search for the sm higgs boson in the 4 leptons channel with the atlas...

Search for the SM Higgs boson in the 4leptons channel with the ATLAS detector

Nicolas Morange,on behalf of the ATLAS Collaboration

MCTP Spring 2012 Higgs Symposium

17 April 2012

Higgs Boson Production in the 4ℓ channelHiggs production :

gg fusion dominant

VBF comes next (∼ 10%), relativecontribution increases with mH.

4 leptons channel :

Large BR for ZZ above threshold

Small fraction once BR(ZZ→ 4ℓ),ℓ= e,μ included

Cross-sections ∼ 1−10 fb :� σ(H→ γγ) at low mass

Nicolas Morange (IRFU) H→ 4ℓ search with ATLAS 17/04/2012 2 / 24

H→ 4ℓ : characteristics

4 leptons (e,μ) in the final state :

⇒ Full reconstruction of the Higgs,with excellent experimentalresolution (∼ 2 GeV at low mass)

⇒ Clean channel, low backgroundrates

Backgrounds :

Irreducible SM ZZ continuum

With leptons from Heavy-Flavourdecays : Zbb̄, tt̄

With fake leptons : Z+light jets,multijets

Cross-section in 1−10 fb over whole search range :Important channel from 115 to 600 GeV.

Low mass : complementary to γγ

High mass : Higgs width sizeable, experimental resolution less critical

Experimental challenges

High lepton reconstruction efficiency and acceptance

Good resolution on leptons

Good measurement of lepton properties (cut reducible backgrounds) : isolations,impact parameters, electron shower shapes

Search driven by lepton performance.


The ATLAS detectorInner Detector : (|η|< 2,5, B=2T)Si Pixels, Si Strips, Transition Radiation trackerprovides precise tracking, vertexing, e/π separationσ/pT ∼ 3,810−4(GeV)⊕0,015

EM Calorimeter : (|η|< 3,2)Pb-LAr accordion structureprovides e/γ trigger,identification and measurementσ/E∼ 10%/

pE⊕0,7%

Hadronic Calorimeter :Scint/Fe tiles in the central (|η|< 3,2)W/Cu-LAr in the fwd region (|η|< 4,9)provides trigger,

jet measurement and EmissT

σ/E∼ 50%/pE⊕3%

Muon Spectrometer : (|η|< 2,7)air core toroids with gas chambersprovides μ trigger and standalone momentum measurementresolution < 10% up to ET ∼ 1 TeV


Data-taking in 2011

Very efficient data-taking

5.25 fb−1 recorded by ATLAS in2011, with peak luminosity up to3.651033 cm−2s−1

Very high efficiency of allsubdetectors

⇒ 4.9 fb−1 analysed in the 4e channel

⇒ 4.8 fb−1 analysed in the 2e2μ and4μ channels

Pile-up conditions increasing withinstantaneous luminosity :<μ>∼ 6→<μ>∼ 12⇒ Simulations corrected to matchobserved distribution

Day in 2011

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> = 6.3µ * = 1.5 m, <β


Electron performance

Reconstruction :

Good quality track matched to anEM cluster

Track refitted, takingBremsstrahlung possibility intoaccount : improve track parametersin bending plane

Energy scale and resolution :

Measured using Z decays

Simulations corrected to matchresolution in data

Energy scale flat with pile-up

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DataFit result

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RMS: 0.032%

RMS: 0.058%

ATLAS Preliminary

1 = 4.9 fbtdL∫=7 TeV, sData 2011,


Electron Identification

Identification :Set of cuts to reduce fakes

track quality

track-cluster matching

cluster shower shapes, transverseand longitudinal

Performance :

Over 90 % efficiency

Efficiencies measured in data withW, Z and J/ψ

Simulations corrected to matchdata

Small efficiency dependance withpile-up, well reproduced bysimulations

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MC Loose++Data Loose++

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MC Tight++Data Tight++


Muon performance

Reconstruction :Two types of muons used

Good quality track in ID, combinedwith spectrometer track

Good quality track in ID, tagged inspectrometer

Rejection of cosmic rays throughimpact parameter cuts (1 mm)

Efficiencies :

Very high (∼ 98%, measured with Zdecays)

Well reproduced by simulations

Simulations corrected to match data

Resolution :

Measured with Z decays

Data/MC difference due tonon-nominal alignment of MS and ID

⇒ Simulations corrected to matchdata

Small degradation with pile-up


Selection of a candidate

Trigger :

Single-lepton : muon pT > 18 GeV, electron ET > 20−22 GeV

Di-lepton : 2 muons pT > 10 GeV, 2 electrons ET > 12 GeV

Lepton selection :

pT > 7 GeV

|η|< 2.47 (electrons), |η|< 2.7 (muons)

