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Search for Standard Model Higgs in ZH  l + l  bb channel at D Ø

Chicago. p. p. 1.96 TeV. Booster. CDF. DØ. Tevatron. p sou rce. Main Injector & Recycler (new). Search for Standard Model Higgs in ZH  l + l  bb channel at D Ø. Shaohua Fu Fermilab For the D Ø Collaboration DPF 2006, Oct. 29 – Nov. 3 Honolulu, Hawaii.

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Search for Standard Model Higgs in ZH  l + l  bb channel at D Ø

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  1. Chicago p p 1.96 TeV Booster CDF DØ Tevatron p source Main Injector & Recycler (new) Search for Standard Model Higgsin ZH  l+lbb channel at DØ Shaohua Fu Fermilab For the DØ Collaboration DPF 2006, Oct. 29 – Nov. 3 Honolulu, Hawaii

  2. Tevatron and DØ Detector Used almost the whole Run IIa data ~ 1fb1 • Run II goal: 4 ~ 8 fb1 in 2009 • Analyses presented here used 840-920 pb1 • DØ detector in Run IIa • Run IIb: Layer0 for SMT, trigger upgrade Shaohua Fu, Fermilab

  3. Constraints on SM Higgs • Standard Model Higgs is the key to Electro-Weak symmetry breaking and gives masses to elementary particles, with its own mass unpredicted • Limit from direct searches at LEP2: mH > 114.4 GeV at 95% C.L. • Indirect limit from fits to precision EW measurements from LEP, SLC, and Tevatron: mH < 166 GeV at 95% C.L. (< 199 GeV if LEP2 limit included) • Indirect best fit value: 85 +3928 GeV at 68% C.L. • A light Higgs is favored LEPEWWG July 2006 Shaohua Fu, Fermilab

  4. SM Higgs Production and Decay • SM Higgs production at Tevatron • Gluon fusion  ~ 0.8-0.2 pb (mH 115-180 GeV) • Associated production with a W or Z boson  ~ 0.1-0.03 pb • Dominant decays • Low mass (mH<135 GeV): Hbb, high mass (mH>135 GeV): HW+W • This analysis: ZH  l+lbb, in e+e and+ channels Excluded at LEP Shaohua Fu, Fermilab

  5. Data Selection (Signed) Track B Impact Parameter Decay Length Hard Scatter • Integrated luminosity = 920 (840) pb–1 for e+e (+) sample • Using all EM (Muon) triggers with efficiency ~100% (75%) for e+e (+) sample • e+esample • 2 electrons pT > 15 GeV, || < 1.1 or 1.5 < || < 2.5, central track match • Z candidate: 65 GeV < Mee < 115 GeV • + sample • 2 muons pT > 15 GeV, central track matched (|| < 2.5), and isolated • Z candidate: 70 GeV < M < 110 GeV, Z pT > 20 GeV • At least 2 jets • pT > 15 GeV, || < 2.5 • b-tagging • Secondary vertex • Large impact parameter of the tracks • Neural Net tagging algorithm • ~60% efficiency and ~3% light-jet fake rate (b-tagging and taggability) Shaohua Fu, Fermilab

  6. Backgrounds • Z( l+l) + jets • Including Z( l+l) + light-parton jets and Z( l+l) + c jets • Z( l+l) + b jets • Hard to reduce Z+bb background, which will pass b-tagging just like signal • Top pair, WZ, ZZ, etc. • Small contribution, e.g. tt l+lbb is reduced by l+l invariant mass cut • MC Normalization • Z( l+l) + jets normalized to #data under Z peak, other MC normalized to Z+jets using NLO cross sections • Multijet background – QCD process • e+echannel: selecting QCD enhanced sample from data by inverting the electron shower shape and track-matching requirement, then fitting Mee distribution to normalize QCD sample. • + channel: fitting M distribution of data with a Gaussian (Z) and an exponential (Drell-Yan + QCD) to get the fraction of QCD, depending on jet multiplicity and b-tagging. Shaohua Fu, Fermilab

  7. Data and Backgrounds • Z peak for Z( e+e) + 2 jets (before b-tagging) • 2900 events within 65 GeV < Mee < 115 GeV • 102 events estimated for QCD background • Total background estimated to be 2860  470 events • Signal ZH (mH=115 GeV): 0.78 0.03 events Mee Z pT Shaohua Fu, Fermilab

  8. Data and Backgrounds • Z( l+l) + 2 jets (before b-tagging): e+e and + combined • 5386 events observed in data • 5610 930 events expected as total background • Signal ZH (mH=115 GeV): 1.50 0.06 events • Leading and second lepton pT distributions: Shaohua Fu, Fermilab

  9. Data and Backgrounds • Z( l+l) + 2 jets (before b-tagging): e+e and + combined • Leading and second jet pT distributions: • Then apply b-tagging Shaohua Fu, Fermilab

  10. With 0 b-tagged jet #events in di-jet range 60 (70) ~ 110 GeV for e+e (+) channel Shaohua Fu, Fermilab

  11. Exclusive Single b-tagging #events in di-jet range 60 (70) ~ 110 GeV for e+e (+) channel Shaohua Fu, Fermilab

  12. Inclusive Double b-tagging #events in di-jet range 60 (70) ~ 110 GeV for e+e (+) channel Shaohua Fu, Fermilab

  13. Systematic Uncertainties • Sources of uncertainties • Lepton identification efficiency uncertainty: 4% • Jet energy scale correction uncertainty: 1-7% for backgrounds, and 1-2% for ZH signals • b-tagging uncertainty (double b-tagging): 7-8% for backgrounds and signals • NLO cross section uncertainty: 15% for Z+jets, 50% for Z+bb, 6-8% for other backgrounds • QCD normalization uncertainty: 30% • Total uncertainties • Correlations among each background uncertainty taken into account • For double b-tagging: 35% on total background, 9% on ZH signals Shaohua Fu, Fermilab

  14. Cross Section Limits * *CDF results with NN selection • e+e and + combined • No excess observed, so set cross section limits • Modified frequentist approach (CLS), using di-jet mass distribution • 95% C.L. upper limits on ZH cross section: 3.3-1.6 pb for mH=105-155 GeV Shaohua Fu, Fermilab

  15. Summary LEP 8 Int. Luminosity per Exp. (fb-1) 4 Tevatron • ZH  l+lbb searches performed at DØ in e+e and+ final states, using about 1 fb–1 integrated luminosity • No excess of events is observed, thus upper limits on ZH cross section are set as 3.3-1.6 pb for mH=105-155 GeV • ZH mH=115 GeV cross section limit is about 30 times of the SM production • To improve sensitivity • Optimization techniques (e.g. Neural Net selection) • Better detector understanding • Layer0 silicon detector  better b-tagging • Combine all channels • More data! Shaohua Fu, Fermilab

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