T. Kono (ATLAS) - Latest Results on Searches for Dark Matter Candidates with the ATLAS Detector at the LHC

LATEST RESULTS ON SEARCHES FOR DARK MATTER CANDIDATES WITH THE ATLAS EXPERIMENT AT THE LHC

Takanori Kono (University of Hamburg/DESY)

BW2011 Workshop, Serbia28.08 – 01.09, 2011

28.08.2011 - 01.09.2011 BW2011 Workshop, Serbia 1

Outline• Standard model and supersymmetry• LHC and ATLAS• SUSY searches in ATLAS

• 0-lepton + missing transverse energy (w/wo b-tag)• 1-lepton/2-lepton + missing transverse energy • 2 photon + missing transverse energy• Long-lived particle search• Medium-lived particles with displaced vertex• eμ resonance

• Conclusion


R-parity violatingscenarios

Standard Model of particle physics28.08.2011 - 01.09.2011 BW2011 Workshop, Serbia 3

• Standard Model (SM) is a beautiful theory which describes nature with great precision

• Quarks, leptons and force-carrying particles• Electroweak symmetry breaking (EMSB) is

essential to describe observed particle masses and interactions• Masses of W/Z bosons• Coupling between W/Z and fermions• Higgs field drives EWSB

Unanswered questions in SM• 3 generation structure• Particle mass hierarchy and mixing• Matter-antimatter asymmetry• Stability of the Higgs mass• What is dark matter? (evidence from cosmology)

Extensions of the SM• Many extensions to the SM have been proposed

• Fourth generation• New symmetry• Extra-dimensional models• …..

• Supersymmetry (SUSY)• SUSY provides an elegant solution to the hierarchy problem by

introducing a super-partner to every SM particle (fermion – boson)• Provides a natural dark matter candidate• Unification of gauge couplings

• If SUSY is accessible, new particles are expected to be produced copiously at the LHC• Production of gluinos and squarks and their decays


Supersymmetry (SUSY)28.08.2011 - 01.09.2011 BW2011 Workshop, Serbia 5

• SUSY• Introduces a superpartner to each SM particle

which differs ½ in spin• SUSY must be a broken symmetry

• Different symmetry breaking mechanism leads to different phenomenologies

• Minimal Supersymmetric SM (MSSM)• Minimal extension of the SM• 124 free parameters

• mSUGRA (5 parameters)

parameter potential Higgs SUSY ofsign :parameter potential Higgs SUSY ofsign :tan

coupling trilinear:scaleGUT at the massfermion common :

scaleGUT at the massscalar common :

0

2/1

0

sign

Amm

Most experimental results are interpreted in the context of mSUGRA

Large Hadron Collider (LHC)28.08.2011 - 01.09.2011 BW2011 Workshop, Serbia 6

2011 running• Proton-proton collision at the center-

of-mass energy of 7 TeV• 50 ns bunch spacing• Peak luminosity : 2.37×1033 cm-2s-1

• Delivered luminosity : >2.5 fb1

• Average pileup: ~8 collisions/bunch

• 2 general-purpose experiments (ATLAS, CMS)• Experiments dedicated for specific physics

(ALICE, LHCb, TOTEM, LHCf)

ATLAS experiment


Inner Detector• Pixel (pixel detector)• SCT (silicon strip detector)• TRT (transition radiation tracker)

Muon spectrometer • MDT, CSC : precise momentum measurement• RPC, TGC : trigger chambers

Calorimeter• LAr : EM calorimeter• Tile: Fe/Scintillator tile

Magnet system• 2 T solenoid• 0.5 T toroid

• Large acceptance and hermiticity• High granularity readout and

excellent resolution

Physics at the LHC28.08.2011 - 01.09.2011 BW2011 Workshop, Serbia 8

σ (proton-proton)Total inelastic pp collision Minimum bias events

QCD jet production including heavy flavors

W/Z boson productiontop pair production

Higgs, SUSY and other new physics searches

With >2 fb-1 of data we are already probing this region

Searches for SUSY at the LHC


Characteristic SUSY production and decay• In R-parity is conserved, SUSY particles are produced in pairs and the lightest SUSY particle (LSP) becomes stable• R=(-1)3(B-L)+2s

• No direct observation of SUSY particles, but only SM particles are reconstructed directly• No mass peaks

• LSP escapes the detector undetected producing a missing transverse energy (ET

miss)• Evidence of SUSY is done by

establishing an excess of events in some region of phase space• Crucial to understand the

contribution from SM processes

Overview of SUSY searches in ATLASChannel Signature Integrated lumi.0-leptons+jets+ET

miss ≥2-4 jets, large ETmiss, meff 1.04 fb-1

1-leptons+jets+ETmiss ≥3 jets, e or μ, large ET

miss, meff 1.04 fb-1

2-leptons+ETmiss Exactly 2 leptons, large ET

miss 1.04 fb-1

0-leptons+bjets+ETmiss ≥3 jets, large ET

miss, meff 0.83 fb-1

Diphoton+ETmiss γγ+ET

miss+X 1.07 fb-1

Long-lived particle Slowly moving particle 37 pb-1

eμ resonance eμ+X 0.87 fb-1

Medium-lived particle μ+displaced vertex 35 pb-1


missT

n

i

iTeff Epm

1

• Interpretation of search results in the context of• mSUGRA/CMSSM model

• M0, M1/2, A0, tanβ, sgn(μ)• Simplified model

ETmiss + jets + 0-lepton channel


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Z/γ+jets γ/leptons as jets, Z+jetsνν+jets

Top Hadronic tau decay

QCD Mis-measurement of jets or ν from heavy flavor decay

All background estimations are data-driven

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Total expected

62.3±4.3±9.2 55.0±3.8±7.3 984±39±145 33.4±2.9±6.3 13.2±1.9±2.6

Data 58 59 1118 40 18

Limit on fiducial cross section

22 (fb) 25 (fb) 429 (fb) 27 (fb) 17 (fb)

No excess was observed

L=1.04 fb-1

ETmiss + jets + 0-lepton : Interpretation


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Squark-gluino-neutralino model (m

=7 TeVs, -1 = 1.04 fbintL

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observed 95% C.L. limitsCL

median expected limitsCL

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2010 data PCL 95% C.L. limit

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observed 95% C.L. limitsCL median expected limitsCL

exp. limit 68%, 99% CLReference point2010 data PCL 95% C.L. limit

GeV 1075masses , if

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mSUGRA/CMSSM, A0=0, tanβ=10, μ>0

Pheonomenological MSSM

General simplified model• Squarks assumed to be heavy• Gluino-gluino pair production

ETmiss + bjets + 0-lepton


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L=0.83 fb-1

Signal region 3J-A 3J-B 3J-C 3J-DTotal expected 356+103

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2-lepton channel results SS, OS 28.08.2011 - 01.09.2011 BW2011 Workshop, Serbia 20

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miss>250 GeV• OS SR2 : ET

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miss>100 GeV, jet pT>100,70,70,70 GeV

Limits on A×ε×σ

Flavor subtraction, OS28.08.2011 - 01.09.2011 BW2011 Workshop, Serbia 21

1111111 22ee

eNNeeNS

electron, muon trigger efficiencyratio of electron and muon reconstruction efficiencies and acceptances

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Diphoton + ETmiss


• Generalized model of gauge-mediated SUSY breaking (GGM) with a bino-like lightest neutralino(95 % C.L.)• σ<0.02 – 0.04 pb• m(gluino)>776 GeV

• Universal Extra Dimension (UED)(95 % C.L.)• σ<0.015 – 0.027 pb• 1/R>1224 GeV on the UED

compactification radius

G~01

• In gauge-mediated SUSY breaking (GMSB) models, the LSP is the gravitino

• Signature: 2γ + ETmiss +X

L=1.07 fb-1

[GeV]χ∼m200 400 600 800 1000 1200

[GeV

]g~

m

400

500

600

700

800

900

1000

1100

1200

= 7 TeVs

-1Ldt = 1.07 fb∫

< 0.1 mmNLSPτ = 2, cβGGM: bino-like neutralino, tan

NLSPg~

LimitsATLAS Expected CL

LimitsATLAS Observed CL

σ 1 ±)-1 Limit (36 pbsATLAS Observed CL

ATLAS Preliminary

Massive long-lived particle (LLP) search28.08.2011 - 01.09.2011 BW2011 Workshop, Serbia 23

• SUSY particles produced at the LHC may have a long lifetime• The particle can travel >10 meters • This may happen when the mass difference

between the LSP is very small• Long-lived slepton

• For example, stau as the NLSP• Behaves like a heavy muon in the detector

• Long-lived squarks/gluinos• Hadronizes with normal quarks or gluons R-hadrons

Search strategy• Look for a slowly traveling particle reaching the muon spectrometer• Measure the velocity (β) m=p/(βγ)

• Timing resolution : 1-2 ns with the Tile calorimeter, 3-4 ns with the muon spectrometers

