1. Suyong Choi (SKKU) SUSY 2008 1 Standard Model Higgs Searches at DØ Suyong Choi SKKU, Korea for DØ Collaboration

2. Suyong Choi (SKKU) SUSY 2008 Suyong Choi (SKKU) PASCOS 2006, Columbus, Ohio 2 Higgs in the Standard Model 1.Mass of elementary particles – coupling to massive particles stronger 2.Electro-weak symmetry breaking Higgs in the Standard Model 1.Mass of elementary particles – coupling to massive particles stronger 2.Electro-weak symmetry breaking Particles of the Standard Model

3. Suyong Choi (SKKU) SUSY 2008 3 Limits on M H Current limits –Direct searches at LEP M H > 114 GeV @ 95% CL –Fits to electroweak data M H < 160 GeV @ 95% CL –M H <190 GeV if direct search result included  Light Higgs favored Tevatron –Direct Searches : rule out or find evidence –Precision m t and M W measurements M H =87 +36 -27 GeV

4. Suyong Choi (SKKU) SUSY 2008 4 SM Higgs Production at the Tevatron Though Higgs production copious, not all channels are accessible gg→H –Useful for M H >140 GeV –H→WW→ll –Background: WW qq  W/Z+H –M H <140 GeV –WH→l bb –ZH→llbb, bb –Background: W+bb, Z+bb, top Sensitivity studies have shown that all channels must be studied, CDF + D0 combination is essential pb

5. Suyong Choi (SKKU) SUSY 2008 Tevatron Collider Status Excellent performance –Steady increase in instantaneous luminosity –>85% data collecting efficiency 5 Results presented today are based on 1.0 ~ 2.3 fb -1 of data 4.3 fb -1 3.7 fb -1

6. Suyong Choi (SKKU) SUSY 2008 W Associated Production WH → bb Sensitive for M H <140 GeV –Large  x Br 6 b jets Two b-jets form a resonance

7. Suyong Choi (SKKU) SUSY 2008 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. Suyong Choi (SKKU) SUSY 2008 Neural Network Selection Variables for ANN –p T of two jets –Opening angle of jets –Dijet system p T and mass –p T (lepton +  ) Observation in agreement with background only hypothesis

9. Suyong Choi (SKKU) SUSY 2008 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 9 Expected limits SourcesError Luminosity6% Trigger3~10% Jet energy scale2~6% B-tagging4~6% Background x- section 16~20%

10. Suyong Choi (SKKU) SUSY 2008 WH Search Results

11. Suyong Choi (SKKU) SUSY 2008 11

12. Suyong Choi (SKKU) SUSY 2008 12 Z Associated Production ZH → + - bb Clean Small cross section x Br –M H <140 GeV b jets

13. Suyong Choi (SKKU) SUSY 2008 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% SourcesError Jet energy scale1~7% B-tagging7%

14. Suyong Choi (SKKU) SUSY 2008 Higgs Limits from ZH → + - bb Analysis

15. Suyong Choi (SKKU) SUSY 2008 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. Suyong Choi (SKKU) SUSY 2008 Missing E T + jets Data: 2.1 fb -1 MET > 50 GeV Jets –2 or 3 jets, p T >20 GeV –2 leading jets should not be back-to-back W+jets and multijets dominant –Multijet background due to mismeasured jet E T signal x 500

17. Suyong Choi (SKKU) SUSY 2008 Multivariate Analysis Boosted decision tree result using 26 variables after b-tagging

18. Suyong Choi (SKKU) SUSY 2008 ZH → bb Search Result Systematic uncertainties Limits –best limit in W/Z+H SourcesError Luminosity6.1% Trigger5.5% Jet energy scale1~7% B-tagging6% Background x- section 6~16% Heavy flavor fractions 50%

19. Suyong Choi (SKKU) SUSY 2008 19 H → WW → + - H → WW → + - Important for m H >140 GeV –Final state: 2 leptons + MET –Cannot reconstruct M H Data: 2.3 fb -1 –1.1 (IIa data) + 1.2(IIb data) –ee, e ,  Selection –2 oppositely charged leptons –Large MET –Di-lepton mass –min( M T (e), M T (  ) ) –  –H T  reduce Z, W+jets, tt-bar Analysis optimized for each M H Signal 1.1fb -1

