Standard Model Higgs Searches at ATLAS

H->ZZ/WW

for the ATLAS Collaboration

Paul Thompson (Birmingham)

Higgs Hunting 2011, Orsay, France, July 27th-29th, 2011

1P. Thompson ATLAS SM searches H->WW/ZZ

• H->ZZ (llqq,llvv,llll)

• H->WW (lvlv,lvqq)

(ATLAS-CONF-2011-111)

Higgs Hunting 2011, Orsay, France, July 27th-29th, 2011

SM H->ZZ:

Higgs Production at the ATLAS

• Sizeable branching ratio, especially for llqq

and llvv

• Presence of at least 1 on shell Z, clean final

states

• Take advantage of high decay fraction and high

branching fraction especially for lvqq(e,µ) and lvlv(ee,µµ,eµ) – clean dilepton signature

SM H->WW:

2P. Thompson ATLAS SM searches H->WW/ZZ

Update for Summer 2011:

• Published 2010 data for 35pb-1

• Excellent LHC performance in 2011, delivered 1.3fb-1

peak lumi 1.5×1033cm-2s-1 (already out of date!)

• Analyses presented here use >1fb-1

Signature lepton pair(ee,µµ)+2 jets:

H->ZZ->llqq

• Main background Z+jets

Search in 200 < mH < 600 GeV:

• Optimised high/low mass region mH>300 GeV

• “Tagged” 2 b tags, “Untagged” 0/1 b tag

• High mass: pT>45 GeV, ∆φll<π/2, ∆φjj<π/2

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More details in talk from Carl Gwilliam

3P. Thompson ATLAS SM searches H->WW/ZZ

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4P. Thompson ATLAS SM searches H->WW/ZZ

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Ldt=1.04fb∫llqq→ZZ→H

H->ZZ->llqq : Exclusion Limits

• Combined

exclusion limit

from tagged and

untagged samples

• Standard ATLAS:

CLs modified

frequentist

formalism with

profile likelihood

5P. Thompson ATLAS SM searches H->WW/ZZ

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• Expected limit 2.7-9 xSM cross section, observed 1.7 xSM @mH=360 GeV

• These channels approaching σSM with 1fb-1

profile likelihood

test statistic

• Good sensitivity

across mass range

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Signature lepton pair(ee,µµ)+MET:

H->ZZ->llvv

• Main backgrounds top and diboson

• Signal contributions from H->WW->lvlv

Search in 200 < mH < 600 GeV:

• Optimised “high”/“low” mass region

mH>280 GeV

6P. Thompson ATLAS SM searches H->WW/ZZ

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• Two high pT isolated leptons

• 3rd lepton and b-jet vetoes

• MET>66 GeV (high mass MET>82 GeV)

• ∆φ(jet,MET)>0.3, high mass also ∆φ(Z,MET)>1

• 1<∆φll<2.64, high mass ∆φ<2.25

• Final discriminant is transverse mass

ZZ->llvv : Backgrounds

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7P. Thompson ATLAS SM searches H->WW/ZZ

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W+jets: same sign lepton pairs

• Top background. MC expectation verified

with data

• W+jets background, MC normalized in

control region

• Diboson backgrounds from theory ~10%

• Data driven QCD background ‘Multijet’

(small)

H->ZZ->>llvv : Results

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8P. Thompson ATLAS SM searches H->WW/ZZ

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PreliminaryATLAS

9P. Thompson ATLAS SM searches H->WW/ZZ

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PreliminaryATLAS

• Sensitivity across mass range

• Expected limit 1.2-7 x SM cross section

• Exclude SM Higgs between 360 < mH < 420 GeV

H->ZZ*->llll

Very clean signature but low rate

Primary Lepton pair (closest to Z) mass distribution Secondary Lepton pair

• Main background is ZZ production, Z+jets, top suppressed by isolation/impact parameter

• 110 < mH < 600 GeV, sensitivity best 200-400 GeV

Event Selection:

• 2 pairs of same flavour, opposite charge leptons (4e,4µ,2e2µ)

• Lepton mass selection: |m12-mZ|<15 GeV and m34<115 GeV and m34>mThreshold(m4l)

10P. Thompson ATLAS SM searches H->WW/ZZ

H->ZZ->llll : Backgrounds

• Theory uncertainty 15%

• Luminosity error 3.7%

Dominant ZZ* production normalized to MC prediction

tt background using MC, verified in control region

11P. Thompson ATLAS SM searches H->WW/ZZ

Z+jets normalized on data control sample

• Select Z(->ll), and no isolation, impact parameter

cuts on second

• Separate into different components(heavy flavour,

electroweak, fakes)

• µµ dominated by Zbb

• ee: Z+light

• Uncertainties 20-40%

H->ZZ->4l : Results

• Signal shown for three values of

mH (150, 220, 480 GeV)

• Sum of background expectation

shown

• Shape follows expectation,

local fluctuations observed

Summary of event yield in 4l invariant mass:

