Experiment

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  1. Search for the Higgs boson in the decay mode Remi ZAIDAN (The University of Iowa) On behalf of The ATLAS Experiment US LHC UsersOrganization Annual Meeting - Fermilab 2012

  2. decay mode: Motivation • Observation of a new Higgs-like particle: • m= 126 GeV • Observed decay into vector bosons: , and • Not yet observed direct decay into fermions: and arXiv:1207.7214 *p0 = probability of the background only hypothesis Remi ZAIDAN - USLUO Annual Meeting 2012

  3. decay mode: Motivation • : • Highest branching ratio at low masses arXiv:1201.3084 Remi ZAIDAN - USLUO Annual Meeting 2012

  4. Higgs production • Direct production: • gg fusion and VBF: By far the highest cross-sections • Suffers from a huge QCD multi-jets backgrounds • Associated production: • , and : smaller cross-section but cleaner signal arXiv:1101.0593 Remi ZAIDAN - USLUO Annual Meeting 2012

  5. Signal channels Remi ZAIDAN - USLUO Annual Meeting 2012

  6. A word about Jets Remi ZAIDAN - USLUO Annual Meeting 2012

  7. Jet calibration and uncertainties • Jet reconstruction: • Anti-kt(R=0.4) reconstructed at the EM scale with a global calibration to the hadronic scale • Use tracks in the jet to reject jets from pile-up Uncertainties on the energy scale Jet energy resolution arXiv:1112.6426 Remi ZAIDAN - USLUO Annual Meeting 2012

  8. b-jet identification (b-tagging) • Spatial tagging: • B hadrons have a significant flight path length: • E(B) ~ 50 GeV ⇒ L ~ 5 mm • Secondary vertex in jets. • Tracks with high impact parameter to the primary vertex. • Single Neural Network based discriminant • Performance: • b-tagging efficiency: 70% • Light jet rejection*: ~130 • c-jet rejection: ~5 ATLAS-CONF-2012-043 *rejection= 1/efficiency Remi ZAIDAN - USLUO Annual Meeting 2012

  9. Analysis:Selection, backgrounds and limits Remi ZAIDAN - USLUO Annual Meeting 2012

  10. Main backgrounds W+jets, Z + jets Dibosons:WW QCD Multi-jets Top: , single top Remi ZAIDAN - USLUO Annual Meeting 2012

  11. Selection: • Signal definition: • One high isolated lepton • Used for the trigger • High missing transverse energy • 2 high b-tagged jets • Veto events with extra leptons or jets • Strategy: • Search for excess in the invariant mass of the two b-jets distribution • Split into 4 bins to enhance sensitivity: • Different signal to background ration in each bin • and Remi ZAIDAN - USLUO Annual Meeting 2012

  12. Background estimation: • QCD Multi-jets: from data • ETmiss templates in inverted lepton isolation region for WHlnb • Diboson: from MC • Top+jets and W+jets: • Shape from MC distribution • Normalization from control region: • GeV&GeV • W+3 jets arXiv:1207.0210 Remi ZAIDAN - USLUO Annual Meeting 2012

  13. Results: • Exclusion limits: • 3.3 to 5.9 times the SM Limits are expressed in terms of the cross-sections divided by the SM prediction arXiv:1207.0210 arXiv:1207.0210 Remi ZAIDAN - USLUO Annual Meeting 2012

  14. Selection: • Signal definition: • 2 high same flavor isolated leptons: • Opposite sign if muons. • Used for the trigger • Low missing transverse energy • 2 high b-tagged jets • Compatibility of the di-lepton invariant mass with a Z decay • Veto events with extra leptons or jets • Strategy: • Search for excess in the invariant mass of the two b-jets distribution • Split into 4 bins to enhance sensitivity: • Different signal to background ration in each bin • and Remi ZAIDAN - USLUO Annual Meeting 2012

  15. Background estimation: • QCD Multi-jets: from data • mllside-bands • Contribution < 1% • Diboson: from MC • Top+jets and Z+jets: • Shape from MC distribution • Normalization from control region: • GeV&GeV • side-bands arXiv:1207.0210 Remi ZAIDAN - USLUO Annual Meeting 2012

  16. Results: • Exclusion limits: • 7.7 to 14.4 times the SM arXiv:1207.0210 arXiv:1207.0210 Remi ZAIDAN - USLUO Annual Meeting 2012

  17. Selection: • Signal definition: • Very high missing transverse energy • Used for the trigger • Confirmed by a missing momentum measured in the inner detector • 2 high b-tagged jets • Back to back with the • Veto events with extra leptons or jets • Strategy: • Search for excess in the invariant mass of the two b-jets distribution • Split into 3 bins: • Different signal to background ration in each bin • and Remi ZAIDAN - USLUO Annual Meeting 2012

  18. Background estimation: • Top+jets and W,Z+jets: • Shape from MC distributions • Normalization propagated from the other channels • Diboson:from MC • QCD Multi-jets: from data • 3 control regions using 2 variables: arXiv:1207.0210 arXiv:1207.0210 Remi ZAIDAN - USLUO Annual Meeting 2012

  19. Results: • Exclusion limits: • 3.7 to 10.3 times the SM arXiv:1207.0210 arXiv:1207.0210 Remi ZAIDAN - USLUO Annual Meeting 2012

  20. Selection: • Signal definition: • One high isolated lepton • Used for the trigger • High missing transverse energy • At least 4 high jets • Strategy: • Events categorized according to the number of b-tagged and non-b-tagged jets: • Different signal to background ration in each bin • Kinematic likelihood fit to reconstruct the pair • Search for excess in the invariant mass distribution of the remaining two b-jets Remi ZAIDAN - USLUO Annual Meeting 2012

