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Searches for the Higgs Boson at the TEVatron

Daniela Bortoletto Purdue University. Searches for the Higgs Boson at the TEVatron. THE STANDARD MODEL HIGGS. SM unifies weak and electro-magnetic interactions. Experimentally: weak gauge bosons are massive  EWK symmetry breaking. BEH mechanism.

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Searches for the Higgs Boson at the TEVatron

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  1. Daniela Bortoletto Purdue University Searches for the Higgs Boson at the TEVatron D. BortolettoMoriond QCD

  2. THE STANDARD MODEL HIGGS SM unifiesweakandelectro-magneticinteractions Experimentally:weak gauge bosons are massive  EWK symmetrybreaking BEHmechanism • Finding the Higgs boson is essential to confirm the validity of the BEH mechanism • The search is difficult since mH is not predicted in SM • Since the Higgs decays very quickly (10-24 s)  it can be observed only through its decays into other particles • The Higgs couples to mass and decays preferentially to the heaviest objects kinematically allowed D. Bortoletto Moriond QCD

  3. Higgs boson phenomenology • Higgs decay modes and searches in 1975: D. Bortoletto Moriond QCD D. Bortoletto, RPM, Berkeley 3

  4. THE TEVATRON Peak instantaneous luminosity L=4.31 1032 cm-2 sec-1 End of operation September 2011 • Proton-antiproton collider with 1.96TeV center-of-mass energy • 396 ns between bunches Fermilab Tevatron CDF 1 km D0 ≈ 12 fb-1 delivered ≈ 10 fb-1 acquired by the experiments D. BortolettoMoriond QCD

  5. HIGGS PRODUCTION AND DECAY 135 GeV 100 GeV LOW MASS HIGH MASS 1 TeV • Four main production mechanisms at hadron colliders - - qq'qq' H (VBF) ggH qqWH qqZH Branching fraction too small for discovery at the TEVATRON D. Bortoletto Moriond QCD

  6. The Higgs challenge S/B • Many of the background processes have cross section orders of magnitude larger than the Higgs W Z WγZγ WW tt WZ t ZZ Expect 167 SM Higgs events (reconstructed and selected) and ~200,000 events from SM backgrounds for mH=125 GeV/c2 D. BortolettoMoriond QCD Potential Higgs signal is TINY Maximize signal acceptance Excellent modeling of background processes Use multivariate analysis techniques (MVA) to fully exploit all kinematic differences

  7. Main Higgs channels at the Tevatron • Low Mass MH < 135 GeV/c2 WH®lnbb ZH®llbb ZH®nnbb WH®(l)nbb Select: Strategy: • Maximize lepton reconstruction and selection efficiencies • Maximize efficiency for tagging b-quark jets • Optimize dijet mass resolution • High MassMH > 135 GeV/c2 • Main channel: ggH WW which is also important at low mass • High PT leptons and Missing transverse energy 7 D. Bortoletto Moriond QCD 0,1,2 leptons and/or missing Et Two or three high Et jets 7

  8. Tevatron Higgs searches Total H WWll  WH  l bb ZH bb ZH llbb D. Bortoletto Moriond QCD

  9. Higgs analyses strategies Verify modeling of background Select data sample Control regions Apply loose selections Multivariate techniques Improve S/B Improve S/B Channel 1 Template 1 Signal region Template 2 Channel 2 ……. ……. Separate into channels based on S/B Systematics and correlations Limits or signal significance D. Bortoletto Moriond QCD

  10. Improvements since summer 2011 • Both experiment are: • Validating the Higgs search techniques in WZ/ZZ→ X + bb searches (talks on Thursday) • Cross section is ~5 times higher • Using 25% more luminosity in many analysis • New techniques, improved MVA and modeling to increase the sensitivity • Additional triggers and leptons • CDF • New multivariate b-tagger optimized for H ®bb jets (HOBIT)with ~20% more acceptance Light Jets b-jets HOBIT SECVTX D. Bortoletto Moriond QCD

  11. ZH®llbb Z+qq - like other • MVA Improvements • Many backgrounds processes are present the llbb selection • The individual processes have different kinematics • We utilize the three expert networks to assign events to distinct regions in the final event discriminant used in the extraction of upper limits. WZ, ZZ - like ZH - like tt-like other D. Bortoletto Moriond QCD

