Status of Standard Model Higgs Searches ATLASpartsem/fy12/2011-12-20-higgs.pdf · 12/20/2011 ·...

Status of Standard Model Higgs Searches ATLAS

Scott SnyderFor the ATLAS Collaboration

Brookhaven National Laboratory, Upton, NY, USA

Dec 20, 2011BNL

IntroductionSM Higgs search channels

I WW → `ν`νI γγI ZZ → 4`

Combination

Conclusions/outlook

Scott Snyder (BNL) Status of Standard Model Higgs Searches ATLAS BNL Dec 20, 2011 1 / 46

LHC

LHC performance has greatlyexceeded expectations!

2011 goal set in spring

Goal for 2011: 1 fb−1

Accomplished: ∼ 5 fb−1

Integrated luminosity:

July (EPS):

1.2 fb−1

Aug. (LP):

2.5 fb−1

Delivered: 5.6 fb−1

Recorded: 5.3 fb−1

90%–96% of recorded data usablefor analysis (depending on analysis).

Peak lumi at ATLAS: 3.6× 1033 cm−2 s−1


ATLAS detector


Pileup

Large luminosity means having many interactions in the same crossing (“pileup”)

Up to ∼ 20 — worse than expected forthis luminosity.

∼ 2(20) events/crossing forL = 1033 (1034).

50 ns bunch spacing, otherparameters pushed beyond design.

–Aug Sep–Oct

Z → µµ

Event with 20 vertices.(Ellipses enlarged 20× forvisibility.)

Challenges for reconstruction, trigger, and computing requirements.Precise modeling of pileup is essential.


Trigger

Single-lepton triggers used where possible, augmented with others (eg,multi-lepton triggers) where needed.

Triggers adjusted over the course of 2011 to adjust to the increasing luminosity.

Triggers for Higgs analyses mostly stable, with a few increases of trigger threshold.

!"#"$%$&'"()

*+,-./0#)1230.0#)

4,.)1.(%)1$"))"#5&*6

!%0#)1'7238)"-)

!"#!$&6/"55,/&92,/(."0#&:;<<

Typical recorded rates: e/γ: 100 Hz; Muons: 150 Hz; jets/τ/E/T : 100 Hz


SM Higgs production and decay

HqqH

WHZHttH

pp → X + H

γγ: < 130 WW → `ν`ν: 125–180ZZ → 4`: 125–300 ZZ → ``νν: 300–600

Much progress has been made in theoretical predictions (LHC xsect WG etc)

Calculations to NNLO in most cases; uncertainties <∼ 20%


Previous ATLAS/CMS combinationCombination as of Nov., using data through Aug. 11 (∼ 2.3 fb−1 each)

LEP 95% CL:mH > 114.4 GeV

Tevatron 95% exclusion:100 < mH < 109 GeV;156 < mH < 177 GeV

Excluded at 95% CL: 141–476 GeVExcluded at 99% CL: 146-443 GeV (except a few small regions within 200-300 GeV).Expected exclusion: 124–520 GeV. Max deviation from BG-only @ 144 GeV (3σ).


Summary of ATLAS SM Higgs searches

Substantial progress over the past year in understanding the detector andin improving reconstruction and simulation.

Channel mH range Int lum Major Signal events S/B after Expected

(GeV) fb−1 bkgs after cuts cuts σ/σSMsensitivity

H → γγ 110–150 4.9 γγ, γj , jj 70 0.02 1.6–2H → ττ → `` + ν 110–140 1.1 Z → ττ , top 0.8 0.02 30–60H → ττ → `τhad 100–150 1.1 Z → ττ 10 5E-3 10-25W/ZH → bb`(`) 110–130 1.1 W/Z + jets, top 6 5E-3 15–25

