1. LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 1 Higgs Searches at DØ LCWS06, Bangalore, India March 9-13, 2006 Marcel Demarteau Fermilab For the DØ Collaboration

2. LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 2 LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 2 Tevatron Experimental Program Tevatron Run I: –1992 - 1996 –∫ L dt = 120 pb -1 –E CM = 1.8 TeV Tevatron Run IIa: –Δt = Δt(Run I) (2001 - 2006) –∫ L dt = 10 * ∫ L dt (Run I) –E CM = 1.96 TeV Tevatron Run IIb: –Δt < Δt(Run IIa) (2006 - 2009) –∫ L dt ≈ 5 * ∫ L dt (Run IIa) –E CM = 1.96 TeV Outline: –Introduction –Low mass Higgs search –High mass Higgs search –MSSM Higgs search –Prospects

3. LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 3 LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 3 Constraints on Higgs Mass in SM Limit from direct searches at LEP: m H > 114.4 GeV (95% CL) Indirect limit from fits to precision EW data from LEP, SLC and Tevatron –m H < 219 GeV (95% CL) with –m t = 172.7 ± 2.9 GeV (CDF, D0, Run I+II) Indirect best fit value –m H = 91 +45 -32 GeV

4. LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 4 LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 4 SM Higgs Boson Production and Decay Dominant Decays: –Low mass: bb; High mass: W + W - Search strategy: –M H < 135 GeV: associated production qq → WH / ZH production, with H → bb Dominant backgrounds: Wbb, Zbb and tt –M H > 135 GeV: direct production gg → H (or WH), with H → WW* Dominant backgrounds: WW and WZ production σ (pb) Excluded at LEP

5. LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 5 LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 5 Search Channels for SM Higgs Notes: –WH channel in leptonic modes with b-tag –ZH channel all hadronic decay mode –WW exploit scalar nature of Higgs –WWW like-sign di-leptons ReachSearch ChannelInt. Lumi Low Mass 382 pb -1 261 pb -1 High Mass 299 - 325 pb -1 363 - 384 pb -1

6. LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 6 LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 6 WH → e bb Event selection: –central electron p T > 20 GeV, |  |<1.1 –veto second lepton –Missing E T > 25 GeV –Two b-jets p T > 25 GeV, |  |< 2.5 at least one 1 b-tag (impact parameter b-tag) Backgrounds: –W+jets, multi-jet, tt, single top, WZ Single b-tag sample dominated by background: –Used as control sample Limit extracted from double b-tagged sample using di-jet mass distribution - inclusive (≥1) b-tag

7. LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 7 LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 7 WH → e bb Limit set from di-jet invariant mass spectrum –4 events observed in mass window 85 GeV < m bb <135 GeV –2.37 ± 0.59 events expected background –Thus, upper limit on WH cross section of 8.6 pb. -

8. LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 8 LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 8 ZH → bb Event rate:  · BR  0.01 pb   WH → ℓ bb) Event selection: –Missing E T > 25 GeV –2 aco-planar b-jets, E T > 20 GeV, |  | < 2.5 –b-tagging lifetime probability algorithm: jets having tracks with large impact parameters (JLIP) B-tagging efficiency ~43% with 0.5% mistag rate after quality cuts Backgrounds: –Physics: W+jets, Z+jets, top, WZ, ZZ No isolated track, H T < 200 GeV –H T defined as the scalar sum of jets’ E T –Instrumental: multijets w/mistag Jet aco-planarity, various asymmetries Strategy –Trigger on events with large missing H T –Estimate “instrumental” bckg. from data –Search for an event excess in di-b-jet mass distribution -- ≥ 0 b-tag

9. LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 9 LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 9 ZH → bb: Background Treatment -- Jet1 Jet2 ETET HTHT E T for mis- measured jet Signal MC Data Definition of various missing energy / momentum variables Form various asymmetries For signal events and bb are balanced: asymmetries peak at 0. Measure instrumental background from the sidebands

10. LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 10 LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 10 ZH → bb Set limit on cross section from observed number of events in double tagged sample as function of Higgs mass Higgs Mass (GeV) Window (GeV) 105 [70,120] 115 [80,130] 125 [90,140] 135 [100,150] Data4322 Acceptance (%) 0.29  0.070.33  0.080.35  0.090.34  0.09 Total bkgd. 2.75  0.882.19  0.721.93  0.661.71  0.57 Expected limit (pb)8.87.56.06.5 Limit @ 95% C.L. (pb)12.29.37.7 8.5 Background% Wjj/Wbb32 Zjj/Zbb31 Instrumental16 Top15 WZ/ZZ6 For M H = 115 GeV bin Systematic uncertainty –26% on signal acceptance –33% on background –dominated by uncertainty on b-tag eff. --

11. LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 11 LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 11 H → WW * → ℓ + ℓ - H → WW * → ℓ + ℓ - Dominant production mechanism for higher mass Higgs 3 decay channels, oppositely charged leptons: ee, e ,  Leptonic final state allows for excellent event modeling Event Selection: –p T > 15 (10) GeV for leading (trailing) lepton –missing E T > 20 GeV –sum of p T of leptons and missing E T –H T < 100 GeV Backgrounds: –WW, WZ, ZZ, Drell-Yan, W+jets, W+ , tt, multijets –Z-events removed through  (ℓℓ) cut –tt-events removed through H T cut ee ee

12. LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 12 LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 12 H → WW * → ℓ + ℓ - H → WW * → ℓ + ℓ - Opening angle between leptons (∆  ℓℓ <2.0) is useful discriminant –Two leptons from Higgs tend to move in parallel due to scalar nature of Higgs Limit set on  · BR for all three channels combined as function of Higgs mass

13. LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 13 LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 13 WH → WWW * → ℓ ± ℓ ±  qq Select like-sign isolated leptons –one of W’s from Higgs decay has same-sign lepton as associated W Event Selection: –Two isolated e/  p T > 15 GeV –Third lepton veto –Missing E T > 20 GeV Backgrounds largely avoided –Physics: WZ production –Instrumental: dominant W+jets “charge flips” accounted for by estimating flip probability from data: ratio of like to unlike sign events at high invariant mass (M ll >70 GeV) –P flip = (9.7 ± 3.1) 10 -4 (ee) –P flip = (11.7 ± 2.6) 10 -4 (  )

14. LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 14 LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 14 WH → WWW * → ℓ ± ℓ ±  qq “fermiophobic Higgs“ model –Br(H → WW) close to 100 % for Higgs masses down to ~ 100 GeV –L. Braucher, R. Santos hep-ph/9907434.

15. LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 15 LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 15 Current Limits SM Higgs

16. LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 16 LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 16 Limits / SM

17. LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 17 LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 17 MSSM Neutral Higgs Search In MSSM three neutral Higgs particles predicted, h 0, H 0, A 0 At tree level Higgs coupling to down-type quarks, i.e. b-quarks, is enhanced with respect to the SM, proportional to tan  –Production cross-section rises as tan  2 –Larger  for part of parameter space than SM Search assumptions: –No difference between A 0 and h 0 /H 0 –Mass degeneracy 100-130 GeV: h 0, H 0, A 0 higher mass: h 0, A 0 or H 0, A 0 Total signal cross-section twice that of the A 0 boson Search strategy: –multi-jet high E T sample –3 or more jets b-tagged J. Campbell, R. Ellis, F. Maltoni, S. Willenbrock Alternate, consistent, calculation by: S. Dawson, C. Jackson, L. Reina, D. Wackeroth

18. LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 18 LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 18 MSSM Neutral Higgs Search At least 3 tagged b-jets; Look for excess in di-jet mass spectrum –Signal rates and kinematics are normalized to NLO calculations –Background shape determined from double b-tagged data, applying tag rate function to non-b-tagged jets –Two scenarios considered No mixing in stop sector: X t = A t –  cot  = 0 (m = – 0.2 TeV) Maximal mixing: X t = √6×M SUSY, M SUSY = 1 TeV Fitting outside signal region (±1  of peak)

19. LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 19 LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 19 Prospects Development of new Neural-Network b- tagging algorithm which yields 34% increase in b-jet ID efficiency for the same fake rate Construction of new small radius silicon detector, with expected improvement of impact parameter resolution of ~50% for 10 GeV p T Layer 0 is being installed as we speak !

20. LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 20 LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 20 Summary Full range in mass of Higgs boson being explored in many different channels Current limits still far from Standard Model predictions Significant progress anticipated from: –Analysis of full data sets –Refinements in current analyses and development of new analysis tools –Combination of all different search channels –Combination of results from two collider experiments –Exploitation of new silicon, Layer 0, detector !

21. LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 21 LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 21 Backup

22. LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 22 LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 22 ZH → bb For m H = 115 GeV, expect 2.19 events in di-jet mass window 80 GeV < M jj < 130 GeV –observe 3; thus cross section limit of 9.3 pb at 95% confidence level Systematic uncertainty dominated by uncertainty on b-tagging efficiency: –26% on signal acceptance –33% on background 0 btag 2 btags 1 btag --

23. LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 23 LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 23 Total Data : 2140 Expect : 2125 ZH → bb: Before b-tagging

24. LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 24 LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 24 ZH → bb: Single b-tag Total Data : 132 Expect : 145

25. LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 25 LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 25 ZH → bb: Double b-tag Total Data : 9 Expect : 6.4

26. LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 26 LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 26 Prospects LEP Tevatron ∫Ldt, fb -1 Updated in 2003 in the low Higgs mass region –better understanding of detector –optimization of analyses Sensitivity in the mass region above LEP limit starts at ~2 fb -1 Current results show that optimal sensitivity, as assumed by past Higgs working group studies, has not yet been reached (typically by factor 2-3) –selection efficiency –b-tagging efficiency –trigger efficiency –larger backgrounds –mass resolution –…… But, …

27. LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 27 LCWS06, Bangalore, March 2006, Marcel DemarteauSlide 27 Impact Parameter Resolution