search for the sm higgs boson in the 4 leptons channel with the atlas...
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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.
Nicolas Morange (IRFU) H→ 4ℓ search with ATLAS 17/04/2012 3 / 24
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
Nicolas Morange (IRFU) H→ 4ℓ search with ATLAS 17/04/2012 4 / 24
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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Nicolas Morange (IRFU) H→ 4ℓ search with ATLAS 17/04/2012 5 / 24
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
MCee→Z
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RMS: 0.058%
ATLAS Preliminary
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Nicolas Morange (IRFU) H→ 4ℓ search with ATLAS 17/04/2012 6 / 24
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 Tight++Data Tight++
Nicolas Morange (IRFU) H→ 4ℓ search with ATLAS 17/04/2012 7 / 24
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
Nicolas Morange (IRFU) H→ 4ℓ search with ATLAS 17/04/2012 8 / 24
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
Nicolas Morange (IRFU) H→ 4ℓ search with ATLAS 17/04/2012 9 / 24
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%.
Nicolas Morange (IRFU) H→ 4ℓ search with ATLAS 17/04/2012 10 / 24
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 %)
Nicolas Morange (IRFU) H→ 4ℓ search with ATLAS 17/04/2012 11 / 24
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.
[GeV]µµµµm
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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
Nicolas Morange (IRFU) H→ 4ℓ search with ATLAS 17/04/2012 12 / 24
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
ℓ̄1
ℓ1ℓ̄2
ℓ2
q
q̄
ℓ̄1
ℓ1
ℓ̄2
ℓ2q
g
gℓ̄1
ℓ1ℓ̄2
ℓ2
Nicolas Morange (IRFU) H→ 4ℓ search with ATLAS 17/04/2012 13 / 24
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
Nicolas Morange (IRFU) H→ 4ℓ search with ATLAS 17/04/2012 14 / 24
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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µµ + e+
/eµ+µ→Z
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µµ + e+
/eµ+µ→Z
Nicolas Morange (IRFU) H→ 4ℓ search with ATLAS 17/04/2012 15 / 24
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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+ eee+
/eµ+µ→Z
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1Ldt = 4.8 fb∫ = 7 TeVs
+ eee+
/eµ+µ→Z
Nicolas Morange (IRFU) H→ 4ℓ search with ATLAS 17/04/2012 16 / 24
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 %
Nicolas Morange (IRFU) H→ 4ℓ search with ATLAS 17/04/2012 17 / 24
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
Nicolas Morange (IRFU) H→ 4ℓ search with ATLAS 17/04/2012 18 / 24
Distributions
After the full selection :Comparisons of data with SM backgrounds, separated into reducible and irreduciblecontributions.
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4l→(*)
ZZ→H
DATA(*)
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Syst.Unc.
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Invariant mass of first pair [GeV]34m
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4l→(*)
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Syst.Unc.
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Invariant mass of second pair
Good agreement between data and SM expectations
Nicolas Morange (IRFU) H→ 4ℓ search with ATLAS 17/04/2012 19 / 24
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
Nicolas Morange (IRFU) H→ 4ℓ search with ATLAS 17/04/2012 20 / 24
Event display
2e2μ candidate. m4ℓ = 124.3 GeV.Masses of pairs : 76.8 GeV and 45.7 GeV
Nicolas Morange (IRFU) H→ 4ℓ search with ATLAS 17/04/2012 21 / 24
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
[GeV]Hm110 120 130 140 150 160 170 180
SM
σ/σ
95%
CL lim
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sCLObserved
sCLExpected
σ 1 ±
σ 2 ±
ATLAS
4l→(*)
ZZ→H1Ldt = 4.8 fb∫
=7 TeVs
sCLObserved
sCLExpected
σ 1 ±
σ 2 ±
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SM
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95%
CL lim
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sCLObserved
sCLExpected
σ 1 ±
σ 2 ±
ATLAS
4l→(*)
ZZ→H1Ldt = 4.8 fb∫
=7 TeVs
sCLObserved
sCLExpected
σ 1 ±
σ 2 ±
Nicolas Morange (IRFU) H→ 4ℓ search with ATLAS 17/04/2012 22 / 24
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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ZZ→H1Ldt = 4.8 fb∫
=7 TeVs
σ2
σ3
110
Observed p0 value and expected one inpresence of a signal
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Sig
nal str
ength
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Best fit
) < 1µ(λ2 ln = 7 TeVs
1 Ldt = 4.74.9 fb∫
ATLAS Preliminary llll→ZZ*→H
Observed signal strength
Nicolas Morange (IRFU) H→ 4ℓ search with ATLAS 17/04/2012 23 / 24
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
Nicolas Morange (IRFU) H→ 4ℓ search with ATLAS 17/04/2012 24 / 24
Lepton kinematics
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η spectrum of leptons of candidates
Nicolas Morange (IRFU) H→ 4ℓ search with ATLAS 17/04/2012 2 / 4
Invariant mass for different channels
[GeV]4lm100 150 200 250
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µ4→(*)
ZZ→H
DATABackground
=125 GeV)H
Signal (m=150 GeV)
HSignal (m
=190 GeV)H
Signal (mSyst.Unc.
ATLAS
Invariant mass of 4μ candidates [GeV]4lm
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µ2e2→(*)
ZZ→H
DATABackground
=125 GeV)H
Signal (m=150 GeV)
HSignal (m
=190 GeV)H
Signal (mSyst.Unc.
ATLAS
Invariant mass of 2e2μ candidates
[GeV]4lm100 150 200 250
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4e→(*)
ZZ→H
DATABackground
=125 GeV)H
Signal (m=150 GeV)
HSignal (m
=190 GeV)H
Signal (mSyst.Unc.
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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qqH (NLO)→qq
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Nicolas Morange (IRFU) H→ 4ℓ search with ATLAS 17/04/2012 4 / 4