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On the Trail of the Higgs Boson Meenakshi Narain.

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Presentation on theme: "On the Trail of the Higgs Boson Meenakshi Narain."— Presentation transcript:

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 SM Higgs Production at Tevatron  [pb] (m H =100 GeV) typical production cross-sections gg H WH ZH 1.0 0.3 0.18 WZ Wbb 3.2 11 tt tb+tq+tbq 7.5 3.4 QCDO(10 6 ) Gluon fusionAssociated Production WZ/ZH production is cleanest

5 SM Higgs Decays and BRs Divide into two regions Low Mass – 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 32

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  l bb 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  l bb

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 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 dodo primary vtx secondary vtx L xy e or  in jet b secondary vtx  2 tracks tagged if L xy /  Lxy 

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

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 Thanks to CDF and DØ collaborations 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.

22 WH: Leptonic Channels Distributions

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

24 b-tagging displaced vertices: RunI SVX algorithms on RunII detector (3D Si, large  ) + soft lepton tagging (~10%) ~55-60% fakes: dodo primary vtx secondary vtx L xy secondary vtx  2 tracks tagged if L xy /  Lxy  e or  in jet b DØ used only  ‘s in top analyses M.Roco

25 Low Mass Higgs Searches Channels:

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