H → WW (∗) → `ν`ν 110–300 2.1 WW , top, Z + jet 20 (@130) 0.3 0.3–8

H → ZZ (∗) → 4` 110–600 4.8 ZZ∗, top, Zbb 2.5 (@130) 1.5 0.7–10

H → ZZ (∗) → ``νν 200–600 2.1 ZZ , top, Z + jets 20 (@400) 0.3 0.8–4H → ZZ → ``qq 200–600 2.1 Z + jets, top 2–20 (@400) 0.05–0.5 2–6H → WW → `νqq 240–600 1.1 W + jets, top, jets 45 (@400) 1E-3 5–10

γγ and ZZ → 4` channels updated with full luminosity.WW → `ν`ν also updated.(Three channels with the best sensitivity in the allowed low-mass region.)


H → WW (∗) → `ν`ν∫L dt = 2.1 fb−1

Covers 110 < mH < 300 GeV.

Relatively large σ ∼ 200 fbI Most sensitive channel over

125–180 GeV.

But 2ν: no mass reconstruction.

Initial selection: 2opposite-charge leptonsinconsistent with Z/J/ψ decay,large E/T .

0- and 1-jet bins; b-jet veto.

Additional selections to reduceWW : pT (``), m(``), ∆φ(``).

Final selection:0.75mH < mT (``,E/T ) < mH .

Backgrounds: WW , top, W /Z + jetsCompare data to background MC insignal-free control regions; derive scalefactors by which MC should be corrected.

Control MC Observedregion expectation in data

WW + 0 jets 296± 36 296WW + 1 jet 171± 21 184top 1-jet 270± 69 249

jetsN

0 2 4 6 8 10

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nts

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10001200140016001800200022002400

Data stat)⊕ SM (sys

Diboson Top

Z+jets H [150 GeV]

W+jets (data driven)

ATLAS-1 L dt = 2.05 fb∫ = 7 TeV, s

νlνl→WW→H

Njets after

`, m(``), E/T cuts

Need good understanding of E/T and background.


H → WW (∗) → `ν`ν

E/T spectrum in data for µµ well-described by background model.

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W+jets

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νµνµ→WW→H

For E/T > 50 GeV, mostly realν’s from top.

For E/T < 50 GeV, a mixture offakes and real ν’s from Z + jets.

After all cuts (for mH = 130 GeV)

Observed in data 94 events10 ee, 42 eµ, 42 µµ

Expected BG 76± 11Expected signal 19± 4

Transverse mass spectrum before mT cut:

mT =q

(E ``T + E/T )2 − (P``T + PmissT )2


H → WW (∗) → `ν`ν

Exclusion limits:

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ObservedExpected

σ 1 ±σ 2 ±

-1 Ldt = 2.05 fb∫ = 7 TeVs

νlνl→(*)WW→HATLAS

Consistency with background-onlyhypothesis

[GeV]Hm100 150 200 250 300

0p

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ObservedExpected

σ2

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νlνl→(*)WW→HATLAS

Excluded at 95% CL: 145 < mH < 206 GeV (expected: 134–200 GeV)Maximum deviation is 1.9σ at mH ∼ 130 GeV.


H → γγ∫L dt = 4.9 fb−1


γγ Higgs decay relatively rareI σ × BR ∼ 40 fb

Simple final state: 2 high-pT

isolated photons.I Require

ET (γ1, γ2) > (40, 25) GeV

Events divided into 9 categoriesbased on:

I Photon η (central, crack,endcap).

I Did photon convert?I pT (γγ) perpendicular to γγ

thrust axis.

Main background: continuum γγproduction.

Other backgrounds: γj , jj .

In 4.9 fb−1, expect

70 signal events formH = 125 GeV.

∼ 3000 background events insignal window.

S/B ∼ 0.02.

Analysis requires excellent γγ mass resolution and π0 rejection.


m(γγ): opening angle [m2γγ = 2E1E2(1− cosα)]

Fine EM calorimeter segmentationallows direct θ(γ) measurement.

