Searching for Supersymmetry at the LHC

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Searching for Supersymmetryat the LHC
UC Berkeley Physics 290E, Oct 28 2008
http//www.icepp.s.u-tokyo.ac.jp/kenta/analysis.h
tml
http//home.slac.stanford.edu/pressreleases/2006/2
0060821.htm
Sven Vahsen (Lawrence Berkeley Lab)
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Outline

• Motivation for SUSY
• Limits from Tevatron and LEP
• If SUSY exist, how to find it at the LHC?
• If SUSY discovered, what more can we learn at LHC?

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Motivation
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The SUSY/LHC Dream Scenario

• Discover TeV scale Supersymmetry (SUSY) early!
• Spend LHC SLHC years measuring SUSY !

2012
2014
2016
2018
2020
2010
2008
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The Standard Model of Particle Physics

• Successful theory of fundamental interactions
  since early 1970s
• Survived numerous experimental tests
• Only Higgs missing
• LHC built to look for Higgs and Physics beyond
  the Standard Model

H
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Supersymmetry (SUSY)

• Well motivated extension of the Standard Model
• Standard Model particles have supersymmetric
  partners
• Differ by 1/2 unit in spin
• Higgs sector h,H,A,H/- (skip today)
• EW-gauginohiggsino mixing ? 2 charginos
  4 neutralinos

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SUSY Breaking

• No sparticles observed to date ? if SUSY exists,
  then it is somehow broken
• MSSM gt100 free parameters
• Models of SUSY breaking mechanism
• mSUGRA
• GMSB
• AMSB
• Split SUSY
• ? reduced number of model parameters
• These determine sparticle masses and
  phenomenology

SU
SY
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Why We Like SUSY

• Keeps corrections to Higgs mass small. Requires
  wino and stop masses few hundred GeV
• Unifies gauge couplings at large Q2. Requires
  sparticle masses few hundred GeV
• Can provide plausible WIMP Dark matter
  candidates. Cosmological arguments prefer WIMP
  mass hundred GeV

SM
without SUSY
With SUSY
with SUSY
Energy GeV
W. de Boer, C. Sander Phys.Lett.B (2004)
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R-Parity conservation ? Stable LSP

• R (-1)2j3BL SM particles have R1,
  superpartners R-1
• Even number of SUSY particles for every vertex
• SUSY particles always produced in pairs
• Get decay chains as below
• Lightest SUSY particle (LSP) cannot decay, hence
  potential WIMP Dark Matter candidate

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Is the Dark Matter Supersymmetric?
http//home.slac.stanford.edu/pressreleases/2006/2
0060821.htm

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c01
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SUSY / Dark Matter Connection LSP Relic Density

• WIMPs thermally produced
• Universe expands and cools, WIMP density reduced
  through annihilation
• Eventually density is too low for annihilation
  process to keep up with expansion rate.
• Freeze-out density depends on annihilation
  cross-section
• Need weak cross-section and mWIMP100 GeV to
  get DM density
• Amazingly, SUSY LSP good match!

Freeze-out
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mSUGRA and Dark Matter

• mSUGRA
• SUSY masses unify at GUT-scale m0, m1/2
• tanß, A0, sign(µ)
• Neutralino LSP
• Four regions with W NEUTRALINO ? W DM due to
  enhanced annihilation in early universe

Pseudo-projection no units!
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Exclusion Limits from Tevatron and LEP
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Squark and Gluino Mass Limits

• Best limits from D0
• mgluino gt 308 GeV
• msquark gt 379 GeV
• Evaluated within mSUGRA

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Older Experimental limits

• Many other best limits still from LEP

PDG 2006??
PDG2006
See update on next slide
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If SUSY exist, how to discover it at the LHC?
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Tevatron ? LHC
Geneva
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Will the LHC be a SUSY Factory?

• If SUSY exists at the TeV scale, expect copious
  production of squarks and gluinos
• Just QCD, nearly independent of SUSY model

• s (pp?SUSY) calculated at NLO
• vs14TeV, mSUSY 0.5-1.0 TeV ? spp?SUSY
  1-100 pb

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Tevatron ? LHC

• Win twice when moving to LHC
• sSUSY increases 20000(!) for mgluino400 GeV
• S/B improves
• SUSY-discovery challenge
• reject SM by factor of 1011
• understand SM events that survive SUSY selection

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Experimental Signature

• Two sparticles initially
• Cascade decays down to LSP jets, leptons
• LSP escapes undetected large ETmiss
• ? Canonical SUSY signature ETmiss, high-pT
  jets, often leptons

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mSUGRA bulk region
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Most Generic SUSY signature Missing ET jets

• Event selection
• Jets 1 with pTgt100 GeV
• Jets 2,3,4 with pTgt 50 GeV
• ETMISS gt 100 GeV
• ETMISS gt 0.2 Meff
• Transverse Sphericity gt 0.2
• ?f(jet1,2,3, ETmiss) gt 0.2
• veto events with isolated leptons
• Plot Effective Mass variable
• Meff SpTi ETmiss

mSUGRA bulk region
1 fb-1
Events with several hard, (often miss-measured)
QCD jets Monte Carlo predictions have large
systematic uncertainties
Excess at large Meffective potential discovery of
SUSY
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Measuring Standard Model Backgrounds

• Have to estimate Standard Model background
  passing SUSY selection using data-driven
  techniques
• Example Select Z?ll and replace charged leptons
  by neutrinos
• Obtain shape of ETMISS, Meffective

mSUGRA bulk region
1 fb-1
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Detector Performance Crucial

• QCD multijet is background with largest
  cross-section
• Rejecting this relies on good ETMISS resolution
• Can spoil ETMISS resolution with
• Miss-measured jets in cracks
• Calorimeter noise inefficiencies
• Beam Halo Cosmic Background
• Need to clean up
• use calorimeter timing, ETMISS from tracks vs
  calo, veto bad runs
• Wont be easy to get there, took a lot of time at
  Tevatron

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SUSY Search strategy

• Assuming background measurement have been carried
  out, and detector performance is under control
• apply event selection, look for excess events
  at high Meffective
• Choose exact cuts to maximize number of
  discoverable models
• Works beyond mSUGRA, because other models typical
  produce additional final state particles

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L 1 fb-1 SUSY Discovery Potential

• Full simulation of backgrounds
• Includes expected systematic (JES, background
  estimation)
• Good chance of finding TeV scale SUSY with 1fb-1
  of data
• Dream scenario!
• SUSY at higher mass scales could still show up
  later, but would make detailed studies difficult
• Ultimate LHC reach 3 TeV

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Mass reach versus integrated luminosity

• SUSY search most exciting during first LHC
  years!
• TDR result Reach gluino / squark masses of
• 1.0 TeV with 0.1 fb-1
• 1.5 TeV with 1.0 fb-1
• 2.0 TeV with 10 fb-1
• (Outdated study, but shows nature of luminosity
  dependence)

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Missing ET jets 1 lepton

• Require an additional lepton (electron or muon)
• Drawbacks less inclusive, slightly lower rate
• Advantages
• higher S/B
• Better QCD multijet rejection, simpler background
  composition
• Transverse mass provides additional handle on
  remaining backgrounds
• If standard model backgrounds larger, or detector
  performance worse than expected, 1-lepton mode
  may be more powerful than 0-leptons

(1 lepton)
(0 leptons)
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3rd Generation t- and b-jet modes

• Can require either b-tagged jet or (hadronic) t
  in event selection
• Lower signal efficiency, but higher purity
• Dominant background is ttbar
• Improved discovery potential for high tanß models
  (SU6 on right), which have increased decays
  into 3rd generation

1 fb-1 SU3?
1fb-1
b-jet mode
Tau mode
1fb-1
100 pb-1 SU3?
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GMSB photons

• Lightest SUSY particle (LSP) is Gravitino
• 2nd lightest (NLSP) in GMSB is Neutralino or
  Slepton
• Typical signatures for neutralino case
• Etmiss jets di-photons (prompt or
  non-pointing)

• Prompt photon case
• High-pt photons after SUSY selection almost
  background free
• SUSY Discovery easy for this class of GMSB models

1fb-1 (after SUSY selection) very preliminary
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What if squarks, sleptons are heavy?

• mSUGRA example
• Direct gaugino production dominates
• Signature
• 3 isolated leptons ET-miss
• few jets, at low PT (ISR only)
• Discovery 10-100 fb-1 (under study)

10fb-1 very preliminary
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If SUSY found at LHC, what next?
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Home in on SUSY model

• Inclusive studies provide first constraints on
  model parameters
• Meffective ? Mass scale
• Relative Significance in 0,1,2 lepton channels ?
  m0,m1/2
• 3rd generation ? tan?

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Mass reconstruction

• Most promising Opposite sign, same flavor
  di-leptons from single neutralino decay
• Subtract background (from Standard Model and SUSY
  itself) using flavor information
• ee- µµ - eµ- - e-µ(after efficiency
  correction)
• Low background, relatively high statistics

mSUGRA bulk region
1 fb-1
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• Position of mass-edge sensitive to combination of
  sparticle masses

In favorable models, can measure large part of
mass spectrum at LHC see future talk!
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Do we have a dark matter candidate?

• Calculate neutralino mass m? and cross section
  s?-nucleon
• Compare with direct detection

BULK
FOCUS
NOW
Cross-section (cm2)
ZEPLIN-2
2009?
300 fb-1
FUNNEL
COAN.
2012?
WIMP Mass GeV/c2
300 fb-1
log10 (scpsi / 1pb)
Same WIMP in lab and space?
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Do we have a dark matter candidate?

• Calculate neutralino relic density
• What fraction of the dark matter is neutralinos?
  ? (W ? W DM ?)

• W ? h2 0.192 0.005 (stat)
  0.006 (sys)
• (simulated point is pre-WMAP)

300 fb-1
No. MC Experiments
Wch2
Caveat this precision depends on assuming very
constrained SUSY breaking scenario. In more
general scenario much looser constraints.
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Spin Determination

• Measure spin of new particles to distinguish e.g.
  UED and SUSY
• Method that seems to work for many SUSY models
  direct slepton production
• Occurs through s-channel spin-1 process only
• Characteristic angular distribution in production

Normalised cross-sections
AJB hep-ph/0511115
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Z/?
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Nice idea, but
SUSY PS UED
SUSY data

• Low x-section
• Background from Standard Model and SUSY itself
• Needs lots of data and favorable SUSY model
• Plot on left 500 fb-1 !

AJB hep-ph/0511115
mSUGRA bulk region
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The Dream Scenario

• Discover TeV scale Supersymmetry (SUSY) early!
• Spend LHC SLHC years measuring SUSY !

2012
2014
2016
2018
2020
2010
2008
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Conclusion

• Convincing SUSY search requires detailed
  understanding of Detector and Standard Model
  events passing SUSY selection
• If SUSY exists at TeV-scale (where it seems most
  motivated), LHC should find it early (1fb-1)
• In this case, rates at LHC would be high,
  allowing detailed studies of SUSY in large number
  of final states
• Degree of progress at LHC will then depend on
  Natures benevolence in breaking SUSY
• Favorable scenarios suggest SUSY mass
  reconstruction and precise calculations of m?,
  s???W? possible with 300fb-1 of data
• When combined with Astroparticle Cosmology
  measurements, this would reveal the relation of
  the SUSY LSP to the dark matter
• If SUSY mass scale is higher, LHC may still find
  SUSY, but detailed studies may not be possible

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Backup slides
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A Toroidal LHC Apparatus (ATLAS) All of
detector important for SUSY search!
Muon spectrometer
Hadronic Tile Calorimeter
Solenoid
Toroid
Inner Detector
Electromagnetic Calorimeter
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Fitting for edges after flavor subtraction

• Method sensitive to any sleptons lighter than 2nd
  neutralino

Bulk region
Low mass point
0.5 fb-1
1 fb-1
Two light sleptons, Coannihilation region
SUSY
Standard Model
18 fb-1
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mSUGRA

• Behavior determined by only 4 parameters a sign
  at GUT scale
• m0 common scalar mass
• m1/2 common higgsino/gaugino mass
• A0 common higgs-sfermion-sfermion coupling
• sgn(µ) sign of susy conserving Higgs mass
• tanß vu/vd ratio of Higgs VEVs
• These determine masses and couplings of
  sparticles at weak scale via renormalization
  group equations RGEs

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Predicted Background Uncertainties

• Current ATLAS SUSY group prediction for accuracy
  with which we will estimate Standard Model
  background after SUSY selection
• Used 50 (Z,W,top) and 100(QCD) uncertainties
  for 100pb-1