20. Suyong Choi (SKKU) SUSY 2008 NN Analysis PreselectionAfter final selection

21. Suyong Choi (SKKU) SUSY 2008 21 H → WW Factor 2.4 away from SM For M H =160 GeV Systematic uncertainty Combine distributions from different channels –Statistical uncertainty + Correlated systematics SourcesError Trigger5% Jet energy scale5~10% Muon momentum resolution (  ) 11% Muon ID efficiency-5% +8% WW x-section4%

22. Suyong Choi (SKKU) SUSY 2008 Other SM Higgs Searches WH  WWW –3 lepton final state –Recovers sensitivity M H ~ 140 GeV H   –Not a discovery channel at the Tevatron –Analysis with less model dependence 22

23. Suyong Choi (SKKU) SUSY 2008 Combined DØ SM Higgs Results Correlations of systematic errors taken into account

24. Suyong Choi (SKKU) SUSY 2008 Combined CDF and DØ Results 24 Factor 1.1 away!

25. Suyong Choi (SKKU) SUSY 2008 Prospects for M H <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 25

26. Suyong Choi (SKKU) SUSY 2008 Prospects for M H >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 26

27. Suyong Choi (SKKU) SUSY 2008 Expected Higgs Sensitivity in 2009/2010 Assuming 2 experiments 27 2009 2010 2009

28. Suyong Choi (SKKU) SUSY 2008 Summary We searched for Standard Model Higgs boson in all the sensitive channels using the D Ø data Results in agreement with expected backgrounds In 2008, we may be able to exclude new M H range beyond that of LEP –CDF+D0 results combined Many improvements expected to raise sensitivity in a broad range of M H - most exciting years to come!

29. Suyong Choi (SKKU) SUSY 2008 29 Standard Model Higgs Searches WH ZH H→WW

30. Suyong Choi (SKKU) SUSY 2008 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. Suyong Choi (SKKU) SUSY 2008 31 Search for WH → bb Data: 1.7 fb -1 with e and  Event preselection –lepton: p T >15 GeV –Missing E T :E T >20 GeV –2 Jets: p T >20 GeV |  |<2.5 Veto on –Additional high p T track –4 th jet Background is well understood

32. Suyong Choi (SKKU) SUSY 2008 32 Tevatron @ Fermilab @  s=1.96 TeV Circumference 6 km The only place to directly look for Higgs and supersymmetric particles until LHC

33. Suyong Choi (SKKU) SUSY 2008 33 b-jet Tagging 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 Primary vertex Secondary vertices

34. Suyong Choi (SKKU) SUSY 2008 Neural Network Analysis After final selection Neural network used to maximize sensitivity –p T of leptons –m –  –MET –angles between MET and leptons –minimum transverse mass 34

35. Suyong Choi (SKKU) SUSY 2008 35 Search for H → WW → + - Search for H → WW → + - Important channel for m H >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. Suyong Choi (SKKU) SUSY 2008 Combined SM Higgs Results Use as inputs the discriminant outputs from each analysis

37. Suyong Choi (SKKU) SUSY 2008 Kinematic Distributions of b- jets

38. Suyong Choi (SKKU) SUSY 2008 b-tagging in ZH → + - bb Use both single and double-tagged events –S/B are different Single tag –  b =45%,  j =0.5% Electron Channel Muon Channel Observed7387 Expected 57  21101  38 Higgs ( m H =115GeV) 0.230.31

39. Suyong Choi (SKKU) SUSY 2008 b-tagging in ZH → + - bb Double tag –  b =72%,  j =6% per jet Electron Channel Muon Channel Observed2453 Expected28  8 46  13 Z + jets (udsgc) 1019 Z + bb 12.018.5 Tt-bar 3.15.3 Higgs (m H =115GeV) 0.230.30

40. Suyong Choi (SKKU) SUSY 2008 Neural Network Analysis 9 kinematic variables used 10k signal events and 100k background events NN architecture optimized to yield best significance

41. Suyong Choi (SKKU) SUSY 2008 MET + b-jets Asymmetric b-tagging requirements on the two hadronic jets –maximizes sensitivity 40340 events439 events

42. Suyong Choi (SKKU) SUSY 2008 42 Standard Model Higgs Decays Higgs prefers to decay to massive particle kinematically allowed bb for M H < 140 GeV WW for M H > 140 GeV

43. Suyong Choi (SKKU) SUSY 2008 ZH → + - bb Selection Data: 1.1 fb -1 Dileptons –p T >15 (10) GeV for e (  ) –In well-instrumented region of the detector and isolated –70 GeV < M < 110 GeV Jets –p T >15 GeV and |  |<2.5 –b-tagging After dilepton+jets selection before b-tag