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On the Trail of the Higgs Boson

On the Trail of the Higgs Boson. Meenakshi Narain. Outline. Higgs Primer Overview of Higgs Searches Search Strategies: Standard Model Higgs Low and High Mass regions SUSY Higgs ttbarH production Diffractive Production Future Prospects Conclusions. Constraints on the Higgs Mass.

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On the Trail of the Higgs Boson

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  1. On the Trail of the Higgs Boson Meenakshi Narain

  2. Outline • Higgs Primer • Overview of Higgs Searches • Search Strategies: • Standard Model Higgs • Low and High Mass regions • SUSY Higgs • ttbarH production • Diffractive Production • Future Prospects • Conclusions

  3. Constraints on the Higgs Mass • Direct Searches at LEP • Fits to Precision Electroweak Data • Fit for the Higgs Mass (LEP EWWG 2001)

  4. typical production cross-sections s[pb] (mH=100 GeV) gg H 1.0 WH 0.3 ZH 0.18 WZ 3.2 Wbb 11 tt 7.5 tb+tq+tbq 3.4 QCD O(106) SM Higgs Production at Tevatron Gluon fusion Associated Production WZ/ZH production is cleanest

  5. SM Higgs Decays and BRs • Divide into two regions • Low Mass • H-.bb domintaes • gg->H precluded by QCD background • High Mass • Gauge Boson decays dominate • H->WW becomes promising • Less sensitivity in cross over region

  6. Low Mass Higgs Search • Higgs couples most strongly to massive particles: • Focus on associated production (WH/ZH) • Best Prospects: leptonic W/Z decays • QCD background large for hadronic channels • SM Background processes: • Sensitivity will depend on • b-jet tagging • dijet mass resolution   1  2 3

  7. SM Higgs: Leptonic Channel (1) • Typical Selection: • Main backgrounds: • Event selection optimized to maximize S/B

  8. Expected Events and Sensitivity • Sensitivity crucially depends on dijet mass resolutions

  9. Mass Resolutions: cont’d • Signal significance depends on bb mass resolution • For RunII aim for 10% mass resolution • 30% better than in the previous Run WHlnbb CDF RunI “Calibration for Higgs Search

  10. Mass Resolutions: cont’d • Run I Jet ET resolution vs Fast MC • Optimize b-jet reconstruction and corrections • corrections (partly for b’s): • b/light-q jet calibration • Improvement due to increased +jets statistics • Significant sample of Z bb • Correct for  in bl • Correct for  in jets • Can get 12% at M=120 GeV • If only 12% mass resolution • Required luminosity increases by 20% WHlnbb

  11. Mass resolution issues • Problem is not intrinsic jet resolution • In 2 jet WH events, Mjj is close to gaussian • Mass resolution is about 10% (but, costs 30-70% in efficiency) • With 2 jet requirement relaxed, • Mass resolution is about 15% 3rd jet must be judiciously used!

  12. secondary vtx Lxy primary vtx do e or m in jet b More improvements – b-tagging • b-jet tagging: Will it be good enough? • Displaced Vertices • 3-D vs 2D vertexing possible • Improved impact parameter resolution • (Extrapolation from CDF Run I eff.) • Semileptonic tags • secondary vtx  2 tracks • tagged if Lxy/sLxy >3.0

  13. b-tagging can we improve? • For bb backgrounds: • Relative Luminosity goes as • Eff increase from 60%  65% would result in the same signal significance for 20% less integrated luminosity. LEP2, S.Jin PHENO2000

  14. Multivariate Analysis techniques • Further Improvements from use of Neural Networks, Grid Search, likelihood methods. • Significant gains, compare S/B with and without neural nets

  15. SM Higgs: Leptonic Channel (2) • Main backgrounds: • Event selection optimized to maximize S/B • Typical Selection:

  16. Some distributions:

  17. Use Neural Networks to optimize analysis: • use different networks • one for signal • 4 different ones for bkg

  18. SM Higgs: Leptonic Channel (3) • Small rate but good S/B • Main backgrounds: • Typical Selection:

  19. Neural Network Analysis: • signal • Backgrounds • (4 different networks) • Kinematic fit may enhance sensitivity • Add Taus?

  20. Low Mass Higgs Search • It’s going to be challenging… • A 120 GeV Higgs signal • Total Background

  21. Conclusion • Tevatron Run II • precision studies of top quark properties • LHC… `top factory’ • open possibilities of new measurements e.g. Yukawa coupling, rare decays, CP violation etc. Thanks to CDF and DØ collaborations

  22. WH: Leptonic Channels • Distributions

  23. bb mass reconstruction the extracted signal significance depends on input dijet mass resolution optimized b-jet reconstruction+corrections WHlnbb E. Barberis improvement from use of tracking and preshower in jet reconstruction? (also, different algorithms?) corrections (partly specific to b’s): - corrections for n into jets (bln) - corrections for m into jets - b/light-q jet calibration - b/light-q parton corrections and... -effect of extra interactions on jet reconstruction

  24. secondary vtx ~55-60% Lxy primary vtx do e or m in jet b b-tagging fakes: • displaced vertices: • RunI SVX algorithms on RunII • detector (3D Si, large h) • secondary vtx  2 tracks • tagged if Lxy/sLxy >3.0 M.Roco M.Roco + soft lepton tagging (~10%) DØ used only m‘s in top analyses

  25. Low Mass Higgs Searches • Channels:

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