Combination of Higgs boson searches at ATLAS

Second MCTP Spring Symposium on Higgs Boson Physics,

University of Michigan 16th-20th April 2012

Gemma Wooden, University of Michigan

On behalf of

the ATLAS collaboration

Overview

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• Search for the Higgs boson in multiple channels

• Different channels contribute in different mH regions

• Combine searches to maximise sensitivity

• Search for Higgs production via different mechanisms: – Gluon-gluon fusion

(ggF)

– Vector boson fusion (VBF)

– Associated production (VH)

ggF

VH

VBF

Search channels

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• All channels use the full dataset collected by the ATLAS detector in 2011

• H->gg, H->ZZ->llll as well as H->tt, VH(H->bb) important at low mass

Limit setting procedure

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Likelihood m = signal strength

parameter

No. of signal events

No. of background

events

Nuisance parameters q • Method:

– Test signal hypotheses

– Estimate backgrounds using MC and/or data control regions

– Account for systematic uncertainties (q) using auxiliary measurements (e.g. performance groups)

• Set limits using modified frequentist method (CLs)

• 95% C.L. on m found by adjusting m until CLs = 0.05:

• To quantify the significance of an excess, the local p-value can be calculated using same method but with m set to 0

1 - pb

pm

Combination of channels

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• Must ensure that systematics are handled correctly

• Some will be correlated between channels

qgg

qlnln

q4l

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H->gg

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• Use 9 independent categories of varying sensitivity

• Categories depend on h of photons, whether the photons are (un)converted and the momentum component of the diphoton system transverse to the thrust axis

• Fit mgg to search for presence of a signal

• Mass resolution of around 1.7% for mH = 120 GeV

H->ZZ->llll

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• m4l used as the discriminating variable • Clean signature with low background rate • Irreducible ZZ* background estimated using MC • Z+jets and ttbar backgrounds estimated using data

control regions • Mass resolution of 1.5%(2%) in 4 muon(electron)

channel at mH = 120 GeV

H->ZZ->llnn

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• Use mT as discriminating variable

• Apply different selections in low (mH < 280 GeV) & high (mH >= 280 GeV) regions

• Dominant backgrounds (ZZ, WW, WZ, ttbar) normalised using MC

• Require |mll – mZ| < 15 GeV to avoid overlap with the H->WW->lnln analysis

H->ZZ->llqq

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• Use mllqq as discriminating variable

• Separate, optimised analyses above and below mH = 300 GeV

• Normalise dominant Z+jets background using data

• Divide analysis into tagged and untagged categories: events with two and less than two b-tagged jets respectively

• Analysis is extendable to lower masses

H->ZZ

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• Combination of H->ZZ->llll, H->ZZ->llnn, H->ZZ->llqq

H->WW->lnln

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• Use a fit to the mT distribution

• Split analysis into 0, 1 and at least 2 jet categories

• Use entirely data-driven method to estimate W+jets background

• Normalise top, WW and Z+jets backgrounds with data

• Separate analysis into low, intermediate and high mass regions

H->WW->lnqq

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• Fit the mWW distribution to search for Higgs signal

• M(en) is constrained to the W mass • Mass resolution is quite good in this channel

(~9% at mH = 400 GeV) • Divide search into events with 0 or 1 jets in

addition to the two arising from the W decay • Backgrounds parametrised using forms

motivated by MC studies

H->tt

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• Use three different search channels: – H->tt->ll4n, which uses meff in 0j channel and collinear approx. in 1 and 2j channels

– H->tt->lthad3n, which uses Missing Mass Calculator to calculate di-tau mass

– H->tt->thadthad2n + jet, which uses collinear approx. to calculate the mass

VH(H->bb)

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• Search for the Higgs boson in 3 channels: – ZH->llbb

– WH->lnbb

– ZH->nnbb

• Use leptonic vector boson decays to trigger on and to reduce backgrounds

• mbb is used as the discriminating variable

Systematic uncertainties

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• All uncertainties are considered as either 100% correlated or 100% uncorrelated

Typical theoretical uncertainties

Detector uncertainties

Combination results – limits

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• Expect to exclude the Standard Model Higgs boson at 95% C.L. between: 120 < mH < 555 GeV

• Observed exclusion at 95% C.L.: 110 < mH < 117.5, 118.5 < mH < 122.5, 129 < mH < 539 GeV

• Observed exclusion at 99% C.L.: 130 < mH < 486 GeV

• Observe an excess in the low mass region

Combination results – limits (ii)

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Combination results – local p0

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• Investigate local p0 – the probability for the background to fluctuate and give an excess of events as large or larger than that observed

• Observed local p0 is most consistent with the SM Higgs expectation at mH = 126 GeV

• Observed local significance 2.5σ (expected 2.9σ)

A closer look at the local p0

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Combination results – signal strength

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• Calculate best fit signal strength parameter, m = s/sSM

• m is most consistent with the SM Higgs production cross section at a mass of 126 GeV

• At mH = 126 GeV, the best-fit signal strength is 0.9+0.4-0.3

• Probability of a fluctuation of this magnitude occurring across full range of mH (110 < mH < 600 GeV) is 30% or 10% in the mass range from 110 < mH < 146 GeV

Conclusions

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• Searches for the Higgs boson have been undertaken in a wide range of channels using the full 2011 dataset of up to 4.9 fb-1

• Stringent limits have been placed on the allowed mass of the Standard Model Higgs boson

• Allowed regions: 117.5 < mH < 118.5 GeV, or 122.5 < mH < 129 GeV

• Excess most compatible with a Higgs boson with a mass of 126 GeV – Observe a local significance at mH = 126 GeV of 2.5s

– Expected significance in the presence of a SM Higgs boson is 2.9s

• However, more data is needed to arrive at a definite conclusion • Already looking forward to analysing the new 8 TeV 2012 data,

which is coming in fast

Backup

References

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• Limit setting and combination procedure: ATL-PHYS-PUB-2011-011

• Higgs combination using 1.04-4.9 fb-1: arXiv:1202.1408

• Higgs combination using full 2011 dataset: ATLAS-CONF-2012-019

• H -> gg: arXiv:1202.1414

• H -> ZZ -> llll: arXiv:1202.1415

• H -> ZZ -> llnn: ATLAS-CONF-2012-016

• H -> ZZ -> llqq: ATLAS-CONF-2012-017

• H -> WW ->lnln: ATLAS-CONF-2012-012

• H -> WW -> lnqq: ATLAS-CONF-2012-018

• H -> tt: ATLAS-CONF-2012-014

• VH(H -> bb): ATLAS-CONF-2012-015