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peak lumi 1.5×1033cm-2s-1 (already out of date!)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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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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Top mll sidebands and b-tagTop: eµ events
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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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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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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ZZ→H1
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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
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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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70 Data stat)⊕ SM (sys
WW γ WZ/ZZ/W
t t Single Top
Z+jets W+jets (data driven)
H [150]
ATLAS Preliminary1
L dt = 1.04 fb∫ = 7 TeV, s
+ 0 jetνlνl→WW→H
[GeV]ll
M
50 100 150 200 250
Data
/ M
C
0.5
1
1.5
2
Entr
ies / 0
.3
10
20
30
40
50 Data stat)⊕ SM (sys
WW γ WZ/ZZ/W
t t Single Top
Z+jets W+jets
H [150]
ATLAS Preliminary1
L dt = 1.04 fb∫ = 7 TeV, s
+ 0 jetνlνl→WW→H
(ll) [rad]φ∆
0 0.5 1 1.5 2 2.5 3
Da
ta /
MC
0.5
1
1.5
2
Entr
ies / 1
0 G
eV
5
10
15
20
25
30
35 Data stat)⊕ SM (sys
WW γ WZ/ZZ/W
t t Single Top
Z+jets W+jets (data driven)
H [150]
ATLAS Preliminary1
L dt = 1.04 fb∫ = 7 TeV, s
+ 0 jetνlνl→WW→H
[GeV]T
M
60 80 100 120 140 160 180 200 220 240
Da
ta /
MC
0.5
1
1.5
2
H->WW->lvlv : 1-jet channel
Entr
ies / 1
0 G
eV
5
10
15
20
25
30
35
40 Data stat)⊕ SM (sys
WW γ WZ/ZZ/W
t t Single Top
Z+jets W+jets (data driven)
H [150]
ATLAS Preliminary1
L dt = 1.04 fb∫ = 7 TeV, s
+ 1 jetνlνl→WW→H
[GeV]ll
M
50 100 150 200 250
Data
/ M
C
0.5
1
1.5
2
En
trie
s /
0.3
5
10
15
20
25 Data stat)⊕ SM (sys
WW γ WZ/ZZ/W
t t Single Top
Z+jets W+jets
H [150]
ATLAS Preliminary1
L dt = 1.04 fb∫ = 7 TeV, s
+ 1 jetνlνl→WW→H
(ll) [rad]φ∆
0 0.5 1 1.5 2 2.5 3
Da
ta /
MC
0.5
1
1.5
2
En
trie
s /
10
Ge
V
2
4
6
8
10
12
14
16 Data stat)⊕ SM (sys
WW γ WZ/ZZ/W
t t Single Top
Z+jets W+jets (data driven)
H [150]
ATLAS Preliminary1
L dt = 1.04 fb∫ = 7 TeV, s
+ 1 jetνlνl→WW→H
[GeV]T
M
60 80 100 120 140 160 180 200 220 240
Data
/ M
C
0.5
1
1.5
2
Mll after Z->ττ veto: ∆φll after mll cut: MT after all cuts:
17P. Thompson ATLAS SM searches H->WW/ZZ
Entr
ies / 1
0 G
eV
5
10
15
20
25
30
35
40 Data stat)⊕ SM (sys
WW γ WZ/ZZ/W
t t Single Top
Z+jets W+jets (data driven)
H [150]
ATLAS Preliminary1
L dt = 1.04 fb∫ = 7 TeV, s
+ 1 jetνlνl→WW→H
[GeV]ll
M
50 100 150 200 250
Data
/ M
C
0.5
1
1.5
2
En
trie
s /
0.3
5
10
15
20
25 Data stat)⊕ SM (sys
WW γ WZ/ZZ/W
t t Single Top
Z+jets W+jets
H [150]
ATLAS Preliminary1
L dt = 1.04 fb∫ = 7 TeV, s
+ 1 jetνlνl→WW→H
(ll) [rad]φ∆
0 0.5 1 1.5 2 2.5 3
Da
ta /
MC
0.5
1
1.5
2
En
trie
s /
10
Ge
V
2
4
6
8
10
12
14
16 Data stat)⊕ SM (sys
WW γ WZ/ZZ/W
t t Single Top
Z+jets W+jets (data driven)
H [150]
ATLAS Preliminary1
L dt = 1.04 fb∫ = 7 TeV, s
+ 1 jetνlνl→WW→H
[GeV]T
M
60 80 100 120 140 160 180 200 220 240
Data
/ M
C
0.5
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1.5
2
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