12P. Thompson ATLAS SM searches H->WW/ZZ

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σ 1 ±σ 2 ±

PreliminaryATLAS

4l→(*)

ZZ→H­1

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H->ZZ->4l : Exclusion Limits

• Expectation approaching SM

• Observed limits: 1.8 x SM @mH=200 GeV

• Fluctuations observed, mostly due to

single events

13P. Thompson ATLAS SM searches H->WW/ZZ

SM Expectation

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σ 1 ±σ 2 ±

PreliminaryATLAS

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σ 1 ±σ 2 ±

PreliminaryATLAS

4l→(*)

ZZ→H­1

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sObserved CL

sExpected CL

σ 1 ±σ 2 ±

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PreliminaryATLAS

4l→(*)

ZZ→H­1

Ldt = 1.1 fb∫=7 TeVs

sObserved CL

sExpected CL

σ 1 ±σ 2 ±

H->WW->lvlv

Search in range 110 < mH < 240 GeV. Two isolated leptons and large MET:

• Opposite sign dilepton events: leading lepton (e or µ) pT>25 GeV, subleading pT>20(15) for e(µ)

• Same flavour: mll>15 GeV, |mll-mZ|>15 GeV. For eµ channel just require mll>10 GeV

• Require ET,rel>40 GeV for ee,µµ and ET,rel >25 GeV for eµ

ee channel µµ channel eµ channel

Dominated by Z+jets

before cut

See talk by Aaron

Armbruster

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Topological dilepton

• Events with no jets with pT>25 GeV

and |η|< 4.5. Apply pTll>30 GeV to reduce Drell-Yan

• Events with exacly 1 jet. Apply

b-tag veto to reduce top. Vector

sum of H+j pT <30 GeV and Z->ττ rejection

15P. Thompson ATLAS SM searches H->WW/ZZ

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Topological dilepton

selections, optimised for low

and high mass around 170 GeV:

• mll<50 (60) GeV for low(high) mass

• ∆φll<1.3(1.8) for low(high) mass

Sliding mass cut for final

selection:

• 0.75 x mH < mT < mH

∆φll after mll cut:

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16P. Thompson ATLAS SM searches H->WW/ZZ

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H->WW->lvlv : Backgrounds and Control Regions

• Two largest backgrounds SM WW and top normalized using control regions in data

• W+jets from data template, other smaller backgrounds from, MC

Uncertainties on extrapolation factors:Control regions in data:

18P. Thompson ATLAS SM searches H->WW/ZZ

• Likelihood formed from signal region, 2 control regions for ee,µµ,eµ and 2 jet bins

H->WW->lvlv : Exclusion Limit

[GeV]HM120 140 160 180 200 220 240

SM

σ/σ

95%

CL L

imit o

n

1

10

210Observed

Expected

σ 1 ±σ 2 ±

ATLAS Preliminary

­1 Ldt = 1.04 fb∫

= 7 TeVs

νlνl→(*)

WW→H

19P. Thompson ATLAS SM searches H->WW/ZZ

[GeV]HM120 140 160 180 200 220 240

SM

σ/σ

95%

CL L

imit o

n

1

10

210Observed

Expected

σ 1 ±σ 2 ±

ATLAS Preliminary

­1 Ldt = 1.04 fb∫

= 7 TeVs

νlνl→(*)

WW→H

• Exclude SM Higgs between 158 < mH < 186 GeV at 95% CL

• Expected exclusion 142-186 GeV

H->WW->lvlv : Significance

[GeV]HM120 140 160 180 200 220 240

Sta

tistica

l sig

nific

an

ce

0

2

4

6

8

10Observed

Expected

σ 1 ±σ 2 ±

­1 Ldt = 1.04 fb∫

= 7 TeVs

ATLAS Preliminary νlνl→(*)

WW→H

20P. Thompson ATLAS SM searches H->WW/ZZ

[GeV]HM120 140 160 180 200 220 240

Sta

tistica

l sig

nific

an

ce

0

2

4

6

8

10Observed

Expected

σ 1 ±σ 2 ±

­1 Ldt = 1.04 fb∫

= 7 TeVs

ATLAS Preliminary νlνl→(*)

WW→H

Statistical significance assuming SM Higgs hypothesis

• >2σ excess 126 < mH < 158 GeV

• Largest 2.7σ at 130 GeV

H->WW->lvqq

Search in 240 < mH < 600 GeV range:

• Single high pT lepton pT>30 GeV, MET>30 GeV, b-tag veto

• Require 2 or 3 jets with 1 combination consistent with Mw [ ET>30GeV, |η|<2.8(4.5) ]

• Constraint M(lv)=M(jj)=MW can reconstruct mH=M(lvjj)

Data background fit using 2 exponentials:

21P. Thompson ATLAS SM searches H->WW/ZZ

H->WW-lvqq : Backgrounds

• V+jets shape modelled by MC

22P. Thompson ATLAS SM searches H->WW/ZZ

• Multi-jet obtained from data templates with loose/anti-lepton selection. Normalization

of Multi-jet and V+jets from fit to MET distribution (errors from fit).

