SUSY at the LHC

1
Alan Barr
Just find SM Higgs
Was it reallySUSY?
How can we discovery SUSY at LHC?
What can we sayabout what wevefound?
2
Your mission
SM
SUSY
quarks (LR)leptons (LR) neutrinos (L?)
squarks (LR)sleptons (LR)sneutrinos (L?)
Spin-1/2
Spin-0
AfterMixing
?Z0 W gluon
BinoWino0Wino gluino
BW0
Spin-1
4 x neutralino
Spin-1/2
gluino

h0 H0 A0 H
H0H

2 x chargino
Spin-0
Extended higgs sector (2 doublets)
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Features of RP SUSY?
Production part
standard
2 exotics
Time

• RPV as a conserved QN
• Events build from blobs with 2 exotic legs
• A pair of cascade decays results
• Complicated end result

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General features

• Complicated cascade decays
• Many intermediates
• Typical signal
• Jets
• Squarks and Gluinos
• Leptons
• Sleptons and weak gauginos
• Missing energy
• Undetected LSP
• Model dependent
• Various ways of transmitting SUSY breaking from a
  hidden sector

typical susy spectrum(mSUGRA)
LHC Pt5
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What do we see?
Lifetimes short -gt look for Standard Model decay
relics missing energy
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Example of a search topology
LSP
q
squark
q
_
q
_
BACKGROUND topology (QCD)
q
(and similar)
LSP

• No unique choice of sensitive topology
• Complementary information/sensitivity
• Expect SM backgrounds with similar
  characteristics to signal
• Need to search for excesses

SIGNAL topology
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Practical Problems

• See only SM decay products
• Expect short lifetimes
• Lose information about order of decays
• Jets (other than b and t) indistinguishable
• Loose flavour information for other squarks
• Missing momentum from neutralinos only
  determined perpendicular to beam
• Individual LSP momenta not individually
  measurable
• Z-momentum of initial state unknown (PDFs)
• Cant reconstruct from final state
• Forward jets lost down beam pipe
• Cant form invariant masses of sparticles
• No clean mass peaks for resonances

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Precise measurement of SM backgrounds the problem
Rediscover
Lower backgrounds

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

WW
ZZ
Discover
Higher backgrounds
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Just look for jets?
Big QCD background
Scalar sum of transverse energy / GeV
10
Add some missing energy

• Look for events with jets and missing energy
• Cuts
• at least two jets with
• ETJet1,2 gt 150,100 GeV
• ?Jet1,2 lt 2.5

Meff ?Jets pTi MET

• But with addition of some other cuts
• ? Missing transverse momentum gt 100
  GeV
• ? cuts based on ??i ?(Jet,i)-?(MET))
• R1 ?(??22(?-??1)2) gt 0.5 rad
• R2 ?(??12 (?-??2)2) gt 0.5 rad
• no jet with ??i lt 0.5 rad

QCD dijets
Kill events with missing energy from
miss-measured jets
SUSY
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No MT2Dijet cuts MET ??
Two-Jet
Scalar sum of transverse energy / GeV
Expect discovery distribution to be of something
like this form Excess of some sort of new
physics about SM backgrounds.
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Importance of detailed detector understanding
Et(miss)
Lesson from the Tevatron

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

Rare occurrences hurt
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Inclusive reach in mSUGRA parameter space

• Map of discovery potential corresponding to a 5s
  excess above background in mSUGRA m0 m1/2
  parameter space for the ATLAS experiment.

L 1033 cm-2 s-1
jets ETmiss channel
1 year ? 2200 GeV
1 month ? 1800 GeV
few days (lt one week) ? 1300 GeV
Health warning expecting SUSY discovery in a
few days will seriously damage your credibility
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Different searches

• We will be looking in many different channels
• n jets m leptons missing energy
• - b-jets (common at large tan ß)
• - tau-jets ( )
• Charged stable particles
• NLSP -gt photon gravitino (GMSB)
• R-parity violating modes
• R-hadrons

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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 (gt time)
• Understanding of detector
• Clever experimental technique

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Constraining masses
Frequently- studieddecay chain

• Mass constraints
• Invariant masses in pairs
• Missing energy
• Kinematic edges

Observable
Depends on
Limits depend on angles betweensparticle decays
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Mass determination
Measure edges
Try various masses in equations
Variety of edges/variables

• Basic technique
• Measure edges
• Try with different SUSY points
• Find likelihood of fitting data
• Event-by-event likelihood
• In progress

C.G. Lester

• Narrow bands in ?M
• Wider in mass scale
• Improve using cross- section information

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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
• Lots of effort has been made to find good
  techniques

Look forsensitive variables (many of them)
Extractmasses
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SUSY mass measurements

• LHC clearly cannot fully constrain all parameters
  of mSUGRA
• However it makes good constraints
• Particularly good at mass differences O(1)
• Not so good at mass scales
• O(10) from direct measurements
• Mass scale possibly best measured from
  cross-sections
• Often have gt1 interpretation
• What solution to end-point formula is relevant?
• Which neutralino was in this decay chain?
• What was the chirality of the slepton ?
• Was it a 2-body or 3-body decay?

