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Higgs Searches at DØ

Higgs Searches at DØ. Marcel Demarteau Fermilab For the DØ Collaboration. LCWS06, Bangalore, India March 9-13, 2006. Tevatron Run I: 1992 - 1996 ∫ L dt = 120 pb -1 E CM = 1.8 TeV Tevatron Run IIa: Δ t = Δ t(Run I) (2001 - 2006) ∫ L dt = 10 * ∫ L dt (Run I) E CM = 1.96 TeV

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Higgs Searches at DØ

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

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

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

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

  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 LCWS06, Bangalore, March 2006, Marcel Demarteau

  6. - WH → enbb • Event selection: • central electronpT > 20 GeV, |h|<1.1 • veto second lepton • Missing ET > 25 GeV • Two b-jetspT > 25 GeV, |h|< 2.5at 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-taggedsample using di-jet mass distribution inclusive (≥1) b-tag LCWS06, Bangalore, March 2006, Marcel Demarteau

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

  8. - - ZH → nnbb • Event rate: s · BR  0.01 pb  s(WH → ℓnbb) • Event selection: • Missing ET > 25 GeV • 2 aco-planar b-jets, ET > 20 GeV, |h| < 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, HT < 200 GeV • HT defined as the scalar sum of jets’ ET • Instrumental: multijets w/mistag • Jet aco-planarity, various asymmetries • Strategy • Trigger on events with large missing HT • Estimate “instrumental” bckg. from data • Search for an event excess in di-b-jet mass distribution ≥ 0 b-tag LCWS06, Bangalore, March 2006, Marcel Demarteau

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

  10. - - ZH → nnbb • Set limit on cross section from observed number of events in double tagged sample as function of Higgs mass For MH = 115 GeV bin • Systematic uncertainty • 26% on signal acceptance • 33% on background • dominated by uncertainty on b-tag eff. LCWS06, Bangalore, March 2006, Marcel Demarteau

  11. H → WW* → ℓ+ ℓ- nn • Dominant production mechanism for higher mass Higgs • 3 decay channels, oppositely charged leptons: ee, em, mm • Leptonic final state allows for excellent event modeling • Event Selection: • pT > 15 (10) GeV for leading (trailing) lepton • missing ET > 20 GeV • sum of pT of leptons and missing ET • HT < 100 GeV • Backgrounds: • WW, WZ, ZZ, Drell-Yan, W+jets, W+g, tt, multijets • Z-events removed through Df(ℓℓ) cut • tt-events removed through HT cut ee em mm LCWS06, Bangalore, March 2006, Marcel Demarteau

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

  13. WH → WWW* → ℓ± ℓ± nn 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/m pT > 15 GeV • Third lepton veto • Missing ET > 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 (Mll>70 GeV) • Pflip = (9.7 ± 3.1) 10-4 (ee) • Pflip = (11.7 ± 2.6) 10-4 (mm) LCWS06, Bangalore, March 2006, Marcel Demarteau

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

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

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

  17. MSSM Neutral Higgs Search • In MSSM three neutral Higgs particles predicted, h0, H0, A0 • At tree level Higgs coupling to down-type quarks, i.e. b-quarks, is enhanced with respect to the SM, proportional to tanb • Production cross-section rises as tanb2 • Larger s for part of parameter space than SM • Search assumptions: • No difference between A0 and h0/H0 • Mass degeneracy • 100-130 GeV: h0, H0, A0 • higher mass: h0, A0 or H0, A0 • Total signal cross-section twice that of the A0 boson • Search strategy: • multi-jet high ET 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 LCWS06, Bangalore, March 2006, Marcel Demarteau

  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: Xt = At – mcotb = 0 (m = – 0.2 TeV) • Maximal mixing: Xt = √6×MSUSY, MSUSY= 1 TeV Fitting outside signal region (±1s of peak) LCWS06, Bangalore, March 2006, Marcel Demarteau

  19. 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 pT Prospects • Layer 0 is being installed as we speak ! LCWS06, Bangalore, March 2006, Marcel Demarteau

  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 ! LCWS06, Bangalore, March 2006, Marcel Demarteau

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

  22. - - ZH → nnbb • For mH = 115 GeV, expect 2.19 events in di-jet mass window 80 GeV < Mjj < 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 1 btag 2 btags 0 btag LCWS06, Bangalore, March 2006, Marcel Demarteau

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

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

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

  26. Prospects LEP • Updated in 2003 in the low Higgs mass region • better understandingof 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 • …… ∫Ldt, fb-1 Tevatron • But, … LCWS06, Bangalore, March 2006, Marcel Demarteau

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

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