Optimizing Higgs Analysis at DØ

John Sandy – SULI Intern

(Texas Tech University)

Mentors: Michael P Cooke and Ryuji Yamada

(Fermilab National Accelerator Laboratory)

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Optimizing Higgs Analysis at DØ

Physics – The Standard Model and the Higgs Mechanism

The Standard Model of Particle Physics:

• Three generations of matter• Quarks and Leptons• Force Carriers

Initially predicted that particles should be massless!

Peter Higgs and his colleagues (Englert, Brout, Guralnik, Hagen, Kibble) came to rescue in 1964 and introduced what is now known as the Higgs Mechanism.

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Optimizing Higgs Analysis at DØ

The Higgs Boson

• Excitation of the Higgs field (analogous to photon in E&M field)

• Gives mass to all particles

• Interacts more frequently with more massive particles, attributing more mass to them

The Higgs Field Two particles attempting to move across the Higgs Field.

More massive

Less massive

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Optimizing Higgs Analysis at DØ

Higgs Boson Signal vs. Background

The Higgs boson has been notoriously difficult to search for:

• Complex, mass dependant branching ratios

• Low production rates

• Lots of SM backgrounds

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Optimizing Higgs Analysis at DØ

The DØ Detector

• Inner silicon detector

• Scintillating fiber tracking system

• 2 tesla solenoid

• Uranium liquid argon calorimeter • Large muon system

• 900,000 individual channels

The Tevatron provided proton-antiproton collisions with a center of mass (CM) energy of 1.96 TeV

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Optimizing Higgs Analysis at DØ

Event Displays with D0Cafvis

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Optimizing Higgs Analysis at DØ

The Higgs Boson Search at DØ

My search is focused on four Higgs final states:

H → WW → lνlν

WH → lνbb

ZH → llbb

ZH → ννbb

Pre-selection:

• Triggering

• Lepton Tagging

• “b-tagging”

Electron / muon events

0tag, 1tag, 2tag, loose, medium, tight

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Optimizing Higgs Analysis at DØ

The Higgs Boson Search at DØ

Boosted Decision Trees (BDTs)

Massjj > 120

PassFail

? ?

pp ff

… and so on …

S / B

BDT makes cuts based on signal purity parameters

Never throw out any data, so no chance of losing signal

Bins that pass more cuts have higher signal to background ratio

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Optimizing Higgs Analysis at DØ

Using BDTs to train Multi-Variate Analyses (MVAs)

Create a variable list to build a BDT

Run analysis framework and prune variable list:

• Variable Importance

• Variable Correlation

• Variable Modeling ….

I used BDTs to train MVAs for separating Higgs boson signal from the tt, V + jets, and VV backgrounds

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Optimizing Higgs Analysis at DØ

Variable modeling based on simulation:

Well modeled Poorly modeled

Using BDTs to train Multi-Variate Analyses (MVAs)

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Optimizing Higgs Analysis at DØ

Using BDTs to train Multi-Variate Analyses (MVAs)

BDTs expected signal and background separation

My own training for Higgs vs. VV background discrimination

Multijet training that I am helping train

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Optimizing Higgs Analysis at DØ Final BDTs (Low Mass)

After training, the final BDTs are produced and can be used to separate signal from background

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Optimizing Higgs Analysis at DØ

Final BDTs (High Mass)

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Optimizing Higgs Analysis at DØ

Final BDTs for 2-Tight Tag Events (Low Mass)

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Optimizing Higgs Analysis at DØ

Conclusions

The training of secondary MVAs has shown improvement in other channels (4jet)

Further training may improve modeling

When training is fully completed, improved MVAs will improve overall Higgs search at the Tevatron, allowing for stronger results

Recompiled D0Cafvis will allow DØ collaborators to use event displays in future talks and presentations (already been used at DØ collaboration meeting in UK)

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On July 4th CERN announced a new “Higgs like” particle using γγ and Z-Z channels

DØ and CDF recently published results claiming evidence for such a particle using the bb channel

Optimizing Higgs Analysis at DØ

This work was supported by the Experimental Particle Physics Division at Fermilab National Accelerator Laboratory, U.S Department of Energy Office of Science’s and Office of Workforce Development for Teachers and Scientists. I would like to thank my mentors Ryuji Yamada and Michael P. Cooke. I would also like to thank Anthony Podkowa and Herb Greenlee for their assistance.

Acknowledgments

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Optimizing Higgs Analysis at DØ Backup

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