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Search for SM Higgs with the ATLAS detector

August 29th, 2012

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, MadisonCERN

yaquan.fang@cern.ch

前沿物理工作月北京 -上海 -武汉

OutlineLHC and ATLAS detector. Standard Model, Higgs Mechanism and its

cross-section and branching ratio.Show the most updated results of

2011+2012H→γγH→ZZ→4lH→WW

Combination of 2011+2012Conclusion

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LHC (Large Hadron Collider)

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proton synchrotron : 26 GeV

Super proton synchrotron : 450 GeV

4 TeV

100 meters underground, ring with radius 4.3 kilometersFour experiment ： ATLAS ， CMS, ALICE ， LHCb CM energy : 2012 ， 8 TeV, 2011, 7 TeV

4 TeV 4 TeVin 2012

4.3

kilo

met

ers

Higgs, “God Particle”

ATLAS detector

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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 Collaboration3000 scientists including 1000 graduate students38 countries 174 universities and research labs

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Standard Model

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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, 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 : • (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 whereI am going to…..

Motivation for Higgs Mechanism

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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.

aG

U(1)

SU(3)

Higgs Mechanism

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FFieAieAL4

1)()()( 2*2*2*

We substitute Φ and Aμ with :

evAA

exhv vxi

1

))((2

1 /)(

...4

1

2

1

4

1

2

1)(

2

1 2222243222222' vvFFhAvehAehvhAvehvhL

V(

So a vector gauge boson A 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

(Where μ2<0,λ2>0)

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

correct and promising ?

Peter Higgs in 2008 at CERN

SM Higgs Production and decay for LHC

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Vector Boson Fusion (VBF): Second largest

Associated (small cross-sections)ggFusion : dominant

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Most sensitive channels are : <130 GeV: γγ; 125-300 GeV: ZZ*→4l; 300-600 GeV: ZZ→llvv, 125-180 GeV: WW*→lvlv. H→, H→ are significantly affected by QCD backgrounds. Try associated/VBF modebb

Previous limits from LEP and TEVATRON

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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

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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.

low lum.high lum.

Inclusive (H+0jet)

Analyses in TDR were mostly inclusive

H+2jet

Tag jet

Tag jet

Applied to H,,WW(*)

H+1jet

Tag jet

Tag jet

Not Tagged

Forward jets

Higgs Decay

Not tagged

Not tagged

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

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Analysis strategy : multi-jet analysis

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Higgs Analysis with individual channels from

2011 and 2012 data

H→γγ channel

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PRL coverPhys. Rev. Lett., 2012,108,11803Phys. Lett. B, 2011, 705, 452-470ATLAS-CONF-2011-161ATLAS-CONF-2011-085ATLAS-CONF-2011-071ATLAS-CONF-2011-025ATLAS-CONF-2011-004ATLAS-CONF-2012-079

Signal and backgrounds for H→γγ

Signal : Higgs decays to diphoton via top/W triangleSmall 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)

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diphoton+ ·······

Born Box

Fragmentation

+

Photon-jets+ ·······Di-jet+ ·······

q g

Requirements for H→ channelNeed 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 (et al.) with

rejection above 1000. (9 EM shower shape variables+ isolation are applied to separate reducible backgrounds.(γ-jet,jet-jet).

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Purity: ~70%

Energy Calibration and vertex correction

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) .

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Analysis strategy and selections

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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 pT w.r.t. thrust axis :

which provides a better resolution than pT . 21 TT pp

+ VBF

Signal modelingSignal MC are available at 11 mass points :

100-150 GeV with a 5 GeV step.The shape is described by :

Crystal-ball (CB) + GaussianFor 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.

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Background modeling 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.

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inclusive

Excess around 126 GeV

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Observed excess at mH = 126.5 GeVsignificance w/o look-else-where effect (LEE) : 4.5 σ

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

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

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CMS shows similar excess : 4.1 σ w/o LEE. The mass is 125 GeV.

