Search for the Higgs boson in association with

a Z bosonGiulia Fanti

Michael Mulhearn Selcuk Cihangir

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Physics motivation• Standard Model

– Missing pieceHiggs mechanism• electroweak symmetry breaking • origin of mass• implies existence of Higgs boson

• Higgs decays– mH>135:

– mH<135: • Large background at Tevatron

• ZH: possible production mechanism–

– LEP excluded mH<114 GeV/c2

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Detection• Electrons/positrons

– Tracks in tracker– EM calorimeter

• Jets– Energy in calorimeter– Tracks in the tracker– B-tagging

• Silicon trackers• Displaced vertices

• Muons/antimuons– Exterior muon

chambers

Image courtesy of Fermilab: http://www.fnal.gov/pub/now/live_events/explain_det_dzero.html

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Analysis Techniques

• Preselection– Kinematic cuts, b-tagging

• Start with mbb analysis– Single most discriminating variable

• Multivariate techniques– Neural networks– Matrix element analysis

• Standard Model: matrix elements• Transfer functions: jets, leptons• Hope to improve sensitivity

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mbb analysis: muon channelNN mbb/mjj values Calculated mbb/mjj values

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Further mbb analysis

• Limit setting– Sensitivity to

signal– SM amplification– Collie software

• 95% confidence scaling factor:– Expected: 40.6– Observed: 52.4

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Z-mass comparison

NN zm values:

Calculated zm values:

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Matrix-element analysis: electron transfer functions

• Finite detector resolution• Double gaussian parametrization

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Future directions

• Matrix element analysis– Technique developed on Run IIA data for muon

channel– Muon channel

• Understand kinematics of muon Run IIB data• Have transfer functions for jets, muons

– Electron channel• Fitted electron transfer functions

– Ready to apply ME analysis to both channels

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Acknowledgements

• Mike Mulhearn• Selcuk Cihangir• Sabine Lammers• D0 collaboration and Fermilab• Department of Energy