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SUSY searches at the LHC. Arthur M. Moraes Brookhaven National Laboratory (on behalf of the ATLAS Collaboration). LISHEP 2006 – Workshop on Collider Physics. Itacuruçá, April 5 th 2006. Outline:. Introduction: Why Supersymmetry?. ATLAS and CMS;. SUSY searches at the LHC:

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susy searches at the lhc

SUSY searches at the LHC

Arthur M. Moraes

Brookhaven National Laboratory

(on behalf of the ATLAS Collaboration)

LISHEP 2006 – Workshop on Collider Physics

Itacuruçá, April 5th 2006.



  • Introduction: Why Supersymmetry?
  • ATLAS and CMS;
  • SUSY searches at the LHC:
    • What will SUSY events look like?
    • Inclusive reach in mSUGRA parameter space.
    • SUSY mass scale measurements.
    • Dilepton mass distribution.
    • Right-handed squark mass.
    • SUSY spin measurement.
  • Summary

Supersymmetry (SUSY): why are we interested?

SUSY has many merits:

  • it is elegant;
  • assuming the existence of superpartners with TeV-scale masses, the Strong, Weak and Electromagnetic force strengths become equal at the “GUT scale”;
  • from experimental limits, squarks and gluinos must be heavy (>400GeV)
  • it provides a natural explanation of why the Higgs mass can be low (< 1 TeV).
  • SUSY theories (RPC) provide explanation to the dark matter in the Universe: “neutralinos” (the lightest SUSY particles – LSP).
  • If SUSY is a true symmetry of Nature and it is realized at the TeV scale, it will almost certainly be discovered by ATLAS and CMS.

•Status of the LHC:

  • Priority: complete the LHC project (machine/detectors/LCG) by Spring 2007.
  • April 2007: start machine commissioning (mainly single beam)
  • ~ Summer 2007: two beams in the machine (first collisions!)

LHC (Large Hadron Collider):

•p-p collisions at √s = 14TeV

•bunch crossing every 25 ns(40 MHz)

  • low-luminosity: L ≈ 2 x 1033cm-2s-1(L ≈ 20 fb-1/year)
  • high-luminosity: L ≈ 1034cm-2s-1

(L ≈ 100 fb-1/year)


ATLAS: AToroidal LHC AparatuS

Muon Detectors

Tile Calorimeter

Liquid Argon Calorimeter

ATLAS and CMS are well adapted to anynew physics.

TRT Tracker

Toroid Magnets

Pixel Detector

SCT Tracker

Solenoid Magnet

•Multi-purpose detectors

coverage up to |η| = 5;

design to operate at L= 1034cm-2s-1

  • The most important aspect of any detector used for inclusive SUSY searches are the energy resolutions and hermeticities of the calorimeters.
  • Mismeasurements of jets by the ECAL and HCAL can lead to significantly increased rates of high-ETmiss background QCD events.

CMS: Compact Muon Solenoid


Process Events/sEvents for 10 fb-1

W e 15 ~108

Z ee 1.5~107


tt ~ 1107


bb ~1061012 – 1013

H m=130 GeV 0.02105


ggm= 1 TeV 0.001 104

Black holes 0.0001103

m > 3 TeV

(MD=3 TeV, n=4)

Physics goals and potential for the first year (a few examples…)

Expected event rates for ATLAS (or CMS) at L = 1033 cm-2 s-1

  • Already in the first year, large statistics expected from:
  • known SM processes (understand the detector and physics at the LHC)
  • several new physics scenarios can be tested

Going “beyond the Standard Model” at the LHC

  • ATLAS and CMS will probe physics at TeV mass scale.
  • Goals include searching for the Higgs boson and for other physics “Beyond the Standard Model” (BSM) such as SUSY particles.
    • But we cannot see Higgs/SUSY particles directly as they either decay to lighter (stable) particles or cannot be seen with any known detector.
    • We have to design our detector to look for the stable particles and signs of “invisible” particles…..

ATLAS: illustration of a black-hole event

  • LHC detectors and BSM physics: we do not know what might be found! But “any” new physics must decay to SM particles and/or new stable ones.
  • - Absolutely stable particles must be neutral and weekly interacting. Hence escape detector giving missing ET.

SUSY: what will it look like?







  • The SUSY cross-section at the LHC is dominated by associated strong production of gluinos and squarks. SUSY might be observed with modest integrated luminosity
    • However, much work will be necessary to check our understanding of the detectors and the physics backgrounds before announcing any discovery!
  • SUSY could provide quite spectacular events with many leptons as well as jets and missing transverse energy.
  • mSUGRA: minimal Supergravity used as a standard benchmark model.
inclusive reach in msugra parameter space
Inclusive reach in mSUGRA parameter space
  • mSUGRA framework: five free parameters: m0, m1/2, A0, tan(β), sgn(µ)
  • Map of discovery potential corresponding to a 5σ excess above background in mSUGRA m0 – m1/2 parameter space for the ATLAS experiment.

only statistical errors included;

detector effects simulated using fast simulation;

no pile-up included;

SM background taken into account: tt, W+jet, Z+jet and QCD


  • jets + ETmiss channel (no lepton requirement) gives greatest discovery potential!