H→ZZ channel

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4-μ events

Phys. Lett. B 710(2012) 383-402Phys. Lett. B 707(2012) 27-45Phys. Rev. Lett. 107(2011) 221802ATLAS-CONF-2012-092ATLAS-CONF-2012-017ATLAS-CONF-2012-016ATLAS-CONF-2011-162ATLAS-CONF-2011-150ATLAS-CONF-2011-148ATLAS-CONF-2011-131ATLAS-CONF-2011-048ATLAS-CONF-2011-026

H→ZZ*→4l

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“Golden Channel” : low cross section : expect 20-50 signal events with 10 fb-1, clean (only leptons (e or ) in final state). narrow peak.

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

Simple and loosen selections: 4 leptons: pT

1,2,3,4 > 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

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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 e channel as a control region.

Zjet, ZZ/WZ estimationZZ,WZ from MCNormalization of Zjet: No isolation, impact parameter,

charge requirements on the second lepton pair.

The distribution of M4l and p0 (for background only hypothesis)

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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).

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H→WW→lvlv channel

Phys. Rev. Lett. 108, 11802(2012)Phys. Rev. Lett. 107, 231801(2011)ATLAS-CONF-2012-098ATLAS-CONF-2012-060ATLAS-CONF-2012-018ATLAS-CONF-2011-134ATLAS-CONF-2011-111ATLAS-CONF-2011-005ATLAS-CONF-2010-092

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

where Instead, use

the estimation of backgrounds from 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)

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The distribution of MT (8 TeV) and p0

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Higgs Combination using results from 2011 and 2012 data

Submitted to Physics Letters B

What Goes into the Combination

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On July 4th, 5 sigma was achieved. With the addition of WW a ~ 6 sigma effect is reached

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Signal Strength and mass

•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

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 ?

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We found Higgs

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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.

Thank you

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backup slides

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Masses of the gauge bosons through symmetry breaking

No mass prediction for Higgs .It tends to smaller than a few hundred GeV from a meanful perturbation expansion.

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Shower shape variables for photon and jet

Physics Analysis

LHC

DetectorsConstruction & Commissioning

Trigger &Data Acquisition

Event Reconstruction & Calibration

Event Generation

& Simulation

Performance of the Reconstruction

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2011 Data 2012 Data

2011+2012 Data

Systematics

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20%

14%

Strength for different categories

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Exclusion limit w.r.t SM prediction

04/19/2344Observed exclusion (mH): 112-122.5 GeV, 132-143 GeV

95% CL→ If there is Higgs, there is 5% chance one will make a claim of exclusion by mistake.

Exclusion limit w.r.t Standard Model prediction

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Main systematic uncertaintiesHiggs cross-section : ~ 15%Electron efficiency : ~ 2-8%ZZ* background : ~ 15%Zbb, +jets backgrounds : ~ 40%

Observed exclusions : 135-156, 181-234, 255-415 GeVExpected exclusions : 136-158, 182-400 GeV

H→ZZ→llνv

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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 massObserved exclusions : 320-560 GeVExpected exclusions : 260-490 GeV

two regions of selections

Z mass window, EmissT and ΔΦll selections

H→ZZ→llqqHighest rate among ZZ decaying with leptons.

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

Z+jets (largest), top estimated from sidebandZZ,WZ (MC)

Selection : Two leptons with 83<mll<99 GeVTwo jets with 70<mjj<105 GeVET

miss<50 GeVMore selection for mH>300 GeVDivide into two categories : b-tagged and

untagged

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Observed exclusions : 300-310 , 360-400 GeVExpected exclusions : 360-400 GeV

Irreducible Z0Z0 backgrounds

Higgs decay to Z0Z0

Z

Z

Reducible 4l backgrounds

W+W- backgrounds

Higgs decay to W+W-

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Two leptons + neutrinosNo mass peakEvent counting experiment

Flow chart for Back. Extraction

50 Complete propagation of systematic errors

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