Measured β from real muons

LLP search results28.08.2011 - 01.09.2011 BW2011 Workshop, Serbia 24

Reconstructed mass distributions for • slepton search• R-hadron search

Consistent with BG expectation


R-parity violating SUSY

Heavy long-/medium-lived particles


• Heavy stable particles are predicted in a number of theories including SUSY• Models with stau as the LSP• R-hadron (long-lived gluinos or squarks after hadronization)• SUGRA with R-parity violation

R-parity violating interaction Event selection• Muon pT>45 GeV (also used for the trigger)• Displaced vertex (DV) using tracks with d0>2

mm• DV fit quality : χ2<5• Fiducial region of the pixel detector

• |zDV|<300 mm, |rDV|<180 mm• Distance from the primary vertex > 4 mm

• Ntrk,DV≥4• mDV>10 GeV• Veto vertices in high density material regions

L=33 pb-1

Results on medium-lived particle search


[mm]τc1 10 210 310

Cro

ss-s

ectio

n x

B.F

. [pb

]

-210

-110

1

10

210

0

1χ∼, 494 GeV q~700 GeV

0

1χ∼, 108 GeV q~700 GeV 0

1χ∼, 494 GeV q~1.5 TeV

0

1χ∼, 108 GeV q~150 GeV

preliminaryATLAS

-1Ldt = 33 pb∫preliminary

Number of tracks in vertex2 4 6 8 10 12 14 16 18 20

Ver

tex

mas

s [G

eV]

0

5

10

15

20

25

30

35

40

45

50

Signal Region

ATLAS


Vertex mass [GeV]0 1 2 3 4 5 6 7 8 9 10

Ver

tices

/ bi

n

-110

1

10

210

310

preliminary

data 2010QCD MCW,Z MC

ttbar MC

ATLAS

-1Ldt = 33 pb∫

Number of tracks in vertex2 4 6 8 10 12

Num

ber

of v

ertic

es

-110

1

10

210

data 2010QCD MCW,Z MC

ttbar MC

ATLAS


• No signal observed• Set limit on the cross section

• σ・acceptance・ε<0.09 pb @ 95 % C.L. for m(squark)=150 GeV and 100 % branching ratio

Distributions of mDV and Ntrk

Search for e-μ resonance


• Look for particles decaying into opposite-sign different flavor lepton• R-parity violating SUSY• Extra Z’ gauge boson with lepton

flavor violation• High mass e-μ pair: clear signal with

small SM contribution

singletquark type-nlepton/dow charged :/superfield double SU(2)rk lepton/qua :/

21 : termLagrangian

DEQL

DQLELL kjiijkkjiijk

Limits from e-μ resonance search


[GeV]τν∼m

100 200 300 400 500 600 700 800 900 1000

)[fb

]μ

BR

(e× σ

95%

CL

1

10

210

310

410

= 0.05312

λ = 0.10, 311

’λTheory

= 0.01312

λ = 0.01, 311

’λTheory

Observed Limit

Expected Limit

σ1 ±Expected Limit

σ2 ±Expected Limit

ATLAS 2010 limit

-1 Ldt = 0.87 fb∫ = 7 TeVs

ATLAS Preliminary(a)

[GeV]τν∼m

100 200 300 400 500 600 700 800 900 1000

’ 311

λ

-310

-210

-110

= 0.07312λ = 0.05312λ = 0.01312λ = 0.07(ATLAS 2010 data)312λ = 0.07(D0@Tevatron)312λ

-1 Ldt = 0.87 fb∫

ATLAS Preliminary(b)

eBRpp ~~

σ≤130 fb for mντ=100 GeV

≤11 fb for mντ=1 TeV

Limit on the cross section * BR

L=0.87 fb-1

Summary of ATLAS SUSY limits


Conclusion• LHC is operating wonderfully

• Already L>2 fb-1 collected by ATLAS• ATLAS is exploring new physics in the TeV scale

• Good understanding of the detector and reconstruction to estimate the contribution from SM processes

• Latest results use ~1 fb-1 of data• No excess was observed over SM expectation so far

• Gluino/squark masses are excluded up to ~1 TeV for certain SUSY mass hierarchies

• More data will be collected in 2011/2012. Searches in higher mass region and also in wider SUSY parameter space

• Possibly go to higher energy (14 TeV) after 2013

• All results are accessible at • https://twiki.cern.ch/twiki/bin/view/AtlasPublic• https://twiki.cern.ch/twiki/bin/view/AtlasPublic/SupersymmetryPublicRe

sults



Backup slides

Summary of ATLAS limits (exotics)28.08.2011 - 01.09.2011 BW2011 Workshop, Serbia 33

Education

T. Kono (ATLAS) - Latest Results on Searches for Dark Matter Candidates with the ATLAS Detector at the LHC