SUSY Studies with ATLAS Experiment
Nurcan Ozturk
University of Texas at Arlington
ATLAS Collaboration
2006 Texas Section of the APS Joint Fall Meeting
October 5-7, 2006
Arlington, Texas
Outline
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Introduction
Why Supersymmetry
SUSY Particle Spectrum
SUSY Signatures at the LHC
Data Challenge Activities
Results from Full Simulation
Conclusions
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Introduction
• Large Hadron Collider (LHC) is a 14 TeV proton-proton collider at CERN in
Switzerland. LHC will start taking data in 2007.
• Luminosity goals: 10 fb-1/year (first 3 years)
100 fb-1/year (subsequently)
• Five experiments will operate: ALICE, ATLAS, CMS, LHC-B, TOTEM.
• Supersymmetry will be explored primarily in ATLAS and CMS experiments.
ATLAS Detector
Five-story-high
7000 tons
A Toroidal LHC ApparatuS
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Why Supersymmetry?
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Supersymmetry (SUSY) is one of the most attractive extensions of the
Standard Model (SM) that pairs fermions and bosons.
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Hierarchy Problem: SUSY stabilizes Higgs mass against loop corrections
(gauge hierarchy/fine-tuning problem) g leads to Higgs mass ≤ 135 GeV.
Good agreement with LEP constraints from EW global fits.
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Grand Unification: SUSY modifies running of SM gauge couplings ‘just enough’
to give Grand Unification at single scale.
Dark Matter: R-Parity (R = (-1)3B+2S+L) conservation causes the lightest
supersymmetric particle (LSP) to be stable g provides a solution to dark
matter problem of astrophysics and cosmology.
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SUSY Particle Spectrum
SUSY partners have opposite spin-statistics but otherwise same quantum numbers
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SUSY Signatures at the LHC
A typical decay chain of supersymmetric particles in a proton-proton collision:
p
p
~
g
q
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~
q
~
c0 2
q
~
c0 1
~
l
l
l
Heavy strongly interacting sparticles (gluinos and squarks) produced in
initial interaction
Long decay chains and large mass differences between SUSY states; many
high PT objects are observed (lepton, jets, b-jets)
If R-Parity is conserved cascade decays to stable undetected LSP (lightest
SUSY particle; neutralino in mSUGRA); large ETmiss signatures
If the model is GMSB, LSP is gravitino. Additional signatures from NLSP
~
~0   G
c
(next-to-lightest SUSY
particle) decays; for example photons from 1
~
~
and leptons from l  lG
If R-parity is not conserved LSP decays to 3-leptons, 2leptons+1jet, 3 jets;
ETmiss signature is lost
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mSUGRA Framework
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The minimal SUSY extension of the SM (MSSM) brings 105 additional free
parameters g preventing a systematic study of the full parameter space.
Assume a specific well-motivated model framework in which generic signatures
can be studied.
mSUGRA framework: Assume SUSY is broken
by gravitational interactions g unified masses
and couplings at GUT scale g gives five
free parameters: m0, m1/2, A0, tan(β), sgn(µ)
Reach sensitivity only weakly dependent
on A0, tan(β), sgn(µ).
R-parity assumed to be conserved.
Multiple signatures on most of parameter space:
ETmiss (dominant signature), ETmiss with lepton
veto, one lepton, two leptons same sign (SS),
two leptons opposite sign (OS)
Choose benchmark points in mSUGRA plane
to study SUSY exclusively
5s exclusion contours
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Data Challenge Activities (1)
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Goal:
 Provide simulated data to optimize the detector
 Validate Computing Model, the software, the data model, and to ensure the
correctness of the technical choices to be made
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Analyzing SUSY events is important to test the reconstruction software since
typical SUSY events contain the complete set of physics objects that can be
reconstructed in the detector
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SUSY in ATLAS Data Challenges:
 DC1: July 2002 – March 2003
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Bulk region point, similar to LHCC Point 5
 DC2: June 2004 – December 2004
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DC1 bulk region point (validation of Geant4 and new reconstruction)
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Coannihilation point
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Data Challenge Activities (2)
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Data Challenge for Rome ATLAS Physics Workshop: January- June 2005
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SU1 sample: Coannihilation point
m0 =70 GeV, m1/2 = 350 GeV, A0 = 0 GeV, tanβ = 10, sgn(µ) = +
SU2 sample: Focus point
m0 = 3350 GeV, m1/2 = 300 GeV, A0 = 0 GeV, tanβ = 10, sgn(µ) = +
SU3 sample: DC1 bulk region point
m0 =100 GeV, m1/2 = 300 GeV, A0 = -300 GeV, tanβ = 6, sgn(µ) = +
SU4 sample: Low mass point
m0 = 200 GeV, m1/2 = 160 GeV, A0 = -400 GeV, tanβ = 10, sgn(µ) = +
SU5 sample: Scan of parameter space
SU5.1: m0 = 130 GeV, m1/2 = 600 GeV, A0 = 0 GeV, tanβ = 10, sgn(µ) = +
SU5.2: m0 = 250 GeV, m1/2 = 600 GeV, A0 = 0 GeV, tanβ = 10, sgn(µ) = +
SU5.3: m0 = 500 GeV, m1/2 = 600 GeV, A0 = -400 GeV, tanβ = 10, sgn(µ) = +
SU6 sample: Funnel region point
m0 = 320 GeV, m1/2 = 375 GeV, A0 = 0 GeV, tanβ = 50, sgn(µ)
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Data Challenge Activities (4)
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Data Challenge for Computing System Commissioning (CSC): December
2005-ongoing
Number of successful jobs (Release 11.0.x)
SACLAY , 49443 jobs, 2.55
M events, 7%
LCG , 163849 jobs, 11.53 M
events, 25%
OSG , 184430 jobs, 14.91 M
events, 28%
LCG-CG , 184149 jobs,
13.37 M events, 27%
NORDUGRID , 86149 jobs,
7.47 M events, 13%
K.De, Software workshop, Sept. 2006
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Some Results from Full Simulation
Missing ET Distributions –
Rome Data (1)
Top
SU1
after selection cuts
normalized to 5 fb^-1
Reconstructed
Monte Carlo
As expected, missing ET
provides powerful handle
against SM backgrounds
W+jets
Z+jets
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SU2
SU3
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SU4
SU6
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Missing ET Distributions –
Rome Data (2)
Z+jets
Top
SU1
SU2
SU3
SU4
SU6
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after selection cuts
normalized to 5 fb^-1
Selection cuts applied to
enhance SUSY signal:
• 4 jets with PT > 50 GeV
• 2 jets with PT > 100 GeV
• ET miss > 100 GeV
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Dilepton Invariant Mass –
Rome Data (1)
Top
SU1
before selection cuts
normalized to 5 fb^-1
e+e- + µ+µ- - e+-µ-+
~ 
0
~
c 2  l l  c~10l  l 
Excellent discovery
channel!