Crucial at high pileup with manyvertices over the ∼ 5.6 cm LHCbeam spot:Which one produced the γγ pair?

2. Find vertex zX

Calorimeter “pointing” reducesvertex uncertainty: ∼ 5.6→ 1.5 cm.

With pointing (and conversion vertexlocation), angular term contributesnegligible uncertainty to m(γγ).

Robust against pileup!

z(γ1)− z(γ2)

σ(z) ∼ 1.5 cm


m(γγ): resolution

Calorimeter energy calibration:

From Z , J/ψ, W data and MC

Scale @mZ known to 0.5%.

Linearity better than 1%.

Uniformity: ∼ 1% (barrel),∼ 1.7% (endcap).

e calibration converted to γ viaMC (small systematics fromupstream material).

mee

Width of m(γγ):

Overall: 1.7 GeV.

Best: 1.4 GeV (unconvertedcentral).

Worst: 2.3 GeV (≥ 1γconverted, ≥ 1γ in crack).

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=120 GeVH

, mγγ→ H→gg

Fit

6≤ µ 9≤ µ6 <

12≤ µ9 <

> 12µ


π0 rejectionImportant backgrounds are jj and γjwith j → π0.

Need for π0 rejection motivated fineη segmentation of first EM compart-ment.

(ET ∼ 20–30 GeV)

η-strips

jj : ∼ 500 µb

γj : ∼ 200 nb

γγ: ∼ 30 pb

H → γγ: ∼ 40 fb

Large uncertainties on σ(γj , jj): need to measure with data.


H → γγ background composition

After all cuts: 22489 events in 100 < mγγ < 160 GeV.

Estimate composition using control samples

γγ jγ jj DYγγ jγ jj DY

Num

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of E

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Data-driven estimations

expectedγγj expectedγ

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ATLAS Preliminary

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γγ jγ jj DYγγ jγ jj DY

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20000 Nb. of events Fractionγγ 16000± 1120 (71± 5)%γj 5230± 890 (23± 4)%jj 1130± 600 (5± 3)%DY/Z 165± 8 (0.7± 0.1)%

Purity ∼ 70%γj + jj � γγ

Photon identification efficiency: 85± 5%(From MC, cross-checked with data.)


H → γγ: results

Background in each category fitwith an exponential.

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Data 2011Background model

= 120 GeV (MC)H

SM Higgs boson m

Inclusive diphoton sample

-1 Ldt = 4.9 fb∫ = 7 TeV, s

ATLAS Preliminary

Dominant systematics:

Signal yield: ∼ 20%

mγγ resolution: ∼ 14%

H → γγ pT : ∼ 8%


H → γγ: exclusion limits

[GeV]Hm

110 115 120 125 130 135 140 145 150

S

Mσ/σ

95%

CL

limit

on

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8 limitsObserved CL limit

sExpected CL

σ 1±σ 2±

ATLAS Preliminaryγγ →H

= 7 TeVsData 2011,

-1Ldt = 4.9 fb∫

Max. deviation from background:mH = 126 GeV

Local p0: 0.27% (2.8σ)

SM Higgs expectation: ∼ 1.4σ

Include “look-elsewhere” effect:p0 (110–150 GeV): 7% (1.5σ)

Excluded at 95% CL:

114 ≤ mH ≤ 115 GeV;135 ≤ mH ≤ 136 GeV

Consistency of data withbackground-only expectation:

[GeV]Hm

110 115 120 125 130 135 140 145 150

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0 expected pγγ →SM H

= 7 TeVsData 2011,

-1Ldt = 4.9 fb∫

ATLAS Preliminary

σ1

σ2

σ3


H → ZZ (∗) → 4` (4e, 4µ, 2e2µ)∫L dt = 4.8 fb−1

Covers 110 < mH < 600

Small σ ∼ 2–5 fb. But:I Fully reconstructed mass.I Small background, S/B ∼ 1.

