SUSY Studies with ATLAS Experiment

1
SUSY Studies with ATLAS Experiment
Nurcan OzturkUniversity of Texas at
Arlington ATLAS Collaboration

• 2006 Texas Section of the APS Joint Fall
  MeetingOctober 5-7, 2006
• Arlington, Texas

2
Outline

• Introduction
• Why Supersymmetry
• SUSY Particle Spectrum
• SUSY Signatures at the LHC
• Data Challenge Activities
• Results from Full Simulation
• Conclusions

3
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
4
Why Supersymmetry?

• Supersymmetry (SUSY) is one of the most
  attractive extensions of the Standard Model (SM)
  that pairs fermions and bosons.
• 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.
• Grand Unification SUSY modifies running of SM
  gauge couplings just enough to give Grand
  Unification at single scale.
• Dark Matter R-Parity (R (-1)3B2SL)
  conservation causes the lightest supersymmetric
  particle (LSP) to be stable g provides a solution
  to dark matter problem of astrophysics and
  cosmology.

5
SUSY Particle Spectrum
SUSY partners have opposite spin-statistics but
otherwise same quantum numbers
6
SUSY Signatures at the LHC
A typical decay chain of supersymmetric particles
in a proton-proton collision

• 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 (next-to-lightest
  SUSY particle) decays for example photons from
  and leptons from
• If R-parity is not conserved LSP decays to
  3-leptons, 2leptons1jet, 3 jets ETmiss
  signature is lost

7
mSUGRA Framework

• 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(ß), sgn(µ)
• Reach sensitivity only weakly dependent
• on A0, tan(ß), 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)

• 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
• 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
• SUSY in ATLAS Data Challenges
• DC1 July 2002 March 2003
• Bulk region point, similar to LHCC Point 5
• DC2 June 2004 December 2004
• DC1 bulk region point (validation of Geant4 and
  new reconstruction)
• Coannihilation point

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Data Challenge Activities (2)

• Data Challenge for Rome ATLAS Physics Workshop
  January- June 2005
• SU1 sample Coannihilation point
• m0 70 GeV, m1/2 350 GeV, A0 0 GeV, tanß
  10, sgn(µ)
• SU2 sample Focus point
• m0 3350 GeV, m1/2 300 GeV, A0 0 GeV, tanß
  10, sgn(µ)
• SU3 sample DC1 bulk region point
• m0 100 GeV, m1/2 300 GeV, A0 -300 GeV, tanß
  6, sgn(µ)
• SU4 sample Low mass point
• m0 200 GeV, m1/2 160 GeV, A0 -400 GeV, tanß
  10, sgn(µ)
• SU5 sample Scan of parameter space
• SU5.1 m0 130 GeV, m1/2 600 GeV, A0 0 GeV,
  tanß 10, sgn(µ)
• SU5.2 m0 250 GeV, m1/2 600 GeV, A0 0
  GeV, tanß 10, sgn(µ)
• SU5.3 m0 500 GeV, m1/2 600 GeV, A0
  -400 GeV, tanß 10, sgn(µ)
• SU6 sample Funnel region point
• m0 320 GeV, m1/2 375 GeV, A0 0 GeV, tanß
  50, sgn(µ)

10
Data Challenge Activities (4)

• Data Challenge for Computing System Commissioning
  (CSC) December 2005-ongoing

K.De, Software workshop, Sept. 2006
11
Some Results from Full Simulation
12
Missing ET Distributions Rome Data (1)
after selection cuts normalized to 5 fb-1
Reconstructed Monte Carlo
SU1
Top
As expected, missing ET provides powerful handle
against SM backgrounds
Wjets
SU4
SU2
Zjets
SU3
SU6
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after selection cuts normalized to 5 fb-1
Missing ET Distributions Rome Data (2)
Zjets Top SU1 SU2 SU3 SU4 SU6

• Selection cuts applied to enhance SUSY signal
• 4 jets with PT gt 50 GeV
• 2 jets with PT gt 100 GeV
• ET miss gt 100 GeV

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Dilepton Invariant Mass Rome Data (1)
before selection cuts normalized to 5 fb-1
ee- µµ- - e-µ-
Top
SU1
Excellent discovery channel!
Wjets
SU4
SU2
Zjets
SU6
SU3
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Dilepton Invariant Mass Rome Data (2)
after selection cuts normalized to 5 fb-1
SU1
Top
But need lots of data!
SU2
Wjets
SU4
Zjet
SU3
SU6
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Conclusions

• 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
18
Statistics Rome Data
Sample sigma x BR (pb) Number of AOD files Integrated Luminosity (pb-1)
Top 577 6793 577
W4jets 2400 3693 76
Zjet 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 samples 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

• Two different sets of cuts applied
• before selection cuts, which includes some
  default cuts
• after selection cuts additional cuts to
  enhance SUSY signal
• Default cuts
• 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 chi2lt20
• Selection cuts
• 4 jets with PT gt 50 GeV
• 2 jets with PT gt 100 GeV
• ET miss gt 100 GeV
• Cone 4 jets (R0.4) are used

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mSUGRA Points for Rome Data (1)

• DC1 bulk region point (new underlying event in
  generation)
• m0 100 GeV, m1/2 300 GeV, A0 -300 GeV, tanß
  6, sgn(µ)
• LSP is mostly bino, light lR enhance
  annihilation. Bread and butter region for the
  LHC experiments
• llq distributions, tau-tau measurements, third
  generation squarks (both tau identification and B
  tagging improved)
• Coannihilation point
• m0 70 GeV, m1/2 350 GeV, A0 0 GeV, tanß
  10, sgn(µ)
• LSP is pure bino. LSP/sparticle coannihilation
  .Small slepton-LSP mass
  difference gives soft leptons in the final state
• Focus point
• m0 3350 GeV, m1/2 300 GeV, A0 0 GeV, tanß
  10, sgn(µ)
• LSP is Higgsino, near µ20 bound. Heavy
  sfermions all squarks and sleptons have mass gt2
  TeV, negligible FCNC, CP, gµ-2, etc. Complex
  events with lots of heavy flavor

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mSUGRA Points for Rome Data (2)

• Funnel region point
• m0 320 GeV, m1/2 375 GeV, A0 0 GeV, tanß
  50, sgn(µ)
• Wide H, A for tanß gtgt 1 enhance annihilation.
  Heavy Higgs resonance (funnel) main annihilation
  chain into bb pairs
• Dominant tau decays
• Low mass point at limit of Tevatron RunII reach
• m0 200 GeV, m1/2 160 GeV, A0 -400 GeV, tanß
  10, sgn(µ)
• Big cross section, but events rather similar to
  top
• Measure SM processes in presence of SUSY
  background to show detector is understood
• Scan of parameter space (11 different model
  points)
• mSUGRA points near search limit of 10 fb-1
• Understand limitation of fast simulation
  analyses detector backgrounds, pileup,
  reconstruction errors, etc