  21. Background estimation: • Top and dibosons: from MC • W+jets: • Shape from MC distributions • Normalization from data separating different jet flavors • Use assymmetry: where • Multi-jets: from data • Using Matrix Method with a tight and a loose lepton samples: Remi ZAIDAN - USLUO Annual Meeting 2012

  22. Results: • Exclusion limits: • 7.0 to 33.0 times the SM • 7.0 to 16.4 times the SM for mH < 130 GeV ATLAS-CONF-2012-135 ATLAS-CONF-2012-135 Remi ZAIDAN - USLUO Annual Meeting 2012

  23. Uncertainties and Combination Remi ZAIDAN - USLUO Annual Meeting 2012

  24. Uncertainties • Main experimental uncertainties: • b-jet identification: 0.3 – 5.5% • Jet and ETmiss reconstruction: 1.2 – 12.1% • Background normalization: 1.8 – 4.5% • Other sources: luminosity, pileup, lepton ID, etc… < 3.4% • Theoretical uncertainties: • shapes, cross-sections, PDFs, etc…: 0.3 – 14.9% Remi ZAIDAN - USLUO Annual Meeting 2012

  25. Exclusion limit combination • The combination uses only the VH channels: • 5 mass hypothesis considered: • mH = 110, 115, 120, 125 and 130 GeV • Exclusion limits: • 2.5 to 5.5 times the SM arXiv:1207.0210 Remi ZAIDAN - USLUO Annual Meeting 2012

  26. Outlook and Conclusion Remi ZAIDAN - USLUO Annual Meeting 2012

  27. Ongoing improvements • Improving mbb resolution: • Correcting for the semi-leptonic b-jets decay with muons in jets • Inclusive jet corrections at parton level • Multi-variate approach for signal/background separation: Boosted Decision Tree • Preliminary MC based studies show significant improvement Remi ZAIDAN - USLUO Annual Meeting 2012

  28. Conclusion • Search for Higgs boson decaying into b-quark pair: • Three channels included in the final combination: • WH->lνbb, ZH->ννbb, ZH->llbb • Results from were available at a later stage • Combined limit:σ/σSM= 2.5-5.5 for mH < 130 GeV • Analysis to be updated soon • Additional data still to come: • Already 15 fb-1 collected @ 8 TeV • LHC will run until early 2013 • Improved analysis: • Improved b-jet energy scale and mbb resolution • Analysis based on multivariate techniques Remi ZAIDAN - USLUO Annual Meeting 2012

  29. Back-up Remi ZAIDAN - USLUO Annual Meeting 2012

  30. ATLAS data taking • 2011 data taking: • 93.5% data taking efficiency • = 9.1 interactions/BC • 5.25 recorded luminosity • 2012 data taking • 93.9% data taking efficiency • = 20 interactions/BC • 15.4 recorded luminosity Remi ZAIDAN - USLUO Annual Meeting 2012

  31. Selection: • system: • One charged lepton ( or ): • GeV ; • GeV • GeV • Extra lepton veto: • and • GeV • system: • 2 b-tagged jets: • ; GeV • if GeV • Extra jet veto: • ; GeV • Split into 4 bins: • and Remi ZAIDAN - USLUO Annual Meeting 2012

  32. Selection: • system: • Two opposite sign leptons ( or ): • GeV • GeV • system: • 2 b-tagged jets: • ; GeV • if GeV • Split into 4 bins: • and Remi ZAIDAN - USLUO Annual Meeting 2012

  33. Selection: • system: • Transverse missing energy/momentum: • GeV ; GeV • Extra lepton veto: • and • GeV • system: • 2 b-tagged jets: • ; GeV • if GeV • ifGeV • Extra jet veto: • ; GeV • Split into 3 bins: Remi ZAIDAN - USLUO Annual Meeting 2012

  34. Selection: • Electron channel: • One isolated electron: • GeV • GeV • GeV • Muon channel: • One isolated muon: • GeV • GeV • GeV • At least 4 jets: • GeV • Events categorized according to jet multiplicities: • 9 categories used for the analysis • Kinematic reconstruction: • Kinematic likelihood fit to reconstruct the pair • Remaining two jets are used for candidate Remi ZAIDAN - USLUO Annual Meeting 2012

  35. bins: arXiv:1207.0210 arXiv:1207.0210 arXiv:1207.0210 arXiv:1207.0210 Remi ZAIDAN - USLUO Annual Meeting 2012

  36. bins: arXiv:1207.0210 arXiv:1207.0210 arXiv:1207.0210 arXiv:1207.0210 Remi ZAIDAN - USLUO Annual Meeting 2012

  37. bins: arXiv:1207.0210 arXiv:1207.0210 arXiv:1207.0210 Remi ZAIDAN - USLUO Annual Meeting 2012

  38. Background composition: Remi ZAIDAN - USLUO Annual Meeting 2012

  39. Systematics : channels arXiv:1207.0210 Remi ZAIDAN - USLUO Annual Meeting 2012

  40. Monte Carlo Samples • Signal: • PYTHIA + MRST (LO*) PDFs • Cross-section: LHC Higgs Cross Section Working Group report (arXiv:1101.0593) • Branching ratios: HDECAY • W+b: POWHEG + MSTW 2008 NLO PDFs + PYHTIAparton shower • Z+b, Z+c: SHERPA • W+c, W+j, Z+j: ALPGEN+HERWIG • Diboson: HERWIG • ttbar: MC@NLO + CT10 NLO PDFs + HERWIG • HERWIG: • MRST (LO*) PDFs • ATLAS tune AUET2 Remi ZAIDAN - USLUO Annual Meeting 2012

  41. How to interpret exclusion limits Remi ZAIDAN - USLUO Annual Meeting 2012

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