2 opposite-sign, same-flavorlepton pairs.

pT1,2,3,4 > 20, 20, 7, 7 GeV.

m12 within mZ ± 15 GeV

m34 > 15–60 GeV (dep on mH)

Backgrounds:

ZZ (∗) (irreducible)

At lower masses: Zbb, Z + jets,tt̄.

Reducible backgroundssuppressed with isolation andimpact parameter cuts.

Analysis requirements

High lepton reconstruction/identification efficiencies across pT spectrum.Good lepton energy/momentum resolution.Good control of reducible backgrounds for low mH .

(Need to compare against background-enriched control regions.)


H → ZZ (∗) → 4` (4e, 4µ, 2e2µ): Electron performanceElectron efficiency measured withJ/ψ → ee, W → eν, Z → ee.

Crucial to use data in the low-pT

region, where material effects aremost significant.

Variation of electron efficiency withpileup well modeled by simulation:Z → ee data/MC

Systematics:

6% (pT ∼ 7 GeV)

< 2% (pT ∼ 50 GeV)

σm(H → 4e): 2.5 GeV

[GeV]eeeem

80 90 100 110 120 130 140 150

a.u.

/ 0.

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0.08 ATLAS Simulation Preliminary

4e→ZZ→H

0.06) GeV± = (2.53 σ

= 130 GeVHmgaussian fit

: 0.18σ 2±fraction outside

No Z mass constraint


H → ZZ (∗) → 4` (4e, 4µ, 2e2µ): Muon performance

µ reconstruction efficiency > 95%over 4 < p < 100 GeV.

Recently improved µ alignment:

MC (perfect): 2.31± 0.01 GeVData Spring 2011: 2.89± 0.01 GeVData Summer 2011: 2.45± 0.01 GeV

µ isolation eff., Z → µµ

σm(H → 4µ): 2 GeV

[GeV]µµµµm

80 90 100 110 120 130 140 150

a.u. /

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eV

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0.1 ATLAS Simulation Preliminary

µ4→ZZ→H

0.03) GeV± = (1.98 σ

= 130 GeVHmgaussian fit

: 0.15σ 2±fraction outside

No Z mass constraint


H → ZZ (∗) → 4` (4e, 4µ, 2e2µ): After all selectionsFull mass range

Observe 71 (24 4µ, 30 2e2µ, 17 4e)Expect from background: 62± 9

[GeV]4lm200 400 600

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4l→(*)ZZ→H

DATABackground

=150 GeV)H

Signal (m=190 GeV)

HSignal (m

=360 GeV)H

Signal (m

PreliminaryATLAS

m(4`) < 180 GeV

Observe 8 (3 4µ, 3 2e2µ, 2 4e)Expect from background: 9.3± 1.5

[GeV]4lm100 150 200 250

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4l→(*)ZZ→H

DATABackground

=125 GeV)H

Signal (m=150 GeV)

HSignal (m

=190 GeV)H

Signal (m

PreliminaryATLAS

For mH < 141 GeV (not already excluded), observe 3 events:two 2e2µ (m = 123.6 GeV, m = 124.3 GeV), one 4µ (m = 124.6 GeV)

In 117 < m(4`) < 128 GeV(around mH = 125 GeV):Expect 1.5 events from both signal and BGS/B ∼ 2 (4µ), 1 (2e2µ), 0.3 (4e)Background: ZZ (∗), Z + jets (4e only)

Main systematics:Higgs cross section: ∼ 15%Electron efficiency: ∼ 2–8%ZZ (∗) background: ∼ 15%Zbb,Z + jets backgrounds: ∼ 40%


4µ candidate with m(4µ) = 124.6 GeV

pT (µ−, µ+, µ+, µ−) = 61.2, 33.1, 17.8, 11.6 GeV; m12 = 89.7 GeV, m34 = 24.6 GeV


2e2µ candidate with m(4`) = 124.3 GeV

pT (e+, e−, µ−, µ+) = 41.5, 26.5, 24.7, 18.3 GeV; mee = 76.8 GeV, mµµ = 45.7 GeV


H → ZZ (∗) → 4` backgroundsCheck reducible backgrounds with background-enriched control samples:

2 opposite-sign, same-flavor leptons near mZ ;

2 additional same-flavor leptons passing all selections except isolation and impact parameter

[GeV]34m20 40 60 80 100

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bZbtt

WZSyst.Unc.