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SUSY spin measurements

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

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Measuring spins of particles

• Basic recipe
• Produce polarised particle
• Look at angular distributions in its decay

spin
?
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Revisit Typical sparticle spectrum
Left Squarks-gt strongly interacting -gt large
production -gt chiral couplings
LHC point 5
?20 neutralino2gt (mostly) partnerof SM W0
Right slepton(selectron or smuon) -gt
Production/decay produce lepton -gt chiral
couplings
Right slepton(selectron or smuon) -gt
Production/decay produce lepton -gt chiral
couplings
mass/GeV
?10 gt Stable -gt weakly interacting
?10 neutralino1gt Stable -gt weakly interacting
Some sparticles omitted
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Spin projection factors
P
S
Chiral coupling
Approximate SM particles as massless -gt okay
since m p
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Spin projection factors
P
S
S0
S
Spin-0
Produces polarised neutralino
Approximate SM particles as massless -gt okay
since m p
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Spin projection factors
Fermion
?
p
S
Scalar
Polarisedfermion
Approximate SM particles as massless -gt okay
since m p
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Spin projection factors
P
mql measure invariant mass
S
?
p
S
Approximate SM particles as massless -gt okay
since m p
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lnearq invariant mass (1)
Back to backin ?20 frame
quark
Probability
?
l
lepton
Phase space
Invariant mass
l-
m/mmax sin ½?

• Phase space -gt factor of sin ½?
• Spin projection factor in M2
• lq -gt sin2 ½?
• l-q -gt cos2 ½?

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Production Asymmetry
Twice as much squark as anti-squark pp collider ?
Good news!
Squark
Anti-squark
Note opposite shapes in distributions
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After detector simulation (ATLFAST)
Change in shape due to charge-blind cuts
l-
parton-level 0.6
Events
Charge asymmetry,
spin-0
SUSY
l
detector-level
Invariant mass
-gt Charge asymmetry survives detector
simulation-gt Same shape as parton level (but
with BG and smearing)
? detector effects ? cuts to greatly reduce
SM
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Interesting questions

• Can we test gaugino universality?
• Can we constrain the neutralino mass mixing
  matrix?
• Can we measure sparticle splittings?
• JMR Htt coupling interesting
• Can we predict/confirm dark matter density?
• Can we measure mass scale to better than 10
• Precision measurement/prediction for
  cross-sections?
• Can we confirm spin(s)?

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Extras
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Standard Model backgrounds measure from LHC DATA
Measure in Z -gt µµ
Use in Z -gt ??
R Z -gt nn B 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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W contribution to no-lepton BG
Oe, Okawa, Asai

• Use visible leptons from Ws to estimate
  background to no-lepton SUSY search

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Normalising not necessarily good enough
Distributions are biased by lepton selection ?
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Need to isolate individual components
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Then possible to get it right
Similar story for other backgrounds control
needs careful selection
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Direct slepton spin determination

• Spin important in slepton production
• Occurs through s-channel spin-1 process only
• Characteristic angular distribution in production

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Distributions _at_ parton level
Parallel
Parallel
Perpendicularto beam
redUED

• Spin-0
• SUSY
• Sleptons
• perpendicular to beam
• Spin-½
• UED
• KK leptons
• parallel to beam

bluePS
stotal not to scale
blackSUSY
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Sensitive variables?

• cos ?lab
• Good for linear collider
• Not boost invariant
• Missing energy means Z boost not known _at_ LHC
• Not sensitive _at_ LHC
• ??
• Boost invariant
• Sensitive
• Not easy to compare with theory
• cos ?ll
• 1-D function of ??
• All benefits of ??
• Interpretation as angle in boosted frame
• Easier to compare with theory

(A)
(B)
boost
(C)
N.B. ignores azimuthal angle
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Some results

• SPS5 point
• Below spectrum
• Right results
• Good stat. discrimination

Data inclusive SUSY after cuts