Post on 18-Dec-2014
description
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
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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
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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
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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
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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
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missT
n
i
iTeff Epm
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• 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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0
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Five signal regions are defined to maximize sensitivities for various production mechanisms
Background sourcesW+jets Leptons reconstructed as jets
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
L=1.04 fb-1
Search in 0-lepton : Results
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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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gluino mass [GeV]0 250 500 750 1000 1250 1500 1750 2000
squa
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= 10 pbSUSYσ
= 1 pbSUSYσ
= 0.1 pbSUSYσ
= 0.01 pbSUSYσ
= 0 GeV)LSP
Squark-gluino-neutralino model (m
=7 TeVs, -1 = 1.04 fbintL
0 lepton 2011 combined PreliminaryATLAS
observed 95% C.L. limitsCL
median expected limitsCL
exp. limit 68%, 99% CL
2010 data PCL 95% C.L. limit
[GeV]0m500 1000 1500 2000 2500 3000 3500
[GeV
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>0μ= 0, 0
= 10, AβMSUGRA/CMSSM: tan
=7 TeVs, -1 = 1.04 fbintL
0 lepton 2011 combined PreliminaryATLAS 0 lepton 2011 combined
1
± χ∼LEP2 -1<0, 2.1 fbμ=3, β, tanq~, g~D0
-1<0, 2 fbμ=5, β, tanq~,g~CDF Theoretically excluded
observed 95% C.L. limitsCL median expected limitsCL
exp. limit 68%, 99% CLReference point2010 data PCL 95% C.L. limit
GeV 1075masses , if
GeV 850 GeV, 800C.L. % 95at Exclude
~~
~~
qg
qg
mm
mmGeV 980masses , if
C.L. % 95at Exclude~~ qg mm
Phenomenological MSSM (squark-gluino) grids• Masses from 100 GeV to 2 TeV, m(χ0
1)=0• Limits unchanged if LSP mass raised to 200 GeV
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
• Analysis similar to jets+ channel but requires at least one b-jet• Sensitive to 3rd generation squarks in R-parity conserving scenarios• Define 4 signal regions to maximize sensitivity
GeV 60%) (100 ~~ ,~~
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ETmiss + bjets : Results
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L=0.83 fb-1
Signal region 3J-A 3J-B 3J-C 3J-DTotal expected 356+103
-92 70+24-22 79+28
-25 13+5.6-5.2
Data 361 63 76 12
σ (pb) 95 % C.L. upper limit
0.288 0.061 0.078 0.017
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PreliminaryATLAS
0 lepton, 3 jets
Reference point
)g~)>>m(1,2
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observed limitsCL expected limitsCL
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)-1ATLAS (35 pb
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Gluino masses below 720 GeV excluded for sbottom masses below 600 GeV
Gluino masses between 200-660 GeVexcluded up to LSP mass of 160 GeV
ETmiss + jets + 1-lepton
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Event selection• 1 lepton (electron or muon)
• electron pT>25 GeV or muon pT>20 GeV• Veto second lepton if
• electron pT>20 GeV or • muon pT>10 GeV
• 3 jets (>60, >25, >25 GeV)• ∆φ(jet, ET
miss)> 0.2 for all jets• ET
miss >125 GeV, ETmiss >0.25*Meff
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L=1.04 fb-1
mSUGRA• Gluino mass<875 GeV excluded
Simplified model• LSP masses below 200 GeV are
excluded for gluino mass below 600 GeV (for x>1/2)
Constraints on Bilinear R-parity violating model
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σ1± S
Expected CL
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Expected CL
Preliminary
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• Several leptons may be produced in the cascade decay of SUSY particles
• 2-lepton channel is sensitive to the pair production of weak gauginos
02
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Signal regions (SR)• SS SR1 : ET
miss>100 GeV• SS SR2 : ET
miss>80 GeV, jet pT>50,50 GeV• OS SR1 : ET
miss>250 GeV• OS SR2 : ET
miss>220 GeV, jet pT>80,40,40 GeV• OS SR3 : ET
miss>100 GeV, jet pT>100,70,70,70 GeV
Limits on A×ε×σ
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1111111 22ee
eNNeeNS
electron, muon trigger efficiencyratio of electron and muon reconstruction efficiencies and acceptances
Observed value of SSobs=-3.1
With pseudo-experimentsSs<4.9 (95 % CL)
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2
>300
[GeV]missTE
0 50 100 150 200 250 300 350 400 450 500
Eve
nts
/ 5 G
eV
-210
-110
1
10
210
310
410
-1 Ldt = 1.07 fb∫ATLAS Preliminary
= 7 TeV)sData 2011 (QCD
+Xνe→t, tγ+νe→+jets, Wνe→Wγγ+νl→, Wγγ+νν→Z
= 800 / 400 GeVχ∼ / mg~
GGM m = 140 TeVΛSPS8
UED 1/R = 1200 GeV
= 7 TeV)sData 2011 (QCD
+Xνe→t, tγ+νe→+jets, Wνe→Wγγ+νl→, Wγγ+νν→Z
= 800 / 400 GeVχ∼ / mg~
GGM m = 140 TeVΛSPS8
UED 1/R = 1200 GeV
[GeV]missTE
0 50 100 150 200 250 300 350 400 450 500
Eve
nts
/ 5 G
eV
-210
-110
1
10
210
310
410
Diphoton + ETmiss
28.08.2011 - 01.09.2011 BW2011 Workshop, Serbia 22
• 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
28.08.2011 - 01.09.2011 BW2011 Workshop, Serbia 25
R-parity violating SUSY
Heavy long-/medium-lived particles
28.08.2011 - 01.09.2011 BW2011 Workshop, Serbia 26
• 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
28.08.2011 - 01.09.2011 BW2011 Workshop, Serbia 27
[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
-1Ldt = 33 pb∫preliminary
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
-1Ldt = 33 pb∫preliminary
• 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
28.08.2011 - 01.09.2011 BW2011 Workshop, Serbia 28
• 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
28.08.2011 - 01.09.2011 BW2011 Workshop, Serbia 29
[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
28.08.2011 - 01.09.2011 BW2011 Workshop, Serbia 30
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
28.08.2011 - 01.09.2011 BW2011 Workshop, Serbia 31
28.08.2011 - 01.09.2011 BW2011 Workshop, Serbia 32
Backup slides
Summary of ATLAS limits (exotics)28.08.2011 - 01.09.2011 BW2011 Workshop, Serbia 33