PreliminaryATLAS

-1Ldt = 4.8 fb∫ = 7 TeVs

4l→(*)ZZ→H

Z + µµ

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bZbtt

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PreliminaryATLAS

-1Ldt = 4.8 fb∫ = 7 TeVs

4l→(*)ZZ→H

Z + ee

Low-mass regions dominated by Zbb (muons) and Z + jets (electrons).MC reproduces data well (within uncertainties)Z + e/Z +µ samples used to compare selection efficiencies between data and MC.Good agreement → MC used to estimate background in signal region.


H → ZZ (∗) → 4` exclusion limitsFit signal and background expectations to mass spectrum.

[GeV]Hm110 120 130 140 150 160 170 180

SM

σ/σ95

% C

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it on

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sObserved CL

sExpected CL

σ 1 ±σ 2 ±

PreliminaryATLAS 4l→(*)

ZZ→H-1Ldt = 4.8 fb∫

=7 TeVs

sObserved CL

sExpected CL

σ 1 ±σ 2 ±

[GeV]Hm200 250 300 350 400 450 500 550 600

SM

σ/σ95

% C

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it on

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sObserved CL

sExpected CL

σ 1 ±σ 2 ±

PreliminaryATLAS 4l→(*)

ZZ→H-1Ldt = 4.8 fb∫

=7 TeVs

sObserved CL

sExpected CL

σ 1 ±σ 2 ±

Excluded (95% CL): 135 < mH < 156 GeV and 181 < mH < 415 GeV(except 234–255 GeV)

Expected exclusion: 137 < mH < 158 GeV and 185 < mH < 400 GeV


H → ZZ (∗) → 4`: consistency with BG-only hypothesis

[GeV]Hm100 200 300 400 500 600

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σ2 σ3

PreliminaryATLAS

4l→(*) ZZ→H

-1Ldt = 4.8 fb∫=7 TeVs

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PreliminaryATLAS

4l→(*) ZZ→H

-1Ldt = 4.8 fb∫=7 TeVs

Maximum deviations from background-only expectations

mH (GeV) Local p0 LEE corrected Local signif Expected from SM Higgs125 1.8% ∼ 50% 2.1σ 1.4σ244 1.1% ∼ 50% 2.3σ 3.2σ500 1.4% ∼ 50% 2.2σ 1.5σ

Look-elsewhere effect (LEE) estimated over 110 < mH < 600 GeVScott Snyder (BNL) Status of Standard Model Higgs Searches ATLAS BNL Dec 20, 2011 27 / 46

Combined exclusion limits

Channels: γγ; ττ ; WW (∗) → `ν(`ν, qq); ZZ (∗) → ``(``, νν, qq)Not included: W /ZH → `bb + X

[GeV]HM100 200 300 400 500 600

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σ1 ±σ2 ± = 7 TeVs

-1 Ldt = 1.0-4.9 fb∫ATLAS Preliminary 2011 Data

CLs Limits

[GeV]HM110 115 120 125 130 135 140 145 150

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σ/σ95

% C

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σ1 ±σ2 ± = 7 TeVs


CLs Limits

LEP LHC 11/11

Excluded at 95% CL

112.7 < mH < 115.5 GeV; 131 < mH < 453 GeV, except 237–251 GeV

Expected exclusion if no signal: 124.6–520 GeVExcluded at 99% CL: 133 < mH < 230 GeV; 260 < mH < 437 GeV


Consistency with background-only expectations

[GeV]HM100 200 300 400 500 600

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σ2

σ3

σ4

σ5 = 7 TeVs

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ATLAS Preliminary 2011 Data

Biggest inconsistency with expectedbackground at mH ∼ 126 GeV

SM Higgs: Expect 2.4σ (1.4σ per channel).

[GeV]HM

110 115 120 125 130 135 140 145 150

Loca

l P-V

alue

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Exp. Comb.

Obs. Comb.

4l→Exp. H

4l→Obs. H

γγ →Exp. H

γγ →Obs. H

νlν l→Exp. H

νlν l→Obs. H

ATLAS Preliminary

σ2

σ3

σ4

-1 L dt ~ 2.05-4.9 fb∫

2011 Data

Local significance of excess

1.9× 10−4 (3.6σ)γγ: 2.8σ; 4`: 2.1σ; `ν`ν: 1.4σ

Corrected for look-elsewhere effect

110–146 GeV: 2.5σ (0.6%); 110–600 GeV: 2.2σ (1.4%)


Consistency with SM Higgs

[GeV]HM110 115 120 125 130 135 140 145 150

Sign

al s

treng

th

-3

-2

-1

0

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3Best fit

σ1 ± = 7 TeVs

-1 Ldt = 4.9 fb∫ATLAS Preliminary γγ→H

2011 Dataγγ

[GeV]HM110 115 120 125 130 135 140 145 150

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Best fitσ1 ±

= 7 TeVs

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2011 Data4`

[GeV]HM110 115 120 125 130 135 140 145 150

Sign

al s

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th

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-1

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Best fitσ1 ±

= 7 TeVs


Total

[GeV]HM110 115 120 125 130 135 140 145 150

Sign

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-2

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Best fitσ1 ±

= 7 TeVs

-1 Ldt = 2.05 fb∫ATLAS Preliminary νlνl→WW→H

2011 Data`ν`ν

Excess 2± 0.8 times larger than expected in γγ channel; within 1σ forothers and combined.


Still to come

On timescale of winter conferences:

Update all channels to fullluminosity.

Relax kinematic cuts to increaseacceptance at low mH .

Better S/B discrimination:I Multivariate techniques, more

analysis bins, etc.

Ongoing improvements toobject reconstruction anddetector modeling.

Combination with CMS underdiscussion.

2012 run:

With ∼ 20 fb−1 additionalluminosity per experiment:

5σ discovery at mH ∼ 125(ATLAS alone)

∼ 3σ per channel

Combining ATLAS+CMS:5σ down to ∼ 116 GeV

Doesn’t include:

Analysis improvements.

Possible increase in energy to√s = 8 TeV. (10% gain)


Conclusions

The LHC and ATLAS have performed wonderfully over the past year!

We have searched for a SM Higgs boson at the LHC using up to 4.9 fb−1

of integrated luminosity in many distinct channels over 110–600 GeV;

Small excess observed around mH ∼ 126 GeV

Local significance 3.6σ from the H → γγ (2.8σ), H → ZZ (∗) → 4`(2.1σ), H →WW (∗) → `ν`ν (1.4σ) channels.

SM Higgs expectation: 2.4σ (local). Observed excess compatiblewithin 1σ.

Significance corrected for look-elsewhere effect: ∼ 1% (2.3σ)

More data are needed to draw any definite conclusions.We’re well-positioned for the year to come!

