Transcript Slide 1
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 • Direct Searches at LEP • Fits to Precision Electroweak Data • Fit for the Higgs Mass (LEP EWWG 2001) SM Higgs Production at Tevatron Gluon fusion Associated Production typical production cross-sections s[pb] (mH=100 GeV) gg H WH ZH 1.0 0.3 0.18 WZ Wbb 3.2 11 tt tb+tq+tbq QCD 7.5 3.4 O(106) WZ/ZH production is cleanest 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 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 SM Higgs: Leptonic Channel (1) • Typical Selection: •Main backgrounds: •Event selection optimized to maximize S/B Expected Events and Sensitivity Sensitivity crucially depends on dijet mass resolutions 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 WHlnbb CDF RunI “Calibration for Higgs Search 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 n in bln – Correct for in jets • Can get 12% at M=120 GeV – If only 12% mass resolution • Required luminosity increases by 20% WHlnbb 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! More improvements – b-tagging •b-jet tagging: Will it be good enough? –Displaced Vertices •secondary vtx 2 tracks •tagged if Lxy/s Lxy >3.0 •3-D vs 2D vertexing possible •Improved impact parameter resolution •(Extrapolation from CDF Run I eff.) –Semileptonic tags e or in jet b 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 Multivariate Analysis techniques •Further Improvements from use of Neural Networks, Grid Search, likelihood methods. – Significant gains, compare S/B with and without neural nets SM Higgs: Leptonic Channel (2) • Main backgrounds: •Event selection optimized to maximize S/B •Typical Selection: •Some distributions: •Use Neural Networks to optimize analysis: – use different networks • one for signal • 4 different ones for bkg SM Higgs: Leptonic Channel (3) •Small rate but good S/B •Main backgrounds: •Typical Selection: •Neural Network Analysis: •signal •Backgrounds (4 different networks) •Kinematic fit may enhance sensitivity •Add Taus? Low Mass Higgs Search •It’s going to be challenging… A 120 GeV Higgs signal Total Background 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 WH: Leptonic Channels Distributions bb mass reconstruction the extracted signal significance depends on input dijet mass resolution optimized b-jet reconstruction+corrections WHlnbb 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 (bln) and... - corrections for into jets -effect of extra interactions - b/light-q jet calibration on jet reconstruction - b/light-q parton corrections b-tagging •displaced vertices: RunI SVX algorithms on RunII detector (3D Si, large h) secondary vtx fakes: ~55-60% Lxy primary vtx do •secondary vtx 2 tracks •tagged if Lxy/s Lxy >3.0 M.Roco + soft lepton tagging (~10%) e or in jet b DØ used only ‘s in top analyses M.Roco Low Mass Higgs Searches •Channels: