Higgs Boson Searches in ATLAS - Institut für Kern- und Teilchenphysik

Higgs Boson Searches in ATLAS 

Wolfgang F. Mader 1 

1 Institut für Kern- und Teilchenphysik 

TU Dresden 

Fourth fermion generation and single-top production 2012 

26-28 March 2012 

Leinsweiler, Pfalz, Germany 

Wolfgang F. Mader (TU Dresden) Higgs Boson Searches in ATLAS 1/ 28

Outline 

1 The ATLAS Detector and Performance 

2 Standard Model Higgs Searches 

The Standard Model Higgs Boson 

Low-Mass Higgs Searches 

High-Mass Higgs Searches 

Combination of Standard Model Results 

3 Higgs Searches Beyond the Standard Model 

Fermiophobic Higgs 

MSSM Neutral Higgs 

Charged Higgs 

Doubly-Charged Higgs 


The ATLAS Detector 


Performance of ATLAS and the LHC 

LHC Integrated Luminosity 

] 

-1 

Total Integrated Luminosity [fb 

7 ATLAS Online Luminosity s = 7 TeV 

6 

5 

4 

3 

2 

1 

LHC Delivered 

ATLAS Recorded 

-1 

Total Delivered: 5.61 fb 

-1 

Total Recorded: 5.25 fb 

0 

28/02 30/04 30/06 30/08 31/10 

Day in 2011 

Excellent LHC Performance in 2011 

Peak Luminosity 3.6 · 10 33 cm −2 s −1 

1fb −1 Passed in June 2011 

ATLAS Data Taking Efficiency 

Recording Efficiency [percent] 

120 

110 

100 

90 

80 

70 

ATLAS Online 

Total Efficiency: 93.5% 

s = 7 TeV 

60 

28/02 30/04 30/06 30/08 31/10 

Day in 2011 

Excellent Performance of ATLAS 

Small Fraction of non-Working 

Detector Channels (∼ 3.5%) 

Data-Taking Efficiency ∼ 93.5% 

90 − 96% of Data Used for Analyses 

(Good Quality – Depends on Analysis) 


Performance of ATLAS and the LHC 

Z → µµ Candidate with 20 

Vertices 

] 

-1 

Recorded Luminosity [pb 

4 

10 ATLAS Online 2011, s=7 

TeV 

3 

10 

2 

10 

10 

1 

-1 

10 

-2 

10 

-3 

10 

-1 

Ldt=5.2 fb 

0 2 4 6 8 10 12 14 16 18 20 22 24 

Mean Number of Interactions per Crossing 

Up to 20 Interactions 

∫ 

β * = 1.0 m, < µ > = 11.6 

β * = 1.5 m, < µ > = 6.3 

Comparable to Design Luminosity 

Challenging for 

Trigger 

Computing Resources 

Isolation Criteria 

Vertexing 

E miss 

T and Energy Sclaes 

... 

Continuing Performance Studies to 

Understand Impact of Pile-Up 















The Standard Model Higgs Boson @ LHC 

Production Cross Section (arXiV:1101.0593) 

H+X) [pb] 

→ 

(pp 

σ 

10 

1 

-1 

10 

-2 

10 

pp → 

pp → H (NNLO+NNLL QCD + NLO EW) 

pp → qqH (NNLO QCD + NLO EW) 

pp → WH (NNLO QCD + NLO EW) 

ZH (NNLO QCD +NLO EW) 

pp → ttH (NLO QCD) 

s= 

7 TeV 

100 200 300 400 500 1000 

MH 

[GeV] 

Dominant Production Mode: gg → H 

Sub-Dominant Modes: 

Vector-Boson Fusion: q¯q → Hq¯q 

Higgsstrahlung q¯q → VH (V ∈ {W, Z }) 

Small Contribution from Associated Production: gg → t¯tH 

Total Cross Section ∼ 20 Higher than @ TEVATRON 

LHC HIGGS XS WG 2010 



Branching Fractions (arXiv:1201.3084) 

BR (pb) 

× 

σ 

10 

1 

-1 

10 

-2 

10 

s = 7TeV 

± 

WW → l νqq 

+ - 

WW → l νl 

ν 

+ - 

ZZ → l l qq 

+ - 

ZZ → l l νν 

-3 

10 

± 

WH → l νbb 

+ - 

ZH → l l bb 

+ - 

τ τ 

γγ 

+ - + - 

ZZ → l l l l 

l = e, µ 

ν = νe, 

νµ 

, ντ 

q = udscb 

-4 

10100 

200 300 400 500 

Higgs boson mass (GeV) 

mH ≤ 135 GeV 

Dominant: H → b ¯ b 

Sub-Dominant: H → WW (∗) , H → ττ 

H → γγ very Sensitive despite low Branching Fraction 

mH ≥ 135 GeV 

Dominant: H → WW (∗) 

Cleanest Mode: H → ZZ (∗) (Golden Channel) 

Wolfgang F. Mader (TU Dresden) Higgs Boson Searches in ATLAS 6/ 28 

LHC HIGGS XS WG 2011


Branching Fractions (arXiv:1201.3084) 

BR (pb) 

× 

σ 

10 

1 

-1 

10 

-2 

10 

s = 7TeV 

± 

WW → l νqq 

+ - 

WW → l νl 

ν 

+ - 

ZZ → l l qq 

+ - 

ZZ → l l νν 

-3 

10 

± 

WH → l νbb 

+ - 

ZH → l l bb 

+ - 

τ τ 

γγ 

+ - + - 

ZZ → l l l l 

l = e, µ 

ν = νe, 

νµ 

, ντ 

q = udscb 

-4 

10100 

200 300 400 500 

Higgs boson mass (GeV) 

mH ≤ 135 GeV 

Dominant: H → b ¯ b 

Sub-Dominant: H → WW (∗) , H → ττ 

H → γγ very Sensitive despite low Branching Fraction 

mH ≥ 135 GeV 

Dominant: H → WW (∗) 

Cleanest Mode: H → ZZ (∗) (Golden Channel) 

Mass-Dependent Higgs Searches in 110 < mH < 600 GeV 


LHC HIGGS XS WG 2011


Measurement of Background Processes 

Excellent Agreement with Standard Model Predictions 

Important for Background Estimates in Higgs Boson Searches 


Status of Higgs Boson Searches 

Results Presented @ Summer Conferences 2011 

Higgs Boson Mass Constraints @ 95% CL 

114.4 < mH < 141 GeV 

mH > 476 GeV 


Status of Higgs Boson Searches 

Updates for 2011 Analyses 

Unchanged: H → γγ, H → ZZ (∗) → 4ℓ 

Updated: H → WW (∗) → ℓνℓν (lνq¯q ′ ), H → ZZ (∗) → ℓℓνν(ℓℓq¯q) 

New: H → ττ, H → b¯b 


Systematic Uncertainties (Experiment) 

Treatment of Background 

Estimated in Data from Signal-Free, Background-Enriched Control Regions 

Otherwise: Estimate Taken from Simulation 

W/Z+Jets: ALPGEN (Heavy Flavor also from SHERPA or POWHEG) 

t¯t, Single Top: MC@NLO/AcerMC 

Di-Boson: MC@NLO, SHERPA, PYTHIA 


Systematic Uncertainties (Theory) 

Signal Processes 

MC Generator: 

POWHEG+PYTHIA for ggH and VBF (qq → Hqq) 

PYTHIA for WH/ZH and t¯tH 

Theoretical Calculations: 

ggH : NNLO QCD + NNLL Soft-Gluon Resummation; NLO EW 

VBF : NLO QCD + Approximate NNLO Corrections; NLO EW 

ZH : NNLO QCD + NLO EW 


Statistical Analysis 

Profile-Likelihood Method (Eur.Phys.J.C71:1554,2011) 

L(µ, Θ) = Πj=binPoisson(Nj |µsj(Θ) + bj(Θ))ΠΘGaussian(Θ|0, 1) 

µ : Signal-Strength Parameter (Parameter of Interest – σ/σSM) 

Θ : Vector of Nuissance Parameters 

Nj : Observed Number of Events in Bin j 

sj : Predicted Number of Signal Events in Bin j 

bj : Predicted Number of Background Events in Bin j 

! 

L(µ,ˆΘµ) 

qµ = −2ln 

L(ˆµ, ˆΘ) 

ˆµ : Best Estimator for Signal-Strength Parameter 

ˆΘ : Best Estimator for Nuissance Parameters 

ˆΘµ : Best Estimator for Nuissance Parameters Given Signal-Strength Hypothesis µ 

Exclusion Limits on µ Are Set @ 95% CL Using CLS Method 

(J. Phys. G 28 (2002) 2693-2704) 


Good Mass Resolution 

H → γγ 

H → ZZ (∗) → 4ℓ 

Limted Mass Resolution 

H → WW (∗) → ℓνℓν 

H → ττ 

W /ZH, H → b ¯ b 

Low-Mass Higgs Boson Searches 



H → γγ (arXiv:1202.1414) 

1 st γ: 

2 nd γ: 

ET = 

64.2 GeV 

η = 0.34 

ET = 

61.4 GeV 

η = 0.61 

mγγ: = 126.6 GeV 



H → γγ (arXiv:1202.1414) 

Signal Model 

Christal Ball + Gaussian Tail 

(σCB = 1.4 − 2.3 GeV) 

Mass Scale Uncertainty 0.7 GeV 

(mH = 120 GeV) 

Mass Reconstruction Robust against 

Pile-Up 

Background Model 

Simultaneous Fit to all 9 Categories 

Exponential Function (Free 

Slope/Normalization) 

Modeling Uncertainty (0.1-7.9 

Events) 

Events / GeV 

Data - Bkg 

900 

800 

700 

600 

500 

400 

300 

200 

100 

ATLAS 

Selected diphoton sample 

Data 2011 

Background model 

SM Higgs boson m = 120 GeV (MC) 

H 

-1 

s = 7 TeV, ∫ Ldt = 4.9 fb 

100 0 

100 110 120 130 140 150 160 

50 

0 

-50 

-100 

100 110 120 130 140 150 160 

mγ 

γ [GeV] 



H → γγ (arXiv:1202.1414) 

Signal Model 

Christal Ball + Gaussian Tail 

(σCB = 1.4 − 2.3 GeV) 

Mass Scale Uncertainty 0.7 GeV 

(mH = 120 GeV) 

Mass Reconstruction Robust against 

Pile-Up 

Background Model 

Simultaneous Fit to all 9 Categories 

Exponential Function (Free 

Slope/Normalization) 

Modeling Uncertainty (0.1-7.9 

Events) 

Main Backgrounds 

(Inverted γ ID and Isolation) 

irreducible γγ (30 pb) 

reducible γj (20 nb) 

reducible jj (500 µb) 

Events / GeV 

Data - Bkg 

900 

800 

700 

600 

500 

400 

300 

200 

100 


Selected diphoton sample 

Data 2011 

Background model 

SM Higgs boson m = 120 GeV (MC) 

H 

-1 

s = 7 TeV, ∫ Ldt = 4.9 fb 

100 0 

100 110 120 130 140 150 160 

50 

0 

-50 

-100 

100 110 120 130 140 150 160 

mγ 

γ [GeV] 



H → γγ (arXiv:1202.1414) 

SM 

σ 

/ 

σ 

95% CL limit on 

8 

7 

6 

5 

4 

3 

2 

1 

Observed CLs 

limit 

Expected CLs 

limit H → γγ 

± 1σ 


± 2σ 

Data 2011, s = 7 TeV 

-1 

Ldt = 4.9 fb 

110 115 120 125 130 135 140 145 150 

Observed Exclusion: 113 − 115 GeV and 134.5 − 136 GeV 

Largest Excess @ mH = 126.5 GeV 

∫ 

mH 

[GeV] 

Local Significance: 2.8σ (Global: 1.5σ for 110 < mH < 150 GeV) 



ee Pair: 

µµ Pair: 

H → ZZ (∗) → 4ℓ (arXiv:1202.1415) 

mµµ = 

89.3 GeV 

mee = 

30.0 GeV 

High Mass Resolution (130 GeV : 1.5 − 2%) 

High Lepton Reconstruction Efficiency 

Lepton Performance Well Modeled; Independent of Pile-Up 

0.2 − 2% Uncertainty on Signal Yield; 0.6% Uncertainty on m4e Scale 



H → ZZ (∗) → 4ℓ (arXiv:1202.1415) 

Events/5 GeV 

10 

8 

6 

4 

2 

DATA 

Background 

Signal (m =125 GeV) 

H 


H 


H 

Syst.Unc. 

(*) 

H→ZZ 

→4l 

∫Ldt 

= 4.8 fb 

s = 7 TeV 

-1 


0 

100 150 200 250 

m4l 

[GeV] 

Background Estimation 

ZZ (∗) : From Simulation 

Events/10 GeV 

12 

10 

8 

6 

4 

2 

0 


DATA 

Background 


H 


H 


H 

Syst.Unc. 

H→ZZ 

→4l 

∫Ldt 

= 4.8 fb 

s = 7 TeV 

200 400 600 

m4l 

[GeV] 

Z+Jets: Control Region w/o Charge, Isolation and d0 on 2 nd Lepton Pair 

t¯t: e ± µ ∓ Consistent with mZ + 2 Same-Flavor Leptons 


(*) 

-1


H → ZZ (∗) → 4ℓ (arXiv:1202.1415) 

σ/ 

σ 

SM 


10 

2 

10 

1 


(*) 

H→ 

ZZ → 4l 

-1 

∫Ldt 

= 4.8 fb 

s=7 

TeV 

Observed CL 

Expected CL s 

± 1 σ 

± 2 σ 

110 120 130 140 150 160 170 180 

mH 

[GeV] 

s 

σ/ 

σ 

SM 


10 

2 

10 

1 


(*) 

H→ 

ZZ → 4l 

-1 

∫Ldt 

= 4.8 fb 

s=7 

TeV 

Observed CL 

Expected CL s 

± 1 σ 

± 2 σ 

200 250 300 350 400 450 500 550 600 

mH 

[GeV] 

Expected Limit: 137 − 157 GeV and 184 − 400 GeV 

Observed Limit: 134 − 156 GeV, 182 − 233 GeV, 256 − 265 GeV and 

268 − 415 GeV 

Local Significances: Observed (Expected w/ Signal) 

125 GeV: 2.1σ (1.3σ) 

244 GeV: 2.2σ (3.0σ) 

500 GeV: 1.5σ (2.1σ) 


s


H → WW (∗) → ℓνℓν (ATLAS-CONF-2012-012) 

Events / 10 GeV 

120 

100 

80 

60 

40 

20 

Selection 

ATLAS Preliminary 

-1 

s = 7 TeV, ∫ L dt = 4.7 fb 

(*) 

H→WW 

→lνlν 

+ 0 jets 

Data SM (sys ⊕ stat) 

WW WZ/ZZ/Wγ 

t t 

Single Top 

Z+jets W+jets 

H [125 GeV] 

60 80 100 120 140 160 180 200 220 240 

mT 

[GeV] 


35 

30 

25 

20 

15 

10 

5 


-1 

s = 7 TeV, ∫ L dt = 4.7 fb 

(*) 

H→WW 

→lνlν 

+ 1 jet 

Two Isolated Opposite-Sign Leptons (pT > 20, 25 GeV) 

Large Missing Transverse Energy 

Jet Multiplicity (0/1/2 Jets), b Veto 

Topological Cuts (mℓℓ, ∆φℓℓ) 

Additional Jet-Bin Dependent Cuts (pℓℓ T , �ptot 

T ) 



WW WZ/ZZ/Wγ 

t t 


Z+jets W+jets 

H [125 GeV] 

60 80 100 120 140 160 180 200 220 240 

mT 

[GeV] 





120 

100 

80 

60 

40 

20 


-1 

s = 7 TeV, ∫ L dt = 4.7 fb 

(*) 

H→WW 

→lνlν 

+ 0 jets 


WW WZ/ZZ/Wγ 

t t 


Z+jets W+jets 

H [125 GeV] 

60 80 100 120 140 160 180 200 220 240 

Selection 


mT 

[GeV] 


35 

30 

25 

20 

15 

10 

5 


-1 

s = 7 TeV, ∫ L dt = 4.7 fb 

(*) 

H→WW 

→lνlν 

+ 1 jet 


WW WZ/ZZ/Wγ 

t t 


Z+jets W+jets 

H [125 GeV] 

60 80 100 120 140 160 180 200 220 240 

WW: Control Sample (∆φℓℓ, mℓℓ); For mH > 300 GeV from Simulation 

Top: Control Samples (i) no b-Tagging; (ii) b-Jet Tag Requirement 

Z+jets: |mℓℓ − mZ | < 15 GeV; Correcting for Mis-Modeling of E miss 

T Tail 

W+Jets: Inverted Lepton ID Passing Loose Criteria 

mT 

[GeV] 




σ/ 

σ 

SM 

95% C.L. Limit on 

10 

2 

10 

1 

-1 

10 

ATLAS Preliminary H→WW→lνlν 

Observed 

Expected 

± 1 σ 

± 2 σ 

-1 

Ldt = 4.7 fb 

s = 7 TeV 

100 200 300 400 500 600 

mH 

[GeV] 

Expected Exclusion: 127 − 234 GeV 

Observed Exclusion: 130 − 260 GeV 

∫ 




σ/ 

σ 

SM 

95% C.L. Limit on 

10 

2 

10 

1 

-1 

10 

ATLAS Preliminary H→WW→lνlν 

Observed 

Expected 

± 1 σ 

± 2 σ 

-1 

Ldt = 4.7 fb 

s = 7 TeV 

110 115 120 125 130 135 140 145 150 

mH 

[GeV] 



∫ 



H → ττ → ℓℓ/ℓh/hh + 4/3/2ν (ATLAS-CONF-2012-014) 

Channel Previously not Used in SM Higgs Analyses 

Separate Analyses Accordig to Jet-Category (0/1-Jet, 2-Jet VH, 2-Jet VBF) 


Fake Light Leptons and τ Jets 

Z → ττ from Embedding Technique 

Mass Reconstruction 

Collinear Approximation 

Missing Mass Calculator 


350 

300 

250 

200 

150 

100 

50 

Z→µ 

µ Embedded 

Z→µ 

µ Data 

-1 

L dt = 4.7 fb 

s = 7 TeV 


0 

0 10 20 30 40 50 60 

miss 

ET 

[GeV] 

∫ 


10 

3 

× 10 

8 

6 

4 

2 

eτhad 

+ µ τ 

had 

Z→ττ 

Z→ττ 

Emb. syst. 

s = 7 TeV 

Embedded 

Alpgen MC 

-1 

L dt = 4.7 fb 


0 

0 50 100 150 200 250 300 

MMC mass mττ 

[GeV] 


∫




140 

120 

100 

80 

60 

40 

20 

Channel Previously not Used in SM Higgs Analyses 

Separate Analyses Accordig to Jet-Category (0/1-Jet, 2-Jet VH, 2-Jet VBF) 


Fake Light Leptons and τ Jets 

Z → ττ from Embedding Technique 

Mass Reconstruction 

Collinear Approximation 

Missing Mass Calculator 

ee + eµ 

+ µ µ H+2-jet VH 

Data 

30 x H(120) →ττ 

Z→ττ 

Z→ee, 

µ µ 

tt+single-top 

WW/WZ/ZZ 

Fake leptons 

Bkg. uncert. 

-1 

L dt = 4.7 fb 

s = 7 TeV 


0 

0 50 100 150 200 250 300 350 400 

Collinear mass mττ 

[GeV] 

∫ 


45 

40 

35 

30 

25 

20 

15 

10 

5 

eτhad 

+ µ τ 

had 

H+2-jet VBF 

Data 

5 x H(120) →ττ 

Z→ττ 

(OS-SS) 

Others (OS-SS) 

W+jets (OS-SS) 

Same Sign 

Bkg. uncert. 

-1 

L dt = 4.7 fb 

s = 7 TeV 


0 

0 50 100 150 200 250 300 350 400 

MMC mass mττ 

[GeV] 

∫ 


100 

80 

60 

40 

20 

τhadτhad 

H+1-jet 

Data 

5 x H(120) →ττ 

Z→ττ 

Multi-jet 

Others 

Bkg. uncert. 

-1 

L dt = 4.7 fb 

s = 7 TeV 


0 

0 50 100 150 200 250 300 

Collinear mass mττ 

[GeV] 


∫



SM 

σ 

/ 

σ 

95% CL Limit on 

25 

20 

15 

10 

5 

H → ττ 

Observed CLs 

Expected CLs 

± 2σ 

± 1σ 


∫ Ldt ≈ 4.7 fb 

s = 7 TeV 

0 

100 110 120 130 140 150 

Observed Limit ∼ 3 − 10 times Standard Model 

No Significant Excess in 100 < mH < 150 GeV 

-1 

mH 

[GeV] 



W/Z H, H → b ¯ b (ATLAS-CONF-2012-015) 

Events Events / 10 GeV 

Previously not Used in SM Higgs Analyses 

W /Z and H Recoil w/ Significant pT 

Analysis Sub-Divided in 4 p W/Z 

T Categories OR ... (50/100/200 GeV) 

(WH → ℓνb¯b and ZH → ℓℓb¯b) 

... 3 E miss 

T Categories (120/160/200 GeV) 

(ZH → ννb¯b) 

Exactly 2 b-Tagged Jets w/ E b1 

T 

mb¯b Discriminating Variable 

120 

100 

80 

60 

40 

20 


∫ 

-1 

L dt=4.7 fb , s = 7 TeV 

ZH → llbb 

Data 2011 

Signal × 5 

(m =120 GeV) 

H 

Total BG 

Top 

Z+jets 

Diboson 

0 

0 50 100 150 200 250 

m [GeV] 

b b 

> 45 GeV and Eb2 

T 

Events / 10 GeV GeV 

250 

200 

150 

100 

50 

> 25 GeV 

450 

400 

350 


-1 

∫ L dt=4.7 fb , s = 7 TeV 

WH → lνbb 

Data 2011 

Signal × 5 

(m =120 GeV) 

H 

Total BG 

Top 

300 

Z+jets 

W+jets 

Diboson 

Multijet 

0 

0 50 100 150 200 250 

m [GeV] 


b b


W/Z H, H → b¯b (ATLAS-CONF-2012-015) 

SM 

σ 

/ 

σ 

95% C.L. limit on 

14 

12 

10 

8 

6 

4 

2 


Observed (CLs) 

Expected (CLs) 

± 1σ 

± 2σ 

s=7 

TeV, ∫ 

VH(bb), combined 

-1 

Ldt =4.6-4.7 fb 

0 

110 115 120 125 130 

Combined Exclusion ∼ 2 − 5 times Standard Model 

mH 

[GeV] 


H → ZZ (∗) → ℓℓνν 

H → ZZ (∗) → ℓℓq¯q 

H → WW (∗) → ℓνq¯q ′ 

High-Mass Higgs Boson Searches 



H → ZZ (∗) → ℓℓνν (ATLAS-CONF-2012-016) 

Most Sensitive Channel in the High-Mass Range 

Discriminating Variable: Transverse Mass 

Pile-Up Dependent due to E miss 

T : 


100 

80 

60 

40 

20 

ATLAS Preliminary s = 7 TeV 

H → ZZ → llνν 

Low pile-up data 

-1 

Data ∫ L dt = 2.3 fb 

Total BG 

Top 

ZZ,WZ,WW 

Z,W 

Signal (m = 200 GeV) 

H 

0 

150 200 250 300 350 400 450 

mT 

[GeV] 


70 

60 

50 

40 

30 

20 

10 

0 



High pile-up data 

-1 


Total BG 

Top 

ZZ,WZ,WW 

Z,W 


H 

200 300 400 500 600 700 

mT 

[GeV] 




Most Sensitive Channel in the High-Mass Range 

Discriminating Variable: Transverse Mass 

Pile-Up Dependent due to E miss 

T : 


100 

80 

60 

40 

20 



Low pile-up data 

-1 


Total BG 

Top 

ZZ,WZ,WW 

Z,W 


H 

0 

150 200 250 300 350 400 450 


mT 

[GeV] 


70 

60 

50 

40 

30 

20 

10 

0 



High pile-up data 

ZZ: From Simulation (11% Normalization Uncertainty) 

WZ: Estimated from Three-Lepton Events 

-1 


Total BG 

Top 

ZZ,WZ,WW 

Z,W 


H 

200 300 400 500 600 700 

mT 

[GeV] 

t¯t: From Simulation (Veryfied in eµ+ ≥ 1b-Jet Data or mℓℓ Sidebands) 

W+Jets: Like-Sign ee and eµ Events in mℓℓ Sideband + 0 b-Jets 

Z+Jets: Low ∆φ(�p miss 

T , �pjet 

T ) 




SM 

σ 

/ 

σ 


4.5 

4 

3.5 

3 

2.5 

2 

1.5 

1 

0.5 


H→ZZ→llνν 

-1 

∫Ldt=4.7fb 

, 

s=7TeV 

Observed 

Expected 

± 1σ 

± 2σ 

200 250 300 350 400 450 500 550 600 



mH 

[GeV] 



H → ZZ (∗) → ℓℓqq (ATLAS-CONF-2012-017) 

Entries / 20 GeV 

Analyses Categorized According to b-Tag 

mℓℓjj as Discriminating Variable 

240 

220 

200 

180 

160 

140 

120 

100 

80 

60 

40 

20 

0 


∫ 

-1 

L dt=4.71 fb , 

s = 7 TeV 

data 

Signal × 5 

(m =400 GeV) 

H 

Total BG 

Z + jets 

Top 

Diboson 

200 300 400 500 600 700 800 

[GeV] 


mlljj 


8 

7 

6 

5 

4 

3 

2 

1 


∫ 

-1 

L dt=4.71 fb , 

s = 7 TeV 

data 

Signal 

(m =400 GeV) 

H 

Total BG 

Z + jets 

Top 

Diboson 

0 

200 300 400 500 600 700 800 

[GeV] 

Z+Jets/Top: Normalized from mjj Sidebands; Shape from Simulation 

Di-Boson: Taken from Simulation 

Multijets: Reverted Electron-ID Criteria 


mlljj


H → ZZ (∗) → ℓℓqq (ATLAS-CONF-2012-017) 

SM 

σ 

/ 

σ 


10 

8 

6 

4 

2 

0 


H→ 

ZZ→ 

llqq 

-1 

Ldt = 4.7 fb , s=7 

TeV 

∫ 

Observed 

Expected 

± 1 σ 

± 2 σ 

200 300 400 500 600 

mH 

[GeV] 


Observed Exclusion: 300 − 310 GeV and 360 − 400 GeV 



H → WW (∗) → ℓνqq (ATLAS-CONF-2012-018) 

Analysis Split into 0/1-Jet and 2-Jet (VBF) Category 

Discriminating Variable mℓνqq with mℓν = mW and mqq = mW 

Search for Bump above Steeply-Falling Background 

(Modeled from Fit to Data) 

Major Systematic Uncertainties: Jet-Energy Scale (10 − 20%); Pile-Up 

(10 − 15%) 


Data / MC 

-1 ∫ L dt = 4.7 fb ATLAS Preliminary 

3 

10 Data 

s = 7 TeV 

Total bkg (stat ⊕ sys) 

H + 0j, H→ 

eν 

qq 

W+jets 

Z+jets 

Dibosons 

2 

10 

Multi-jet 

Top 

SM Signal (m = 400 GeV) 

H 

10 

1 

3 

2.5 

2 

1.5 

1 

0.5 

0 

200 300 400 500 600 700 800 900 1000 

m 

[GeV] 

200 300 400 500 600 700 800 900 1000 

mWW 

[GeV] 


Data / MC 

2 

10 

10 

1 

3 

2.5 

2 

1.5 

1 

0.5 

0 

-1 ∫ L dt = 4.7 fb 

s = 7 TeV 

H + 2j, H→ 

µ ν qq 


Data 

Total bkg (stat ⊕ sys) 

W+jets 

Z+jets 

Dibosons 

Multi-jet 

Top 

SM Signal x10 

mH 

= 400 GeV 

200 300 400 500 600 700 800 900 1000 

m 

[GeV] 

200 300 400 500 600 700 800 900 1000 

mWW 

[GeV] 



H → WW (∗) → ℓνqq (ATLAS-CONF-2012-018) 

Analysis Split into 0/1-Jet and 2-Jet (VBF) Category 

Discriminating Variable mℓνqq with mℓν = mW and mqq = mW 

Search for Bump above Steeply-Falling Background 

(Modeled from Fit to Data) 

Major Systematic Uncertainties: Jet-Energy Scale (10 − 20%); Pile-Up 

(10 − 15%) 

σ/ 

σ(SM) 

95% C.L. limit on 

2 

10 

10 

1 

Expected 

± 1σ 

± 2σ 

Observed 

s=7 

TeV 

-1 

L dt=4.7 fb 

H→ 

WW→ 

lν 

jj ATLAS Preliminary 

-1 

10 

300 350 400 450 500 550 600 

mH 

[GeV] 

Limit ∼ 2 − 10 Times Standard Model 

∫ 


Combination 

SM 

σ 

/ 

σ 


10 

1 

-1 

10 

Exp. Obs. Exp. Obs. 

-1 -1 

H→γγ 

H(4.9 

→γγfb(4.9 

) fb ) 

-1 -1 

H→ττ 

H(4.7 

→ττfb(4.7 

) fb ) 

-1 -1 

H → bb H →(4.6-4.7 

bb (4.6-4.7 fb ) fb ) 

-1 -1 

H→ 

WW H→ 

→ WW lνlν→ 

(4.7 lνlνfb(4.7 

) fb ) 

ATLAS 2011 Preliminary 

-1 -1 

H→ 

WW H→ 

→ WW lνqq 

→(4.7 

lνqq 

fb(4.7 

) fb ) 

-1 -1 

H→ 

ZZH 

→ →llll 

ZZ→(4.8 

llll fb(4.8 

) fb ) 

-1 -1 

H→ 

ZZH 

→ →llqq 

ZZ→(4.7 

llqq fb(4.7 

) fb ) 

-1 -1 

H→ 

ZZH 

→ →ll 

ZZ νν→ 

(4.7 llννfb(4.7 

) fb ) 

-1 

L dt ~ 4.6-4.9 fb , 

s=7 

TeV 

100 200 300 400 500 600 

∫ 

CLs limits 

mH 

[GeV] 



Combined Exclusion Limit 

SM 

σ 

/ 

σ 


10 

1 

-1 

10 

ATLAS Preliminary 2011 Data 

Obs. 

Exp. 

± 1 σ 

± 2 σ 

CLs Limits 

∫ 

-1 

Ldt = 4.6-4.9 fb 

s = 7 TeV 

100 200 300 400 500 600 

mH 

[GeV] 

Excluded Mass Range 

SM 

σ 

/ 

σ 

Expected (95% CL): 120 < mH < 550 GeV 


10 

1 

-1 

10 


Obs. 

Exp. 

-1 

Ldt = 4.6-4.9 fb 

± 1 σ 

± 2 σ 

s = 7 TeV 

CLs Limits 

110 115 120 125 130 135 140 145 150 

mH 

[GeV] 

Observed (95% CL): 110 < mH < 117.5 GeV and 118.5 < mH < 122.5 GeV 

and 129 < mH < 539 GeV 

Observed (99% CL): 130 < mH < 486 GeV 


∫


Combined Exclusion Limit 

SM 

σ 

/ 

σ 


10 

1 

-1 

10 


Obs. 

Exp. 

± 1 σ 

± 2 σ 

CLs Limits 

∫ 

-1 

Ldt = 4.6-4.9 fb 

s = 7 TeV 

100 200 300 400 500 600 

mH 

[GeV] 


SM 

σ 

/ 

σ 



10 

1 

-1 

10 


Obs. 

Exp. 

-1 

Ldt = 4.6-4.9 fb 

± 1 σ 

± 2 σ 

s = 7 TeV 

CLs Limits 

110 115 120 125 130 135 140 145 150 

mH 

[GeV] 


and 129 < mH < 539 GeV 



∫


Testing µ = 1 Hypothesis 

CLs 

1 

-1 

10 

-2 

10 

-3 

10 

-4 

10 

-5 

10 

-6 

10 


Obs. 

Exp. 

100 200 300 400 500 600 

mH 

[GeV] 


∫ 

-1 

Ldt = 4.6-4.9 fb 

s = 7 TeV 

95% 

99% 

CLs 

1 

-1 

10 

-2 

10 

-3 

10 

-4 

10 

-5 

10 



∫ 

Obs. 

Exp. 

-1 

Ldt = 4.6-4.9 fb 

s = 7 TeV 

110 115 120 125 130 135 140 145 150 

mH 

[GeV] 


and 129 < mH < 539 GeV 



95% 

99%


Breakdown of the Observed Excess 

0 

Local p 

1 

-1 

10 

-2 

10 

-3 

10 

-4 

10 

-5 

10 

-6 

10 

-7 

10 


Obs. 

Exp. 

100 200 300 400 500 600 

mH 

[GeV] 

Excess @ 126 GeV 

∫ 

-1 

Ldt = 4.6-4.9 fb 

s = 7 TeV 

1σ 

2σ 

3σ 

4σ 

5σ 

0 

Local p 

1 

-1 

10 

-2 

10 

-3 

10 

-4 

10 

-5 

10 

-6 

10 

-7 

10 

Observed local Significance: 2.6σ (2.9σ Expected) 

Probability to Observe such Fluctuation in 

... 110 < mH < 600 GeV: 30% 

... 110 < mH < 146 GeV: 10% 

∫ 


Obs. 

Exp. 

-1 

Ldt = 4.6-4.9 fb 

s = 7 TeV 

110 115 120 125 130 135 140 145 150 

mH 

[GeV] 


1σ 

2σ 

3σ 

4σ 

5σ


Excess in Individual Channels 

0 

Local p 

1 

-1 

10 

-2 

10 

-3 

10 

-4 

10 

-5 

10 

-6 

10 

-7 

10 


Obs. γγ+4l 

Exp. γγ+4l 

Obs. γγ+4l+lνlν 

Exp. γγ+4l+lνlν 

Obs. 

Exp. 

110 115 120 125 130 135 140 145 150 

mH 

[GeV] 

Excess mainly in H → γγ and H → ZZ (∗) → 4ℓ 

Local Significance: 3.4σ 

-1 

Ldt = 4.6-4.9 fb 

s = 7 TeV 

NO Excess in H → WW (∗) → ℓνℓν, H → ττ, and H → b ¯ b 

∫ 


1σ 

2σ 

3σ 

4σ 

5σ


Excess in Individual Channels 

0 

Local p 

1 

-1 

10 

-2 

10 

-3 

10 

-4 

10 

-5 

10 

-6 

10 

-7 

10 


Obs. γγ+4l 

Exp. γγ+4l 

Obs. γγ+4l+lνlν 

Exp. γγ+4l+lνlν 

Obs. 

Exp. 

