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Search for SM Higgs with the ATLAS detector. 方亚泉 威士康辛大学麦迪逊分校 欧洲核子研究中心 University of Wisconsin, Madison CERN [email protected] 前沿物理工作月 北京-上海-武汉. August 29th, 2012. Outline. LHC and ATLAS detector. Standard Model, Higgs Mechanism and its cross-section and branching ratio.

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Search for SM Higgswith the ATLAS detector




University of Wisconsin, Madison


[email protected]



August 29th, 2012


  • LHC and ATLAS detector.

  • Standard Model, Higgs Mechanism and its cross-section and branching ratio.

  • Show the most updated results of 2011+2012

    • H→γγ

    • H→ZZ→4l

    • H→WW

  • Combination of 2011+2012

  • Conclusion

Lhc large hadron collider
LHC (Large Hadron Collider)

4 TeV

Super proton synchrotron : 450 GeV

  • 100 meters underground, ring with radius 4.3 kilometers

  • Four experiment :ATLAS,CMS, ALICE,LHCb

  • CM energy : 2012,8 TeV, 2011, 7 TeV

proton synchrotron : 26 GeV

4.3 kilometers

Higgs, “God Particle”

4 TeV

4 TeV

in 2012

Atlas detector
ATLAS detector

  • Long: 44 meters,12.5 meters in radius, ~7000 tons.

  • (One Eiffel tower ,~100 jet 747).

  • components built within 35 countries :

  • Muon Spectrometer, Hadronic Calorimeter,

  • Electromagnetic (EM) Calorimeter ,Inner Detector,

The ATLAS Collaboration

3000 scientists

including 1000 graduate students

38 countries

174 universities and research labs


Standard model
Standard Model

  • Standard Model explains what and how

  • the matter is built at the subatomic level :

  • Subatomic particle :

    • 6 quarks : u, d, c, s, t, b

    • 3 leptons e, μ, t and 3 neutrino

  • Three fundamental forces to describe the

  • interactions between particles :

    • Electromagnetic (EM) force

    • Weak

    • Strong

  • Three sets of mediators to mediate the forces :

    • g (EM).

    • W/Z, Higgs (Weak).

    • gluons (Strong).

  • NOT covered:

    • Dark matter (energy).

    • Neutrino Oscillations and its non-zero mass.

    • Gravitons not included in the frame (GUT).

It cannot tell who I am and where

I am going to…..

Motivation for higgs mechanism
Motivation for Higgs Mechanism

  • Gauge Symmetry

    Lagrangian is invariant under local phase transformation

    • QED : local gauge invariance → massless photon field Aμ

    • QCD: local gauge invariance → 8 massless vector gluon fields

    • Weak interaction: massive W/Z instead of massless (1983).

      • A solution: spontaneous breaking of a local gauge symmetry (introduce mass without breaking gauge invariance) (1960s).

      • Or ignore the experiment factor that massive W/Z mediators have been discovered.



Higgs mechanism



Higgs Mechanism

Peter Higgs in 2008 at CERN

correct and promising ?

(Where μ2<0,λ2>0)

We substitute Φ and Aμwith :

So a vector gauge boson Am and massive scalar h (higgs particle) are produced

Similarly, for SU(2), three massive gauge fields (W± ,Z) and one massive scalar H are produced

Higgs particle : The last particle in SM that hasn’t been shown experimentally.

Sm higgs production and decay for lhc
SM Higgs Production and decay for LHC

Associated (small cross-sections)

Vector Boson Fusion (VBF): Second largest

ggFusion : dominant

  • Most sensitive channels are : <130 GeV: γγ; 125-300 GeV: ZZ*→4l;

  • 300-600 GeV: ZZ→llvv, 125-180 GeV: WW*→lvlv.

  • H→tt, H→ are significantly affected by QCD backgrounds. Try associated/VBF mode


Previous limits from lep and tevatron
Previous limits from LEP and TEVATRON

  • Before LHC’s 2011 results, some Higgs mass regions have been excluded by

  • TEVATRON (July, 2010) and LEP.

  • LEP : excludes <114.4 GeV.

  • TEVATRON : excludes 158-175 GeV.

Data taken with atlas detector in 2011 2012
Data taken with ATLAS detector in 2011-2012

high lum.

low lum.

  • In 2011-2012, LHC operated successfully with high luminosity.

  • peak lumi : 3.65X1033 /cm2/s (2011), 6.8X1033/cm2/s (2012).

  • present buch space 50 ns, 30 collision/bunch crossing

  • Precise understanding pile-up effect is crucial for the analyses.

    • Especially for analyses related with ETmiss and jets.

Tag jet

Forward jets

Tag jet



Higgs Decay

Analysis strategy : multi-jet analysis

Slicing phase space in regions with different S/B seems more optimal when inclusive analysis has little S/B



Inclusive (H+0jet)

Tag jet

Not tagged

Tag jet

Not Tagged

Not tagged

Analyses in TDR were mostly inclusive

Applied to H,,WW(*)

H channel
H→ dataγγ channel

Phys. Rev. Lett., 2012,108,11803

Phys. Lett. B, 2011, 705, 452-470







PRL cover

Signal and backgrounds for h
Signal and backgrounds for H→ dataγγ

  • Signal :

    • Higgs decays to diphoton via top/W triangle

    • Small branching ratio as page 9 shows.

    • Expect ~400 events (120 GeV) with 10 fb-1 before any selection.

  • Backgrounds :

    • Irreducible

      • Born, Box

    • Reducible :

      • Photon-jet/di-jet with one/two jets faking as a photon/photons.

  • Advantage : side-band to fit the signal (the most important channel)







+ ·······


+ ·······



+ ·······


Requirements for h channel
Requirements for H data→ channel

  • Need good energy and angular resolution

    to achieve ~1-2% resolution in the Higgs

    mass reconstruction.

    • σ/mH ~ 1.4%

  • Need good particle identification :

    ~85% for real photon and reject the large

    QCD background (p0 et al.) with

    rejection above 1000. (9 EM shower

    shape variables+ isolation are applied to

    separate reducible backgrounds.


Purity: ~70%

Energy calibration and vertex correction
Energy Calibration and vertex correction data

  • Energy Calibration:

    • MC-based calibration

      (experience from beam-test)

    • After that, energy scale correction

      obtained from electrons using Z→ee

      events from data.

  • Vertex reconstruction :

    • Unconverted photon :

      1st+2nd layer EM calorimeter

    • Converted photon :

      1st layer EM calorimeter

      + track from converted e+/e-

    • Robust against pileup (not use primary vertex) .

Analysis strategy and selections
Analysis strategy and selections data


  • Selection : Two photons passing trigger, identification,

    isolation with pTγ1,γ,2 >40, 30 GeV.

  • Strategy :

    • Based on different ratio of S/B and

      resolution, divide events into 9 categories:

      unconverted – converted

      pseudorapidity (central, transition, rest)

      and pTt lower/higher than 40 GeV.

      where pTt is nothing but the transverse

      component of pTgg w.r.t. thrust axis :

      which provides a better resolution than pTgg .

Signal modeling
Signal modeling data

  • Signal MC are available at 11 mass points :

    • 100-150 GeV with a 5 GeV step.

  • The shape is described by :

    • Crystal-ball (CB) + Gaussian

      • For 120 GeV, resolution of CB is from 1.4 to 2.3

        for different categories with inclusive 1.7.

    • For those mass points not available, derived from parameterization.

  • Signal events passed the inclusive selection :

    ~80 events with mH = 110-125 GeV for 7 TeV/4.8 fb-1 ~110 events for 8 TeV/5.9 fb-1.

Background modeling
Background modeling data

  • Background shape is determined by a fit

    with single-exponential like in the mass

    range from 100 to 160 GeV.

    • The mis-modeling is treated as systematic

      uncertainty on number of signal events

      (“spurious” signal).

    • Simultaneous fit on all categories with the same mass.


9 Categories

Excess around 126 gev
Excess around 126 GeV data

  • p0 : If there is no Higgs, one could make a wrong claim (there is Higgs) with a probability p0 .

Observed excess at mH = 126.5 GeV

significance w/o look-else-where effect (LEE) : 4.5 σ

The difference between black (separate VBF) and red curves shows that VBF is

crucial for July 4th discovery.

The observation of H→γγ disfavors spin 1 particle (Landau Yang theorem)

Comparison with cms results
Comparison with CMS results data

CMS shows similar excess : 4.1 σ w/o LEE.

The mass is 125 GeV.

H zz channel
H→ dataZZ channel

Phys. Lett. B 710(2012) 383-402

Phys. Lett. B 707(2012) 27-45

Phys. Rev. Lett. 107(2011) 221802










4-μ events

H zz 4l
H→ZZ*→4l data

  • “Golden Channel” :

    • low cross section : expect 20-50 signal events

      with 10 fb-1, clean (only leptons (e or m) in final state).

    • narrow peak.

      • but constrained by natural H width for mH>>200 GeV.

  • Simple and loosen selections:

    • 4 leptons: pT1,2,3,4 > 20,20,7,7 GeV; m12 = mZ ± 15 GeV; m34 > 15-60 GeV for 7 TeV.

  • Backgrounds:

    • ZZ(*) (irreducible)

    • Z+jet (in particular bb), tt (Prompt lepton requirements : isolation and impact parameter).

  • Challenge of the analysis :

    • Good reconstruction and identification of low pt lepton.

    • Reducible backgrounds have to be estimated from data.

    • Low statistics with current luminosity ~ 5-10 fb-1.

Signal and the estimation of different backgrounds
Signal and the estimation of different backgrounds data

  • The resolution of mH for signal is fairly good.

    • The case of 8 TeV is slightly worse than that of 7 TeV.

  • tt contribution:

    • use em channel as a control region.

  • Zjet, ZZ/WZ estimation

    • ZZ,WZ from MC

    • Normalization of Zjet: No isolation, impact parameter, charge requirements on the second lepton pair.