More info: https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2011-163 (+references)


Backup


H → γγ by category 1

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[GeV]γγm

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TtCP6: Converted central, high p

-1 Ldt = 4.9 fb∫ = 7 TeV, s

ATLAS Preliminary

[GeV]γγm

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TtCP8: Converted rest, high p

-1 Ldt = 4.9 fb∫ = 7 TeV, s

ATLAS Preliminary


H → γγ: Background model

!Scott Snyder (BNL) Status of Standard Model Higgs Searches ATLAS BNL Dec 20, 2011 36 / 46

H → γγ: Previous results

June 2011: PLHC July 2011: EPS


H → γγ

thrust axis

pT

ggp

Tt

pTl

pT

g1pT

g2


H → γγ: Events per category


m(γγ): opening angle [m2γγ = 2E1E2(1− cosα)]

Fine EM calorimeter segmentationallows direct θ(γ) measurement.

Crucial at high pileup with manyvertices over the ∼ 5.6 cm LHCbeam spot:Which one produced the γγ pair?

2. Find vertex zX

Calorimeter “pointing” reducesvertex uncertainty: ∼ 5.6→ 1.5 cm.

Robust against pileup!

Without pointing, mass resolutionwould be ∼ 20% worse with > 10pileup events.

[GeV]γγm110 112 114 116 118 120 122 124 126 128 130

/ 0.

5 G

eVγγ

1/N

dN

/dm

0

0.02

0.04

0.06

0.08

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0.12ATLAS Preliminary

(Simulation)*=1 mβ

=120 GeVH

, mγγ→ H→gg

Fit

truth vertex

2T

pΣ

Calo/Conv pointing


H → ZZ (∗) → 4` kinematics

[GeV]12m70 80 90 100 110

Eve

nts/

5 G

eV

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-1Ldt = 4.8 fb∫ = 7 TeVs

4l→(*)ZZ→H

DATA

(*)ZZ

t,tbZ,Zb

PreliminaryATLAS

[GeV]T

p20 40 60 80 100 120 140

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DATA

ZZ

t,tbZ,Zb

ATLAS Preliminary-1Ldt= 4.8 fb∫

= 7 TeVs

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nts/

10 G

eV

0

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-1Ldt = 4.8 fb∫ = 7 TeVs

4l→(*)ZZ→H

DATA

(*)ZZ

t,tbZ,Zb

PreliminaryATLAS

η-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5

Eve

nts/

0.4

0

10

20

30

40

50DATAZZ

t,tbZ,Zb

ATLAS Preliminary-1Ldt= 4.8 fb∫

= 7 TeVs


H → ZZ (∗) → ``νν∫L dt = 2.1 fb−1


σ ∼ 20 fb larger than 4`.I But larger background.I Still most sensitive channel for

mH > 300 GeV.

Selection:I Opposite-sign ee or µµ pair

with |m`` −MZ | < 15 GeV.I No b-tagged jets.I E/T not pointing along a jet.I Further cuts on E/T and ∆φ``

dep. on hypothesized mH .

mH < 280 GeV mH > 280 GeVE/T > 66 GeV E/T > 82 GeV1 < ∆φ`` < 2.64 ∆φ`` < 2.25

∆φE/T ,``< 1

Backgrounds:ZZ : modeled from MC.

Z + jets, top: modeled from MC, checkedagainst data control samples.

Multijet BG calculated from data.

Missing ET distribution:

0 50 100 150 200 250 300

Eve

nts

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eV

110

210

310

410

510

610

710

0 50 100 150 200 250 300

Eve

nts

/ 5 G

eV

110

210

310

410

510

610

710 -1 L dt = 2.05 fb∫Data

Total BackgroundTopZZ,WZ,WWZOther Backgrounds

= 260 GeV)H

Signal (m = 400 GeV)

HSignal (m

ATLAS Preliminary = 7 TeVs

νν ll→ ZZ →H

[GeV]missTE

0 50 100 150 200 250 300

Dat

a / M

C

0.81

1.2

[GeV]missTE

0 50 100 150 200 250 300

Dat

a / M

C

0.81

1.2

Background modeled using alpgen. Small

disagreement at low end goes away if pythia

used instead. Difference taken as a systematic.Scott Snyder (BNL) Status of Standard Model Higgs Searches ATLAS BNL Dec 20, 2011 42 / 46

H → ZZ (∗) → ``νν

∆φ between leptons from Z decay:smaller for boosted Z s.