• Top and dibosons from MC (errors from energy scale, cross-section, luminosity)

Likelihood fit:

• Non-resonant background from fit with 2 exponentials

• Signal shape from MC

• Fit per jet channel, lepton flavour categories, including systematic effects of detector

and theory uncertainties on signal

H->WW->lvqq : Exclusion Limit

23P. Thompson ATLAS SM searches H->WW/ZZ

• The upper limit at mH=400 GeV is 2.5 times the SM cross-section

• Expected limit is around 5 times the SM cross-section

Conclusions

• ATLAS analysed data samples with 1fb-1 of data for H->ZZ/WW production in a number

of decay channels

• Large data sample means that sensitive to the Higgs Boson across wide mass range:

110 < mH <600 GeV

• H->ZZ->llvv exclude SM Higgs between 360 < mH < 420 GeV

• H->WW->lvlv exclude SM Higgs between 158 < mH < 186 GeV

• Slight excess of events at low mass with around 2.5σ significance

• More data required to see if it a fluctuation or something more interesting

24P. Thompson ATLAS SM searches H->WW/ZZ

• More data required to see if it a fluctuation or something more interesting

• The H->ZZ/WW channels will continue to play crucial role in SM Higgs searches at LHC

Extra Slides

25P. Thompson ATLAS SM searches H->WW/ZZ

H->ZZ/WW signal/bkg

26P. Thompson ATLAS SM searches H->WW/ZZ

Fourth Generation Limits : 2010

27P. Thompson ATLAS SM searches H->WW/ZZ

Excluded cross section to the expected one when a fourth generation of

high mass quarks and leptons with Standard Model-like couplings to the

Higgs boson are included in the cross section calculations

H->WW->lvlv

[GeV]HM120 140 160 180 200 220 240

SM

σ/σ

95%

CL L

imit o

n

1

10

210Observed

Expected

σ 1 ±σ 2 ±

ATLAS Preliminary

­1 Ldt = 1.04 fb∫

= 7 TeVs

+ 0jνlνl→(*)

WW→H

[GeV]HM120 140 160 180 200 220 240

SM

σ/σ

95%

CL L

imit o

n

1

10

210Observed

Expected

σ 1 ±σ 2 ±

ATLAS Preliminary

­1 Ldt = 1.04 fb∫

= 7 TeVs

+ 1jνlνl→(*)

WW→H

Exclusion for 0 and 1 jet

28P. Thompson ATLAS SM searches H->WW/ZZ

[GeV]HM120 140 160 180 200 220 240

SM

σ/σ

95%

CL L

imit o

n

1

10

210Observed

Expected

σ 1 ±σ 2 ±

ATLAS Preliminary

­1 Ldt = 1.04 fb∫

= 7 TeVs

+ 0jνlνl→(*)

WW→H

[GeV]HM120 140 160 180 200 220 240

SM

σ/σ

95%

CL L

imit o

n

1

10

210Observed

Expected

σ 1 ±σ 2 ±

ATLAS Preliminary

­1 Ldt = 1.04 fb∫

= 7 TeVs

+ 1jνlνl→(*)

WW→H

H->WW->lvlv : background only p-value

29P. Thompson ATLAS SM searches H->WW/ZZ

H->WW->lvlv

[GeV]HM120 140 160 180 200 220 240

Sta

tistical sig

nific

ance

0

2

4

6

8

10Observed

Expected

σ 1 ±σ 2 ±

­1 Ldt = 1.04 fb∫

= 7 TeVs

ATLAS Preliminary + 0jνlνl→(*)

WW→H νlνl→(*)

WW→H

[GeV]HM120 140 160 180 200 220 240

Sta

tistica

l sig

nific

an

ce

0

2

4

6

8

10Observed

Expected

σ 1 ±σ 2 ±

­1 Ldt = 1.04 fb∫

= 7 TeVs

ATLAS Preliminary + 1jνlνl→(*)

WW→H νlνl→(*)

WW→H

Significance 0 and 1 –jet

30P. Thompson ATLAS SM searches H->WW/ZZ

[GeV]HM120 140 160 180 200 220 240

Sta

tistical sig

nific

ance

0

2

4

6

8

10Observed

Expected

σ 1 ±σ 2 ±

­1 Ldt = 1.04 fb∫

= 7 TeVs

ATLAS Preliminary + 0jνlνl→(*)

WW→H νlνl→(*)

WW→H

[GeV]HM120 140 160 180 200 220 240

Sta

tistica

l sig

nific

an

ce

0

2

4

6

8

10Observed

Expected

σ 1 ±σ 2 ±

­1 Ldt = 1.04 fb∫

= 7 TeVs

ATLAS Preliminary + 1jνlνl→(*)

WW→H νlνl→(*)

WW→H

H->WW->lvqq

M(lvjj) 0 and 1 jet

31P. Thompson ATLAS SM searches H->WW/ZZ

H->WW->lvjj

M(lvjj) subtracted 0 and 1 jet

32P. Thompson ATLAS SM searches H->WW/ZZ