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

[pb] (mH=100 GeV)

typical production cross-sections

gg HWHZH

1.00.30.18

WZWbb

3.211

tttb+tq+tbq

7.53.4

QCD O(106)

Gluon fusion Associated Production

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

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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

WHlbb

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 in bl– Correct for in jets

• Can get 12% at M=120 GeV

– If only 12% mass resolution • Required luminosity increases

by 20%

WHlbb

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

•3-D vs 2D vertexing possible•Improved impact parameter resolution•(Extrapolation from CDF Run I eff.)

–Semileptonic tags

do

primaryvtx

secondaryvtx

Lxy

e or in jet

b

•secondary vtx 2 tracks•tagged if Lxy/ Lxy

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.

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

Thanks to CDF and DØ collaborations

Tevatron Run IIprecision studies of top quark properties

LHC… `top factory’

open possibilities of new measurements e.g. Yukawa coupling, rare decays, CP violation etc.

WH: Leptonic ChannelsDistributions

bb mass reconstruction

the extracted signal significance depends on input dijet mass resolution

WHlbb

improvement from use of tracking and preshowerin jet reconstruction? (also, different algorithms?)corrections (partly specific to b’s): - corrections for into jets (bl) - 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

b-tagging

•displaced vertices: RunI SVX algorithms on RunII detector (3D Si, large )

+ soft lepton tagging (~10%)

~55-60%

fakes:

doprimaryvtx

secondaryvtx

Lxy

•secondary vtx 2 tracks•tagged if Lxy/ Lxy

e or in jet

b DØ used only ‘s in top analyses

M.RocoM.Roco

Low Mass Higgs Searches•Channels: