Alan Barr University of Oxford

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Discovering (and understanding) SUSY at the
LHC

• Alan Barr University of Oxford

an introduction (with apologies to the many
people whos work I have included unreferenced
and to those whom I have left out)
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LHC physics is about to get very interesting!
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ATLAS control room
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Have lots of cosmics events(these from much
earlier)
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Last chance to visit LHC
Relatively well-known German physicist takes her
chance
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Motivational arguments
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How to make a discovery?
cMSSM

• Which way to search?

Who knows what?
Other SUSY?
ExtraDimensions?
Explorer/experimentalists rule Try to COVER ALL
BASES
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Signature-based hunts
Experiments see Jets, leptons, missing energy,
b-jets

• Astro/cosmo motivation for model-independent
  signatures
• Were pretty sure there are WIMPs out there
• LHC produces Dark Matter something visible
• Invisible particle could be
• Lightest SUSY particle
• Lightest KK particle
• Lightest generic parity-odd particle
• Signature
• Missing energy Xvis Xvis

• Benefit Same search finds multiple different
  models
• Drawback You aint so sure what youve got when
  you find it

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Example SUSY search
Mass (GeV)
Typical SUSY spectrum

• Assume R-parity
• Look for
• Jets from squark gluino decays
• Leptons from gaugino slepton decays
• Missing energy from (stable) LSPs

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SUSY event
Missing transverse momentum
Heavy quarks
Jets
Leptons
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Cross-sections etc
Rediscover
Lower backgrounds
WW
ZZ
Discover
Higher backgrounds
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Precise measurement of SM backgrounds the problem

• SM backgrounds are not that small
• There are uncertainties in
• Cross sections
• Kinematical distributions
• Detector response

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Typical search inclusive distributions

• Trigger on jets missing energy
• Plot effective mass
• Look for non-SM physics at high mass

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Standard Model backgrounds measure from LHC DATA
Measure in Z -gt µµ
Use in Z -gt ??
R Z -gt nnB Estimated R Estimated

• Example SUSY BG
• Missing energy jets from Z0 to neutrinos
• Measure in Z -gt µµ
• Use for Z -gt ??

• Good match
• Useful technique
• Statistics limited
• Go on to use W gt µ? to improve

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Estimating the backgrounds
Good match to true background
Search region
Control Region
More from Davide Costanzolater in this session
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Importance of detailed detector understanding

• Simulation shows events with large fake missing
  energy
• Jets falling in crack region
• Calorimeter punch-through
• Vital to remove these in missing energy tails
• Large effort in physics commissioning

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Reach in cMSSM?
Focus point' region annihilation to gauge bosons
mSUGRA A00, tan(b) 10, mgt0
Slepton Co-annihilation region
Rule out with 1fb-1
'Bulk' region t-channel slepton exchange
WMAP constraints
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Multiple channels for discovery
Below the lines discovered
Differentfinalstates
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What might we then know?

• Can say some things
• Undetected particles produced
• missing energy
• Some particles have mass 600 GeV, with
  couplings similar to QCD
• Meff cross-section
• Some of the particles are coloured
• jets
• Some of the particles are Majorana
• excess of like-sign lepton pairs
• Lepton flavour conserved in first two
  generations
• e vs mu numbers
• Possibly Yukawa-like couplings
• excess of third generation
• Some particles contain lepton quantum numbers
• opposite sign, same family dileptons

• Assume we have MSSM-like SUSY with
  m(squark)m(gluino)600 GeV
• See excesses in these distributions
• Cant say we have discovered SUSY

Slide based on Polesello
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Mapping out the new world
LHC Measurement SUSY Extra Dimensions
Masses Breaking mechanism Geometry scale
Spins Distinguish from ED Distinguish from SUSY
Mixings, Lifetimes Gauge unification? Dark matter candidate? Gauge unification? Dark matter candidate?

• Some measurements make high demands on
• Statistics (? time)
• Understanding of detector
• Clever experimental techniques

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SUSY mass measurements
Tryvariousdecaychains

• Extracting parameters of interest
• Difficult problem
• Lots of competing channels
• Can be difficult to disentangle
• Ambiguities in interpretation
• Example method shown here
• Alternatives also on the market
• Comparable precision

Look forsensitive variables (many of them)
Extractmasses
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Stransverse mass (MT2) method
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Measuring the shapes

• Better precision possible than for endpoints
• Systematic uncertinties need to be controlled

Much work here recently
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SUSY spin measurements
Neutralino spin from angles in decay chains
Slepton spin from angles in Drell-Yan production

• The defining property of supersymmetry
• Distinguish from e.g. similar-looking Universal
  Extra Dimensions
• Difficult to measure _at_ LHC
• No polarised beams
• Missing energy
• Inderminate initial state from pp collision
• Nevertheless, we have some very good chances

lots of other recent work in this area
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Other ways of measuring spin

• Cross-section depends on spin
• If mass scale can be measured then spin can be
  inferred

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Dark matter relic density?

• Use LHC measurements to predict relic density
  of observed LSPs
• Caveats
• Cant tell about lifetimes beyond detector (need
  direct search)
• Studies done so far in optimistic case (light
  sparticles)

• To remove mSUGRA assumption need extra
  constraints
• All neutralino masses
• Use as inputs to gaugino higgsino content of
  LSP
• Lightest stau mass
• Is stau-coannihilation important?
• Heavy Higgs boson mass
• Is Higgs co-annihilation important?
• More work is in progress
• Probably not all achievable at LHC
• ILC would help lots (if in reach)

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Covering all the bases

• Host of other searches
• Light stop squarks
• R-parity violating models
• Dileptons/trileptons with missing energy
• Taus with jets missing energy,
• Single photons
• Diphoton resonances
• Heavy l? resonances
• Heavy flavour excesses
• Monojets
• Same sign Stops

See e.g.CMS Physics TDR II2006 ATLAS
SUSYdiscovery chapter2008
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10 TeV LHC run 2008
10 TeV run need not be just commissioning Lots
of physics and discovery potential
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Conclusions
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Extra rations
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Gauge Mediated SUSY Breaking

• Signature depends on Next to Lightest SUSY
  Particle (NLSP) lifetime
• Interesting cases
• Non-pointing photons
• Long lived staus

• Extraction of masses possible from full event
  reconstruction
• More detailed studies in progress by both
  detectors

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R-hadrons

• Motivated by e.g. split SUSY
• Heavy scalars
• Gluino decay through heavy virtual squark very
  suppressed
• R-parity conserved
• Gluinos long-lived

• Lots of interesting nuclear physics in
  interactions
• Charge flipping, mass degeneracy,
• Importance here is that signal is very different
  from standard SUSY

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R-hadrons in detectors

• Signatures
• High energy tracks (charged hadrons)
• High ionisation in tracker (slow, charged)
• Characteristic energy deposition in calorimeters
• Large time-of-flight (muon chambers)
• Charge may flip
• Trigger
• Calorimeter etsum or etmiss
• Time-of-flight in muon system
• Overall high selection efficiency
• Reach up to mass of 1.8 TeV at 30 fb-1

GEANT simulation of pair of R-hadrons (gluino
pair production)
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Exotic WW scattering

• The ultimate test of electroweak symmetry
  breaking
• Not unitary above 1 TeV if no new physics

BG
BG
signal

• Reconstruct hadronic leptonic W pair
• Require forward jets
• Veto jets in central region

Most difficult case continuum signal
5-? significance with 30 fb-1 in most difficult
case
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Dijet masses Contact Interactions

• Reduce systematics by using ratio à la DZero
• New physics in the central region
• Calibration sample at higher rapidity
• Uncertainties from proton structure not
  negligible
• Improve with LHC data?
• Detector cross-calibration uncertainties to be
  determined from data
• Estimates here

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RS Gravitons heavy bosons
Angular distributions

• Discovery
• Find mass peak
• Characterisation
• Measure spin

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Spectacular states micro Black Holes

• Large EDs
• Micro black hole decaying via Hawking radiation
• Photons Jets
• We will certainly know something funny is
  happening
• Large multiplicities
• Large ET
• Large missing ET
• Highly spherical compared to BGs
• Theory uncertainty limits interpretation
• Geometrical information difficult to disentangle

sphericity
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Black hole interpretation?
Slide from Lester
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Some of the sources

• CMS Physics TDR, Volume II (recent)
• CERN-LHCC-2006-021
• ATLAS Physics TDR (older)
• CERN-LHCC-99-015
• Physics at the LHC 2006
• Programme
• SLAC School 06
• Polesello, Hinchliffe
• SUSY06
• Polesello, Spiropulu
• Missing ET tails
• Paige
• SM background
• Okawa et al,
• WMAP constraints
• Ellis et al
• SUSY mass extraction
• Gjelsten et al

• SUSY Spin
• Barr
• Exotic SUSY
• Parker
• Dark Matter
• Nojiri et al
• R-hadrons
• Kraan et al
• Hellman et al
• WW scattering
• Stefanidis
• GMSB
• Zalewski, Prieur
• RS Graviton Allanach et al,Traczyk
• Black Holes
• Charybdis, Tanaka, Brett, Lester
• WW scattering
• Stephanidis

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Constraining masses with cross-section information

• Edges best for mass differences
• Formulae contain differences in m2
• Overall mass- scale hard at LHC
• Cross-section changes rapidly with mass scale
• Use inclusive variables to constrain mass scale
• E.g. gt500 GeV ptmiss

Lester, Parker, White hep-ph/0508143
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SUSY Dark Matter
mSUGRA A00, tan(b) 10, mgt0
Slepton Co-annihilation region LSP pure Bino.
Small slepton-LSP mass difference makes
measurements difficult.
Ellis et al. hep-ph/0303043
Disfavoured by BR (b ? s?) (3.2 ? 0.5) ?
10-4 (CLEO, BELLE)
'Bulk' region t-channel slepton exchange - LSP
mostly Bino. 'Bread and Butter' region for LHC
Expts.
Also 'rapid annihilation funnel' at Higgs pole at
high tan(b), stop co-annihilation region at large
A0
0.094 ? ? ? h2 ? 0.129 (WMAP)
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More on GMSB

• Negligible contribution from the SM backgrounds
  (consistent with TDR)
• ? Trigger efficiencies of the signal is crucial
  for the discovery potential
• (background rejection, rate estimates would
  be the next step)

G1a (L90TeV)
G1a (L90TeV)
ltAfter Requiringgt Meff gt 400GeV EtMissgt0.1Meff two
leptons
BG Total
BG Total
g1
g2
Leading Photon Pt (GeV)
2nd Leading Photon Pt (GeV)
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Baryonic R-Parity Violation
Decay via allowed where m( ) gt m( )

• Use extra information from leptons to decrease
  background.
• Sequential decay of to through and
  producing Opposite Sign, Same Family (OSSF)
  leptons

Test point
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Leptonic R-Parity Violation
RPV has less missing EtNeutralino -gt stau
taustau -gt tau mu qq Large rate of taus -
smoking gun
Stau LSP
Phillips
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Light stops

• Stop pair production 412 pb (PROSPINO, NLO)
• Dominant (100) stop decay t ? c b ? c01 W
  b
• Final state is very similar to top pair
  production events.
• 4 jets, 2 of which b-jets, one isolated lepton,
  missing energy
• All of them softer (on average) than in top pair
  production
• Invariant mass combinations will not check out
  with top, W masses

M(bjj) 1.8 fb-1
M(bl) 1.8 fb-1
GeV
GeV
Points simulated data Histograms signal events
(MC truth)
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New vector boson W

• Transverse mass plot for W gt µ?