Reconstruction of a candidate :

2 same-flavor, opposite-sign pairs

pT > 20 GeV for 2 leptons

Angular separation ∆R> 0.1 between all leptons

Pair with mass closest to mZ : |m12−mZ |< 15 GeV

Second pair : mmin <m34 < 115 GeV


Additional cutsAim : suppress reducible backgrounds

Lepton isolations. Target : Z+jets, multijets

Relative track and calorimeter isolations in cones of size ∆R< 0.2

Corrected for pile-up

Corrected for the other leptons of the candidate

track isolation : (∑

pT)/pℓT< 0.15

calorimetric isolation : (∑

ET)/EℓT< 0.30

Transverse impact parameters. Target : Zbb̄, tt̄

Used when m4ℓ < 190 GeV

Applied on 2 lowest pT leptons

|d0/σ(d0)|< 3.5 (muons), 6 (electrons)

Efficiencies.

Checked with Z→ ℓℓ decays (isolated leptons), Z+ ℓ and heavy-flavor enriched di-jetsample (non-isolated leptons)

Excellent data/MC agreement : ratio compatible with 1

Dependence on pile-up also well represented

Small systematic uncertainties ;one exception : isolation of low-pT electrons. Uncertainty 5%.


Signal : SimulationGenerators :

Powheg+Pythia for gg and VBF

Pythia for WH and ZH

Cross-sectionsFollow LHC Higgs σ working grouprecommendations :

Use state-of-the-art computations :NNLO+NNLL (QCD) and NLO (EW) forgg, NNLO (QCD) and NLO (EW) for VBF

Decays by Prophecy4f (NLO)

Reweight pT spectrum of gg productionto prediction by HqT

Theory uncertainty : envelope of PDFand αs (8 %), QCD scales (10 %)


Signal : Results of event selection

Reconstruction and selectionefficiencies :

Mass [GeV] 4μ 2e2μ 4e130 27 % 18 % 14 %360 60 % 52 % 45 %

Resolutions at 130GeV :2 GeV (4μ), 2.2 GeV (2e2μ), 2.5 GeV (4e)

4e : lower mean value and tails, due toenergy losses from Bremsstrahlung

Natural width of Higgs dominant from350 GeV on : FWHM∼ 35 GeV atmH = 400 GeV.

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µ4→(*)

ZZ→H

0.03 GeV± = 1.98 σ

: 15%σ 2±fraction outside

= 130 GeVHm

Gaussian fit

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4e→(*)

ZZ→H

0.06 GeV± = 2.53 σ

: 18%σ 2±fraction outside

= 130 GeVHm

Gaussian fit


ZZ∗ background

SM ZZ∗ production : irreduciblebackground.

Generated with Pythia (onlydouble-resonant diagrams)

mZZ spectrum reweighted to MCFMprediction (single-resonantdiagrams, leading gg)

Uncertainties from QCD scales(5 %) and PDF+αs variations (4 %)

Additional normalisation (10 %) +shape uncertainty from varying ggcontribution

q

q̄

Z1

Z2

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ℓ1ℓ̄2

ℓ2

q

q̄

ℓ̄1

ℓ1

ℓ̄2

ℓ2q

g

gℓ̄1

ℓ1ℓ̄2

ℓ2


Reducible backgrounds

Reducible backgrounds contributions derived using data-driven techniques.

tt̄ :

Generated with MC@NLO

Cross-section at NNLO (Hathor)

Checked in control region with e±μ∓ leading pairs

Good data-MC agreement

Sizeable contribution in Z+μμ final states

Z+jets :

Generated with Alpgen+Herwig

Two types of samples :

Z+light jets (incl. c), normalised to inclusive FEWZ NNLO cross-sectionZbb̄, normalised to MCFM NLO cross-sectionOverlap removed between samples

Control regions : Z+ ℓℓ, no isolation nor impact parameter cuts

Dominant background depends on flavor of second pair


Z+μ+μ−

Z+μμ dominated by HF decays

Z+light jets contribution in controlregion estimated using probabilitiesthat tracks fake muons, checked onZ+μ

Then Z+bb̄(→ μμ) estimated bysubtraction : good data/MCagreement

Finally propagation to signal region

Additional MuonsN

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Distributions in control region :

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/eµ+µ→Z

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/eµ+µ→Z


Z+e+e−

Z+ee dominated by light jets

Normalize Z+ee events in MCusing control region with reversedshower shape cut

Compute isolation and impactparameter efficiencies in Z+esamples : good data/MC agreement

Extrapolate to signal region of extra clusteretaR

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Distributions in control region :

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1Ldt = 4.8 fb∫ = 7 TeVs

+ eee+

/eµ+µ→Z


Systematic UncertaintiesNormalizations :

Effects of PDF, αs and QCD scales computed correlated between signal andbackgrounds

Additional systematics for high mass signal (off-shell effects) : 150%×m3H[TeV] for

mH > 300 GeV

Luminosity : 3.9 %

Backgrounds :

tt̄ : 10 % cross-section, 10 % selection efficiencies

Zbb̄ : 40 %

Z+light jets : 45 %origin : statistics in control region, uncertainty on composition of control region,uncertainty on extrapolation

Reconstruction and selection efficiencies :

Muon efficiency : 0.22 % (4μ), 0.16 % (2e2μ), uniform with m4ℓ.