0 0.5 1 1.5 2 2.5 3

Even

ts / 0

.4 ra

d

110

210

310

410

510

610

710

810

0 0.5 1 1.5 2 2.5 3

Even

ts / 0

.4 ra

d

110

210

310

410

510

610

710

810 Total BackgroundTopZZ,WZ,WWZOther Backgrounds

= 260 GeV)H

Signal (m = 400 GeV)

HSignal (m

-1 L dt = 2.05 fb∫Data


νν ll→ ZZ →H

[rad]llΦ∆0 0.5 1 1.5 2 2.5 3

Data

/ M

C

0.81

1.2

[rad]llΦ∆0 0.5 1 1.5 2 2.5 3

Data

/ M

C

0.81

1.2

Z → µµ candidatem(µµ) = 93.8 GeVpT (Z ) = 156 GeVE/T = 161 GeV


H → ZZ (∗) → ``νν

Transverse mass of ``E/T system:

[GeV]Tm

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100 -1 L dt = 2.05 fb∫data

Total BGTopZZ,WZ,WWZ,W

= 260 GeV)H

Signal (m


νν ll→ ZZ →H

mH < 280 GeV

[GeV]Tm

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L dt = 2.05 fb∫data

Total BGTopZZ,WZ,WWZ,W

= 380 GeV)H

Signal (m


νν ll→ ZZ →H

mH > 280 GeV

Final exclusion limit:

[GeV]Hm200 250 300 350 400 450 500 550 600

SM

σ/σ 9

5% C

L lim

it on

0

1

2

3

4

5

6

7

8 ObservedExpected

σ 1±σ 2±

PreliminaryATLAS

=7TeVs, -1Ldt=2.05fb∫ννll→ZZ→H

Excluded at 95% CL

310 < mH < 470 GeV


H → ZZ (∗) → ``qq∫L dt = 2.1 fb−1

Covers 200 < mH < 600 GeV

σ ∼ 10 fbI Large Z + jets background.

Selection:

Reconstruct Z → `` and Z → jj .

E/T < 50 GeV to reject tt̄.

For mH ≥ 300 GeV, additionalrequirements on ∆φ(``), ∆φ(jj),and pT (j).

Sample divided into two bins: twob-tags (“tagged”), and < 2 b-tags(“untagged”).

Tagged sample has ∼ 10× smallersignal but ∼ 10× larger S/B.

Tagged/untagged contribute aboutevenly to final result.

Backgrounds:ZZ : modeled from MC.

Z + jets, top: modeled from MC,normalized using data control samples.

Multijet BG calculated from data.

Dijet mass spectrum (untaggedselection):

[GeV] qq m

0 50 100 150 200 250

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nts

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100×Signal =300 GeV)

H(mOther Background

ATLAS Preliminary


H → ZZ (∗) → ``qq

m(``jj) for mH = 400 GeV

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H(mTotal BackgroundZTopDiboson

ATLAS Preliminary

Untagged

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GeV

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4 -1 L dt=2.05 fb∫data

=400 GeV)H

Signal (m

Total Background

ZTop

Diboson

ATLAS Preliminary

Tagged

(Signal in untagged channel scaled up by ×10.)

Exclusion limits:

[GeV]Hm200 250 300 350 400 450 500 550 600

SM

σ/σ 9

5% C

L lim

it on

0

2

4

6

8

10

12

14

16

18

20 ObservedExpected

σ 1±σ 2±

PreliminaryATLAS

=7TeVs, -1Ldt=2.05fb∫llqq→ZZ→H

Close to excluding SM Higgs aroundmH = 300 GeV.


Status of Standard Model Higgs Searches ATLASpartsem/fy12/2011-12-20-higgs.pdf · 12/20/2011 ·...

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