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W+jets
SU2
Z+jets
SU3
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SU4
SU6
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Dilepton Invariant Mass –
Rome Data (2)
Top
W+jets
Z+jet
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SU1
SU2
SU3
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after selection cuts
normalized to 5 fb^-1
But need lots of data!
SU4
SU6
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Conclusions
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The LHC will be the place to search for SUSY
If TeV scale SUSY exists, ATLAS should find it
Big challenge for discovery will be understanding the performance of
the detector
SUSY discovery is possible in other models which I have not covered
here, however some of UTA group members have been involved:
 Gauge Mediated Supersymmetry Breaking (GMSB)
 Anomaly Mediated Supersymmetry Breaking (AMSB)
 R-Parity Violation
Currently a great effort is being taken in Data Challenges to understand
different SUSY models, and to test the reconstruction software
Exciting times ahead of us with the LHC turn on!
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Backup Slides
Statistics – Rome Data
Sample
sigma x BR (pb)
Number of AOD files
Integrated
Luminosity (pb-1)
Top
577
6793
577
W+4jets
2400
3693
76
Z+jet : ZJ1ee
ZJ1mumu
ZJ1nunu
4730, eff = 0.1003
4730, eff = 0.1058
6140, eff = 0.115
1775
1785
1976
184
175
137
SU1
6.8
3668
26600
SU2
4.9
1156
11555
SU3
19.3
1728
4377
SU4
280
1070
187
SU6
4.5
1308
14293
• Top sample’s cross section is calculated by using what is given in the wiki page: 10K
events corresponds to an integrated luminosity of 17.34 pb-1
• Each AOD file has 49 events
• Each sample is normalized to 5000 pb-1 in all plots
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Event Selection
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Two different sets of cuts applied
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Default cuts:
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Pseudorapidity cuts: ElectronEtaCut: 2.5, MuonEtaCut: 2.5, JetEtaCut: 5.0,
TauEtaCut: 2.5, PhotonEtaCut: 2.5
Transverse momentum cuts: ElectronPtCut: 10 GeV, MuonPtCut: 10 GeV,
JetPtCut: 10 GeV, TauPtCut: 10 GeV, PhotonPtCut: 10 GeV
TauLikelihoodCut: 4
Isolation cuts: 5 GeV for electrons and muons. For muons chi2<20
Selection cuts:
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‘before selection cuts’, which includes some default cuts
‘after selection cuts’ – additional cuts to enhance SUSY signal
4 jets with PT > 50 GeV
2 jets with PT > 100 GeV
ET miss > 100 GeV
Cone 4 jets (R=0.4) are used
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mSUGRA Points for Rome Data (1)
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DC1 bulk region point (new underlying event in generation)
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m0 =100 GeV, m1/2 = 300 GeV, A0 = -300 GeV, tanβ = 6, sgn(µ) = +
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LSP is mostly bino, light lR enhance annihilation. ‘Bread and butter’
~
region for the LHC experiments
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llq distributions, tau-tau measurements, third generation squarks (both
tau identification and B tagging improved)
Coannihilation point
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m0 =70 GeV, m1/2 = 350 GeV, A0 = 0 GeV, tanβ = 10, sgn(µ) = +
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LSP is pure bino. LSP/sparticle coannihilation
.Small
~ 0the
~ final state
slepton-LSP mass difference gives soft leptons c
in
1 1 
Focus point
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m0 = 3350 GeV, m1/2 = 300 GeV, A0 = 0 GeV, tanβ = 10, sgn(µ) = +
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LSP is Higgsino, near µ2=0 bound. Heavy sfermions; all squarks and
sleptons have mass >2 TeV, negligible FCNC, CP, gµ-2, etc. Complex
events with lots of heavy flavor
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mSUGRA Points for Rome Data (2)
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Funnel region point
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m0 = 320 GeV, m1/2 = 375 GeV, A0 = 0 GeV, tanβ = 50, sgn(µ) = +
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Wide H, A for tanβ >> 1 enhance annihilation. Heavy Higgs resonance
(funnel); main annihilation chain into bb pairs
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Dominant tau decays
Low mass point at limit of Tevatron RunII reach
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m0 = 200 GeV, m1/2 = 160 GeV, A0 = -400 GeV, tanβ = 10, sgn(µ) = +
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Big cross section, but events rather similar to top
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Measure SM processes in presence of SUSY background to show
detector is understood
Scan of parameter space (11 different model points)
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mSUGRA points near search limit of 10 fb-1
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Understand limitation of fast simulation analyses; detector backgrounds,
pileup, reconstruction errors, etc
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