110 115 120 125 130 135 140 145 150 

mH 

[GeV] 

Excess mainly in H → γγ and H → ZZ (∗) → 4ℓ 

Local Significance: 3.4σ 

-1 

Ldt = 4.6-4.9 fb 

s = 7 TeV 

NO Excess in H → WW (∗) → ℓνℓν, H → ττ, and H → b ¯ b 

All Channels Combined: 3.4σ 

∫ 


1σ 

2σ 

3σ 

4σ 

5σ














BSM Higgs Searches 

Fermiophobic Higgs (ATLAS-CONF-2012-013) 

Decreased Coupling to Some or All Fermions 

in Higgs Doublet and Triplet Models 

Production via VBF or VH 

Decays to γγ, WW, ZZ, Zγ 

Analysis Identical to that in the SM 

9 Analysis Categories 

(η ⊗ Conversion Status ⊗p γγ(thrust) 

T ) 




Decreased Coupling to Some or All Fermions 

in Higgs Doublet and Triplet Models 

Production via VBF or VH 

Decays to γγ, WW, ZZ, Zγ 

Analysis Identical to that in the SM 

9 Analysis Categories 

(η ⊗ Conversion Status ⊗p γγ(thrust) 

T ) 




Expected Exclusion: 110 − 123.5 GeV 

Observed Exclusion: 110 − 118 GeV and 119.5 − 121 GeV 

Largest Excess @ mH = 125.5 GeV 

Statistical Significance 1.6σ (Including Look-Elsewhere Effect) 



MSSM Phenomenology 

Two Higgs Doublets 

Five Physical Higgs Bosons: 

h/H (CP=+1), A (CP=-1), H ± 

Tree-Level: Two Free Parameters 

(mA, tan β) 

h Standard-Model like 

At least Two Higgs Bosons Mass-Degenerate 

Production Cross Section Function Scales as 

tan 2 β 



MSSM Phenomenology 

Two Higgs Doublets 

Five Physical Higgs Bosons: 

h/H (CP=+1), A (CP=-1), H ± 

Tree-Level: Two Free Parameters 

(mA, tan β) 

h Standard-Model like 

At least Two Higgs Bosons Mass-Degenerate 

Production Cross Section Function Scales as 

tan 2 β 

Production and Decay of MSSM Higgs Bosons 

Neutral Higgs Bosons (h/H/A) 

Gluon-Fusion or b-Quark Associate Production 

g 

g 

φ 

¯ b 

b 

Cross Section O(fb − pb) 

Branching Fraction to τ-Leptons BR = O(10%) 

(b ¯ b (BR ∼ 90%) Channel Extremely Challenging) 

φ 

g 

b 

b 

φ 


g 

g 

¯ b 

b 

φ 

¯q 

q 

g 

¯ b 

b 

φ


MSSM Neutral Higgs (ATLAS-CONF-2011-132) 

Analysis of All Final States (eµ, e/µ τhad, τhadτhad) 

Dominant Backgrounds Estimated from Control Regions 

W/Z+Jets Using Embedding Technique 

QCD from Same-Sign Control Region 

Varying Mass Reconstruction Techniques (mvis, meff, MMC) 



MSSM Neutral Higgs (ATLAS-CONF-2011-132) 

σ× BR 

Assume only one Resonance 

100% gg → H or 100% b ¯ bH 

Useful to Test Arbitrary Models 

mA vs. tan β 

Have to Assume Specific (c)MSSM 

Model 

Here: m max 

h 

Scenario 



Charged Higgs (ATLAS-CONF-2012-011) 

Predicted in Higgs Doublet or Triplet Models 

m H ± < mtop: Produced Predominantly in Top Decays 

tan β > 3: Decays to τν Preferred 

Channels Analyzed 

W → q¯q ′ , H ± → τhadν 

W → q¯q ′ /ℓν, H ± → τlepν 

Lepton+Jets Tau+Lepton Tau+Jets 

cos Θ ∗ ℓ = 4pb · pℓ 

m2 top − m2 −1 (m 

W 

H T )2 „q 

= m2 top + (pℓ T + pb Tpmiss T )2 − pb « 2 “ 

T − p ℓ T 

” 2 

+ pmiss T 



Charged Higgs (ATLAS-CONF-2012-011) 



Doubly-Charged Higgs (arXiv:1201.1091) 

Relevant in Higgs Triplet, Little Higgs, and Left-Right Symmetric Models 

Select Pairs of Same-Charge Muons with pT.20 GeV 

Look for Resonance in Di-Muon Spectrum 

No Significant Excess above Expected Background 


Summary 

Excellent ATLAS Performance from Detector to Physics 

(116 Papers Submitted or Published) 

Scalar/Charged Boson Searches in many Inclusive/Exclusive Channels, Using full 

2011 Data Set 

Allowed Scalar Boson Mass in the Standard Model 

117.5 − 118.5 GeV or 122.5 − 129 GeV 

In SM Low-Mass Region No Exclusion Possible Due to an Excess of Observed 

Events Compared to Background-Only Expectation 

Observed Excess of Events at mH ∼ 126 GeV Compatible with Hypothesis of a 

Standard Model Higgs Boson 

(Statistical Significance not yet Large enough) 

Data to be Collected in 2012 Might Give the Final Answer to SM Higgs Boson

Higgs Boson Searches in ATLAS - Institut für Kern- und Teilchenphysik

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