A Search For the Standard Model Higgs Boson
in the ZH->llbb Final State

Electron + Muon Combination
Summer 2011 Results


We present a search for the Standard Model Higgs Boson using the process ZH->llbb. We use a dataset corresponding to 7.5 (for ZH->eebb) / 7.9 (for ZH->mumubb) inverse femtobarns of integrated luminosity collected with the CDF II detector. This analysis benefits from several new multivariate techniques that have not been used in previous iterations of the ZH analysis channel at CDF. We use a multivariate function to select lepton candidates, increasing signal acceptance while simultaneously keeping fake rates small. We employ an inclusive trigger selection to further increase acceptance. To enhance signal discrimination, we utilize a multi-layer approach consisting of expert discriminants. This multi-layer discriminant method helps isolate the two main classes of background events, t-tbar and Z+jets production. It also includes a flavor separator, to distinguish light flavor jets from jets consistent with the decay of a b-hadron. With this novel multi-layer approach, we proceed to set limits on the ZH production cross section times branching ratio. For a Higgs boson with mass 115 GeV/c^2, we observe (expect) a limit of 4.8 (3.9) times the Standard Model prediction. This analysis combination represents a 40% improvement, relative to the previous (Summer 2010) version of the ZH->llbb analysis.

This analysis combines two independent analyses, which use Z->ee and Z->mu mu decays.
For more information on the independent analyses, and additional plots, use the following links:

ELECTRONS ------------------------ MUONS

Documentation is available via the public notes, available for electrons and muons.

Key Plots For Conferences

Multivariate function for muon identification and selection:

For muons:
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Multi-Layer Discriminant Method

Final Output Shapes of Main Backgrounds and flowchart of this method:
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Final Output Discriminants

For a Higgs mass of 120 GeV/c^2
For electrons:
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For muons:
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Expected and Observed Limits

Combined Limits

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Combined Limits Table

Separate Electron and Muon Channel Limits

Electron Limits Table Muon Limits Table

Overall Improvement to the Analysis

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For more information:

Previous ZH Analysis, 5.7/fb: ZHllbb Summer 2010 Analysis Public Webpage

Contact e-mails: Justin Pilot (pilot@fnal.gov), Sarah Lockwitz (lockwitz@fnal.gov)