  12. ZH®llbb Identify events with enhanced S/B s/b=1/1 WZ, ZZ Tagged events Z+ qq Is the event tt-like? WZ/ZZ like? Z+qq like? ZH tt NO Region 4 NO NO Region 1 Region 3 Region 2 YES YES YES D. Bortoletto Moriond QCD

  13. MET+bb Increasing purity • 50% of signal is fromWHwith lost leptons Medium b Tight b s/ b=0.3% s/ b=1.5% • Add together b-tagger outputs for both jets • Cut on the sum instead of per jet cuts • Use Missing pTTRK to suppress multijet background • Exclude isolated tracks from Missing pTTRK to improve WH acceptance by 10% 25% improvement in sensitivity expected from additional data: 6% D. Bortoletto Moriond QCD

  14. Met +bb • Improve jet energy resolution with Neural network which correlates jet-related variables and returns most probable jet energy based on bottom quark hypothesis • Jet energy is currently used only to determined corrected MET. Selection improves S/B separation b-targeted corrections Multi-jet Signal mass resolution S/B=1/5 Higgs Analysis does not yet use HOBIT. Further improvements expected D. BortolettoMoriond QCD

  15. Limits for H®bb Limits at MH = 115 GeV: Exp: 1.71 x σ(SM) Obs: 1.79 x σ(SM) Limits at MH = 125 GeV: Exp: 2.49 x σ(SM) Obs: 3.29 x σ(SM) TEVATRON Broad excess observed in H→bb Largest Excess: 135 GeV LEE of 2 for range from 100 to 150 GeV/c2 D. Bortoletto Moriond QCD

  16. Limits for H®bb Limits at MH = 115 GeV: Exp: 1.71 x σ(SM) Obs: 1.79 x σ(SM) Limits at MH = 125 GeV: Exp: 2.49 x σ(SM) Obs: 3.29 x σ(SM) TEVATRON Broad excess observed in H→bb Largest Excess: 135 GeV LEE of 2 for range from 100 to 150 GeV/c2 D. Bortoletto Moriond QCD

  17. Tevatron combination: WZ and ZZ • same final state • same set of tagged events • different MVA optimized for WZ and ZZ events W/Z+Z→bb: σobs= (1.01 ± 0.21) x σSM D. Bortoletto Moriond QCD

  18. TEVATRON COMBINATION SM HIGGS TEVATRON 95% C.L. upper limits on SM Higgs boson production − Expected exclusion: 100 < MH < 120 GeV, 141 < MH < 184 GeV − Observed exclusion: 100 < MH < 106 GeV, 147 < MH < 179 GeV D. Bortoletto Moriond QCD

  19. MH=125 GeV MH=125 GeV High s/b region MH=125 GeV Right-to-left Integral of S/B distribution Log 10(S/B) Fits to data, with background subtraction MH=165 GeV MH=165 GeV High s/b region MH=165 GeV D. Bortoletto Moriond QCD

  20. The excess • Simple overlay of H→bb signal prediction for the dijet invariant mass (MH = 120 GeV) • Data and diboson prediction from Tevatron low mass WZ/ZZ measurement • Additional signal is not incompatible • Local p-value distribution for background onlyexpectation • Minimum local p-value: 2.7 standard deviations • Global p-value with LEE factor of 4 range from 100 to 200 GeV/c2 : 2.2 standard deviations D. Bortoletto Moriond QCD

  21. The excess • Simple overlay of H→bb signal prediction for the dijet invariant mass (MH = 120 GeV) • Data and diboson prediction from Tevatron low mass WZ/ZZ measurement • Additional signal is not incompatible • Local p-value distribution for background onlyexpectation • Minimum local p-value: 2.7 standard deviations • Global p-value with LEE factor of 4 range from 100 to 200 GeV/c2 : 2.2 standard deviations D. Bortoletto Moriond QCD

  22. Thank you to Michelle Stancari, Joe Haley, Homer Wolfe,SatishDesai, Wade Fisher, Tom Junk, Eric James, KarolosPotamianons, Quiguna Liu, and many others Conclusions • Tevatron experiments are now analyzing full data set in most channels • More improvements are expected in the near future • The data appears to be incompatible with the background, with a global P-value of 2.2 s.d. ( 2.7 local ) • H→bb only: 2.6 s.d. ( 2.8 local ) • Higgs mass range of 115 < MH < 135 continues to be very interesting • Let us hope that 2012 is the year of the Higgs boson • For additional details see • Tevatron: http://tevnphwg.fnal.gov/results/SM_Higgs_Winter_12/ • CDF: http://wwwcdf.fnal.gov/physics/new/hdg/Results.html • D0: http://wwwd0.fnal.gov/Run2Physics/WWW/results/higgs.html D. Bortoletto Moriond QCD

  23. BACKUP D. Bortoletto Moriond QCD

  24. CONSTRAINTS ON THE HIGGS New CDF 2012 W mass MW= 80387 ± 12 stat ± 15 systMeV/c2 New World Average MW= 80390 ± 16 MeV/c2 • SM parameters ( MW , Mt , Z pole measurements etc) • MH<145 GeV @ 95% CL • MH = 90+29-23 GeV • Many direct searches at the Large Electron Positron Collider, TEVATRON proton anti-proton collider, nd the LHC Exclusions of MH: − LEP < 114 GeV (arXiv:0602042v1) − Tevatron [156,177] GeV( arXiv:1107.5518) − LHC [~127, 600] GeV arXiv:1202.1408 (ATLAS) arXiv:1202.1488 (CMS) D. Bortoletto Moriond QCD

  25. Modeling llbb final discriminant in the pretag region which is background dominated D. Bortoletto Moriond QCD

  26. H → WW Limits at MH = 125 GeV: Exp: 3.14 x σ(SM) Obs: 3.50 x σ(SM) Limits at MH = 125 GeV: Exp: 3 x σ(SM) Obs: 3 x σ(SM) D. Bortoletto Moriond QCD

  27. Limits for H->WW • Final states: ee, μμ and eμ • Exploit spin correlations to control backgrounds • Z → ll is major background for ee and μμchannels • Use Boosted Decision Trees to control backgrounds from Z → ee, μμ • Signal and background composition vary with jet multiplicity • Consider multiple signals: Gluon fusion, Vector boson fusion,H→ ZZ... D. Bortoletto Moriond QCD

  28. CDF and D0 Individual results Winter 2012 Summer 2011 D. Bortoletto Moriond QCD

  29. ZHnnbb • 21% additional luminosity • Small improvements in background rejection • Limits show same basic behavior with 0.5 to 1.0σ increases in significance of excess Summer 2011 Winter 2012 D. Bortoletto Moriond QCD

  30. D. Bortoletto Moriond QCD

  31. WHlnbb • 26% (69%) additional luminosity for 2-jet (3-jet) channels • 5-10% level lepton acceptance/trigger efficiency improvements • New HOBIT b-tagger equivalent to adding another 20% in additional luminosity • Limits show same basic behavior with 1.0 to 1.5σ increases in significance of excess Summer 2011 Winter 2012 D. Bortoletto Moriond QCD

  32. ZHllbb • 23% additional luminosity • More gain from HOBIT in this analysis than WH (original tagging not as sophisticated) • 56% of data events in current analysis were not included in previous analysis! • 37% sensitivity improvement (4.67® 2.95 at mH=120 GeV/c2) Summer 2011 32 Winter 2012 D. Bortoletto Moriond QCD

  33. ZHllbb • Electron channels • Here we observe a significant change Summer 2011 Winter 2012 D. Bortoletto Moriond QCD

  34. ZHllbb • ZHllbbchannel has . . . • lowest backgrounds • smallest expected signal yields (9 events for mH=120 GeV/c2) • Some discriminant bins with large S/B • Low probability for observing events in these bins • A few such events can have substantial effects on observed limits S = 0.16 events, B= 0.06 events 34 D. Bortoletto Moriond QCD

  35. H → WW • 18% additional data • Small signal acceptance improvements (0.1 < ΔRll < 0.2) • No appreciable change in behavior of limits Summer 2011 Winter 2012 D. Bortoletto Moriond QCD

  36. H->ZZ D. BortolettoMoriond QCD

  37. D. Bortoletto Moriond QCD

  38. D. Bortoletto Moriond QCD

  39. Measurement of WZ and ZZ WZ and ZZ events • same final state • same set of tagged events • different MVA optimized for WZ and ZZ events • s(WZ+ZZ)= 4.08 ± 1.32 pb Significance 3.2σ s(WZ+ZZ)= 5.0±1.0±1.3 pb Significance: 3.3σ s(WZ+ZZ): Theory= 4.4±0.3 pb D. BortolettoMoriond QCD

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