The distribution of m 4l and p 0 for background only hypothesis
The distribution of M data4l and p0 (for background only hypothesis)

In 120<mH<130 GeV, Expected Background : 5.1±0.6, Expected signal 5.3±0.8.

Observed events : 13 .

Observed significance around 125 GeV is 3.4σ (expected 2.6σ).

H ww lvlv channel
H→ dataWW→lvlv channel

Event signature : 2 leptons + missing ET (can’t reconstruct the Higgs mass: challenging)


Instead, use

Phys. Rev. Lett. 108, 11802(2012)

Phys. Rev. Lett. 107, 231801(2011)








The estimation of backgrounds from data
the estimation of backgrounds from data data

  • The fake rate of wjet estimated by fakeable obj.

    • using di-jet events (with loose ID) .

  • ttbar survival probability (for 0-jet) with quasi data-driven method by tagging one b-jet:

  • Z+jets : “ABCD” sideband.

  • WW : MC based but:

    (in a high mll control region)

The distribution of M dataT (8 TeV) and p0

Higgs Combination using results from 2011 and 2012 data data

Submitted to Physics Letters B

On July 4 datath, 5 sigma was achieved.

With the addition of WW a ~ 6 sigma effect is reached

Signal Strength and mass data

  • The combination provides the signal strength with 1.4±0.3 SM Higgs prediction.

  • The mass of the discovered particle is 126.0±0.4(stat)±0.4(sys.) GeV.

  • The confidence intervals in the (μ,mH) plane indicates the consistency from different

  • channels.

Conclusion and the impact on the future in hep
conclusion and the impact on the future in HEP data

  • With dedicated work of ~10K scientists for ~20 years, we eventually found Higgs-like particle with more than 5 sigma.

    • H→γγ (4.5σ), H→ZZ→4l (3.6σ), H→WW→lvlv(2.8σ).

    • It is the victory of the Standard Model or it may open a door to the world of new particles.

  • CERN has extended the running of machine towards the end of the year.

    • We are working hard to measure the properties of the new particles such as spin, coupling, etc.

    • Hopefully, we can draw an conclusion whether it is the Standard Model of Higgs soon.

  • The accomplishment of LHC in searching for Higgs definitely

    encourages building new colliders : International Linear Collider (ILC), Compact Linear Collider (CLIC).

    • The former is very possibly hosted by Japan. If so, China will for sure invest much more than 1% . The question is : are we ready for that ?

We found higgs
We found Higgs data

Right after the seminar on July 4th, Sau Lan Wu walked towards Peter Higgs and said :

We have worked for many years looking for you.

You found me.

Masses of the gauge bosons through symmetry breaking
Masses of the gauge bosons through symmetry breaking data

No mass prediction for Higgs .

It tends to smaller than a few hundred GeV from a meanful perturbation expansion.

Physics Analysis data

Performance of the Reconstruction

Event Generation & Simulation

Event Reconstruction & Calibration


Construction & Commissioning

Trigger &Data Acquisition


2011 Data data

2012 Data

2011+2012 Data

Systematics data



Exclusion limit w r t sm prediction
Exclusion limit w.r.t SM prediction data

95% CL→ If there is Higgs, there is 5% chance

one will make a claim of exclusion by mistake.

Observed exclusion (mH): 112-122.5 GeV, 132-143 GeV

Exclusion limit w r t standard model prediction
Exclusion limit w.r.t Standard Model prediction data

Main systematic uncertainties

Higgs cross-section : ~ 15%

Electron efficiency : ~ 2-8%

ZZ* background : ~ 15%

Zbb, +jets backgrounds : ~ 40%

Observed exclusions :

135-156, 181-234, 255-415 GeV

Expected exclusions :

136-158, 182-400 GeV

H zz ll v
H→ZZ→ll dataνv

two regions of selections

  • H→ZZ→llvv is more sensitive at

  • high mass region (both Z on shell).

  • zjets significantly suppressed at high mass region.

  • High pile-up and low pile-up analysis are separated.

Most sensitive for high mass

Observed exclusions :

320-560 GeV

Expected exclusions :

260-490 GeV

Z mass window, EmissT and ΔΦll selections

H zz llqq
H→ZZ→llqq data

Observed exclusions :

300-310 , 360-400 GeV

Expected exclusions :

360-400 GeV

  • Highest rate among ZZ decaying with leptons.

    • on-shell is focused here (ZZ : 200-600 GeV).

  • Backgrounds :

    • Z+jets (largest), top estimated from sideband

    • ZZ,WZ (MC)

  • Selection :

    • Two leptons with 83<mll<99 GeV

    • Two jets with 70<mjj<105 GeV

    • ETmiss<50 GeV

    • More selection for mH>300 GeV

    • Divide into two categories : b-tagged and untagged

Higgs decay to Z data0Z0

Reducible 4l backgrounds

Irreducible Z0Z0 backgrounds



Higgs decay to W data+W-

Two leptons + neutrinos

No mass peak

Event counting experiment

W+W- backgrounds

Flow chart for back extraction
Flow chart for Back. Extraction data

Complete propagation of systematic errors