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Exploring Standard Model Higgs in ZH Decay Channel at DØ

This study presents constraints and analysis on the Standard Model Higgs boson in the ZH decay channel at DØ, Fermilab, using data from the Tevatron collider. The search focuses on the production and decay of the Higgs boson, with a particular emphasis on the ZH -> llbb channel. The analysis includes data selection methods, background estimations, and systematic uncertainties. The study also discusses the exclusion limits set by the analysis and the cross-section limits derived from the data.

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Exploring Standard Model Higgs in ZH Decay Channel at DØ

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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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