Electron efficiency : 2.3 % (4e), 1.6 % (2e2μ) at 600 GeV. 8.0 % (4.1 %) at 110 GeV

Lepton resolution : negligible

Electron energy scale : uncertainty 0.6 % (4e) and 0.3 % (2e2μ) on mass scale

Isolations : negligible, except low-pT electrons (5 %)

Impact parameters : negligible

Modelling of Higgs pT spectrum : 2 %


Results

71 candidate events observed in data, for 62±9 estimated background events

Breakdown per channel, in low-mass (m4ℓ < 180 GeV) and high-mass (m4ℓ > 180 GeV)regions :

Good agreement between data and SM background prediction

Reducible backgrounds negligible in the high mass region

Significant contribution of reducible backgrounds in the low mass region in 4e and2e2μ channels


Distributions

After the full selection :Comparisons of data with SM backgrounds, separated into reducible and irreduciblecontributions.

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Invariant mass of first pair [GeV]34m

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Syst.Unc.

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Invariant mass of second pair

Good agreement between data and SM expectations


Invariant mass

Invariant mass of the candidates, lowmass

Invariant mass of the candidates, fullmass range

Good agreement between data and SM expectations3 excesses of events in the mass range :

around 125 GeV, with 3 events

around 245 GeV

around 500 GeV, with 3–4 events


Event display

2e2μ candidate. m4ℓ = 124.3 GeV.Masses of pairs : 76.8 GeV and 45.7 GeV


Limit-setting

Limits set with CLs formalism.

Test statistic : profile likelihood ratio, evaluated with binned maximum-likelihood fitsof the signal and background distributions to the observed data.

Observed 95 % CL limit curve matches well expected one

Exclusion of SM Higgs in 134–156 GeV, 182–233 GeV, 256–265 GeV and 268–415 GeV

Exclusion expected in 136–157 GeV and 184–400 GeV

Largest deviations from expected limit observed around 125 GeV, 244 GeV and500 GeV

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σ 1 ±

σ 2 ±

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4l→(*)

ZZ→H1Ldt = 4.8 fb∫

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sCLObserved

sCLExpected

σ 1 ±

σ 2 ±

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σ 2 ±

ATLAS

4l→(*)


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sCLObserved

sCLExpected

σ 1 ±

σ 2 ±


Discussion of results

Quantification of excesses : compute p0 values

Use pseudo-experiments, check with asymptotic formulae

1.6 % (2.1σ) at 125 GeV

1.3 % (2.2σ) at 244 GeV

1.8 % (2.1σ) at 500 GeV

Expected in presence of a SM-like signal : 1.3, 3.0 and 1.5σ respectively

Global p0 around 50 % when look-elsewhere effect included

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Sig

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Best fit

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ATLAS Preliminary llll→ZZ*→H

Observed signal strength


Conclusions

Search for the SM Higgs boson inthe 4 lepton channel with 4.8 fb−1 :

Thanks to a very successful run ofthe LHC and efficient data-taking,4.8 fb−1 of data analyzed !

Study of backgrounds shows goodagreement with expectations

71 4 leptons candidates in data

SM Higgs excluded by this channelin 134–156 GeV, 182–233 GeV, and∼ 256–415 GeV

Small excesses at 125, 244 and500 GeV, of probabilities around 2.1– 2.2σ

None of them significant oncelook-elsewhere effect included

Nevertheless intriguing whencompared to other ATLAS results(γγ)

Look forward to 2012 data !

2e2μ candidate, m4ℓ = 123.6 GeV

For more details, analysis published inPhys. Lett. B 710 (2012) 3


http://www.sciencedirect.com/science/article/pii/S0370269312002560

Backup

Backup


Lepton kinematics

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Invariant mass for different channels

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ATLAS

Invariant mass of 4e candidatesNicolas Morange (IRFU) H→ 4ℓ search with ATLAS 17/04/2012 3 / 4

Projected sensitivity for 8TeV

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ATLAS Preliminary (Simulation)


search for the sm higgs boson in the 4 leptons channel with the atlas...

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