Date post: | 21-Dec-2015 |
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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
1
32
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: