Standard Model Higgs Searches at D Ø

1. Standard Model Higgs Searches at DØ Suyong Choi SKKU, Korea for DØ Collaboration

2. PASCOS 2006, Columbus, Ohio Particles of the Standard Model • Higgs in the Standard Model • Mass of elementary particles – coupling to massive particles stronger • Electro-weak symmetry breaking

3. Limits on MH • Current limits • Direct searches at LEP MH > 114 GeV @ 95% CL • Fits to electroweak dataMH < 160 GeV @ 95% CL • MH <190 GeV if direct search result included  Light Higgs favored • Tevatron • Direct Searches : rule out or find evidence • Precision mt and MW measurements MH=87+36-27 GeV

4. Though Higgs production copious, not all channels are accessible gg→H Useful for MH>140 GeV H→WW→llnn Background: WW qqW/Z+H MH<140 GeV WH→lnbb ZH→llbb, nnbb Background: W+bb, Z+bb, top Sensitivity studies have shown that all channels must be studied, CDF + D0 combination is essential SM Higgs Production at the Tevatron pb

5. Tevatron Collider Status • Excellent performance • Steady increase in instantaneous luminosity • >85% data collecting efficiency 4.3 fb-1 3.7 fb-1 Results presented today are based on 1.0 ~ 2.3 fb-1 of data

6. W Associated Production WH→bb • Sensitive for MH<140 GeV • Large  x Br b jets Two b-jets form a resonance b jets

7. WH Search • Single and double b-tagged jet samples are analyzed separately and optimized 193 observed 204  31 expected 2.3 WH expected Expected signal x 10

8. Variables for ANN pT of two jets Opening angle of jets Dijet system pT and mass pT (lepton +) Observation in agreement with background only hypothesis Neural Network Selection

9. WH Search Results • No excess observed  Set limits on cross section x Br • Limits obtained by fitting the NN output • ST and DT treated as independent channels • Systematics Expected limits

10. WH Search Results

12. Z Associated Production ZH→+-bb • Clean • Small cross section x Br • MH<140 GeV b jets b jets

13. Neural Network Analysis • No significant excess • Set limits on  x Br • NN output distributions • Systematic errors and correlations considered • Systematics • Background error : 28% • Signal eff. error : 8%

14. Higgs Limits from ZH→+-bb Analysis

15. ZH→bb • Advantage of large branching fraction of Z→ • MET + 2 jets – cannot reconstruct Z explicitly • Large multijet background • Recovers leptonic decays of WH and ZH, where leptons were not reconstructed explicitly

16. Missing ET + jets • Data: 2.1 fb-1 • MET > 50 GeV • Jets • 2 or 3 jets, pT>20 GeV • 2 leading jets should not be back-to-back • W+jets and multijets dominant • Multijet background due to mismeasured jet ET signal x 500

17. Multivariate Analysis • Boosted decision tree result using 26 variables after b-tagging

18. ZH→bb Search Result • Systematic uncertainties • Limits • best limit in W/Z+H

19. Important for mH>140 GeV Final state: 2 leptons + MET Cannot reconstruct MH Data: 2.3 fb-1 1.1 (IIa data) + 1.2(IIb data) ee, em,  Selection 2 oppositely charged leptons Large MET Di-lepton mass min( MT(e), MT(m) )  H T reduce Z, W+jets, tt-bar Analysis optimized for each MH H→WW→+- Signal 1.1fb-1

20. NN Analysis • Preselection • After final selection

21. Factor 2.4 away from SM For MH=160 GeV H→WW • Systematic uncertainty • Combine distributions from different channels • Statistical uncertainty + Correlated systematics

22. Other SM Higgs Searches • WHWWW • 3 lepton final state • Recovers sensitivity MH ~ 140 GeV • H   • Not a discovery channel at the Tevatron • Analysis with less model dependence

23. Combined DØ SM Higgs Results • Correlations of systematic errors taken into account

24. Combined CDF and DØ Results Factor 1.1 away!

25. Prospects for MH<140 GeV • We achieved 1.7 factor improvement in sensitivity since 2005 • not including gains due to lumi • We expect additional x2 gain in sensitivity • Optimized b-tagging with inner silicon Layer 0 • semileptonic b-tags • dijet mass resolution • lepton efficiencies • refined multivariate analyses

26. Prospects for MH>140 GeV • We achieved 1.7 factor improvement in sensitivity since 2005 (not including gains due to lumi) • We expect additional x1.4 gain in sensitivity • lepton efficiencies • multivariate analyses

27. Expected Higgs Sensitivity in 2009/2010 • Assuming 2 experiments 2010 2009 2009

28. Summary • We searched for Standard Model Higgs boson in all the sensitive channels using the DØ data • Results in agreement with expected backgrounds • In 2008, we may be able to exclude new MH range beyond that of LEP • CDF+D0 results combined • Many improvements expected to raise sensitivity in a broad range of MH - most exciting years to come!

29. Standard Model Higgs Searches WH ZH H→WW

30. The DØ Detector • Tracking • Precision silicon vertex detector • Scintillating fiber tracker • 2T B-field • Calorimetry • Liquid Argon-Uranium • ||<4 • Excellent linearity and resolution • Muon detector • Low punch throughs • 1.8T toroidal B-field • Trying to exploiting full capabilities

31. Search for WH→bb • Data: 1.7 fb-1 with e and  • Event preselection • lepton: pT>15 GeV • Missing ET: ET>20 GeV • 2 Jets: pT>20 GeV |h|<2.5 • Veto on • Additional high pT track • 4th jet • Background is well understood

32. Tevatron @ Fermilab • @ s=1.96 TeV • Circumference 6 km • The only place to directly look for Higgs and supersymmetric particles until LHC

33. D0 developed sophisticated Neural Network based algorithm Lifetime of a b hadron is quite long (a few mm) Superb efficiency Samples and performance derived from data Fakes are due to finite resolutions of the tracking detector Detector view Decay length Impact parameter Measurement errors b-jet Tagging Primary vertex Secondary vertices

34. Neural Network Analysis • After final selection • Neural network used to maximize sensitivity • pT of leptons • m •  • MET • angles between MET and leptons • minimum transverse mass

35. Search for H→WW →+- • Important channel for mH>140 GeV • Final state:2 leptons + MET • Explicit mass cannot be reconstructed • Signal / background separation by exploiting event topology differences • WW decays from a spin 0 particle • leptons prefer to decay in the same direction

36. Combined SM Higgs Results • Use as inputs the discriminant outputs from each analysis

37. Kinematic Distributions of b-jets

38. b-tagging in ZH→+-bb • Use both single and double-tagged events • S/B are different • Single tag • b=45%, j=0.5%

39. b-tagging in ZH→+-bb • Double tag • b=72%, j=6% per jet

40. Neural Network Analysis • 9 kinematic variables used • 10k signal events and 100k background events • NN architecture optimized to yield best significance

41. MET + b-jets • Asymmetric b-tagging requirements on the two hadronic jets • maximizes sensitivity 40340 events 439 events

42. Standard Model Higgs Decays • Higgs prefers to decay to massive particle kinematically allowed • bb for MH < 140 GeV • WW for MH> 140 GeV

43. ZH→+-bb Selection • Data: 1.1 fb-1 • Dileptons • pT>15 (10) GeV for e () • In well-instrumented region of the detector and isolated • 70 GeV < M < 110 GeV • Jets • pT>15 GeV and ||<2.5 • b-tagging • After dilepton+jets selection before b-tag