  • next greatest discovery potential: lepton veto channel (‘0l’).
inclusive reach in msugra parameter space12
Inclusive reach in mSUGRA parameter space
  • Map of discovery potential corresponding to a 5σ 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 (< one week) → ~1300 GeV


Inclusive reach in mSUGRA parameter space

  • Map of discovery potential corresponding to a 5σ excess above background in the mSUGRA parameter space for the CMS experiment.
  • Signal selection: performance similar to that found in ATLAS studies!

Best channel: jets + ETmiss channel(no lepton requirement)

  • A factor two increase or decrease in total background cross-section results in small effect on overall discovery potential (few tens of GeV).
  • Similar reach in gluino and squarks mass should apply to any model in which they decay into an invisible and relatively light LSP.
  • If R parity is violated the presence of additional leptons make the discovery easier!

For example:


SUSY mass scale measurements: MSUSY

m0=70GeV, m1/2=350GeV, A0=0, tan β=10

  • Define effective mass variable (measurement):



SUSY signal

(full simulation)

(pTjeti: transverse momentum of jet i)

  • The effective Mass gives a handle on the SUSY mass scale (Hinchliffe et al., Phys. Rev. D55 (1997) 5520):


low mass scales (~TeV)





SM background

(PS event generator & fast simulation)

  • The peak of the distribution of Meff values for SUSY events should lie at around twiceMSUSY due to the kinematics of the SUSY particle decay processes.
  • Cuts to reject SM background
    • 4 jets with pT > 50GeV
    • 2 jets with pT > 100GeV
    • ETmiss > max(0.2Meff,100GeV)
    • no lepton

SUSY mass scale measurements

  • The presence of a high mass Lightest Supersymmetric Particle (LSP) reduces the number and pT of observed jets.
  • In practice, preferable to consider measurements of an effective mass scale (MeffSUSY) which takes the LSP mass into account.

systematic errors not included;

Scatter plots: correlation between Meff and MeffSUSY


SUSY mass scale measurements

  • Signal / background separation: looks possible but need to verify new multi-parton (ME-PS matching) generators.
  • PS event generators: good in the collinear region but problem in high-pT region!

“no lepton mode”

“1 lepton mode”



Number of events / 400GeV

Number of events / 400GeV

Meff (GeV)

Meff (GeV)

  • Background increases by a factor of 2 – 4 compared to PS prediction.
  • Signal reduced to 20 – 40% of no lepton mode. Backgrounds are suppressed by a factor of 20 – 30!
  • One lepton mode gives clean discovery.
  • Depending on Meff signal/background separation can be severely affected!

Dilepton invariant mass

  • After initial discovery of SUSY the measurement of the sparticle masses will be the next step.
  • In mSUGRA and most SUSY models, all SUSY particles decay to invisible

no mass peaks!

  • Specific decays can often be identified.
  • Use kinematic endpoints to measure mass combinations.
  • Note: ATLAS and CMS have comparable acceptance for e and μ.

Dilepton invariant mass

  • Remove SUSY/SM BG using Opposite Flavor/Opposite Sign (OF/OS) pairs. Plot:
  • Event selection: Electrons and muons with PT ≥ 20 GeV
    • Separate leptons from jets by ΔR > 0.4

m0=100GeV, m1/2=300GeV, A0=-300, tan β=6


  • Only SUSY signal events.
  • No SM background cuts.
  • Fit a triangular shape distribution convoluted with a Gaussian.

mllmax = 100.31 GeV (input)

Medge = 100.25 ± 1.14 GeV (edge position)




  • After SM background cuts & dilepton selection.
  • Reduced statistics but triangular shape still visible.
  • Fitted value after cuts:

mllmax = (99.8±1.2) GeV


mll (GeV)


Dilepton endpoints observable over wide range of mSUGRA parameter space scanned with fast simulation.


  • This technique (kinematic endpoints) can be combined with the analysis of different end-points to constrain sparticle masses!

Right-handed squark mass



  • mSUGRA (R-parity conserved): sparticles are pair produced and cascade decay to the LSP.
  • mSUGRA – right handed squark: usualy large
  • The signal is two hard jets plus large ETmiss
  • Event selection: ETmiss> 200 GeV

Two jets with ET >150 GeV

No reconstructed electrons or muons

  • Calculate the stransverse mass of the two hard
  • jets. The endpoint gives the mass of right-handed
  • squarks


  • Take from dilepton and dilepton+jet measurements.
  • If is known, is obtained from the endpoint of the distribution.
  • Fitted endpoint:
  • Good agreement with actual value:

SUSY Spin Measurement

  • If SUSY signals are observed at the LHC, it will be vital to measure the spins of the new particles to demonstrate that they are indeed the predicted super-partners.
  • Angular distributions in sparticle decays lead to charge asymmetry in lepton-jet invariant mass distributions. The size of the asymmetry is proportional to the primary production asymmetry between squarks and anti-squarks
  • LHC will generate more squarks than anti-squarks (pp collider)!


  • l(near)q invariant mass distribution measure angular distribution of products of decays, and hence spin.


Lepton charge asymmetry

Parton-level result (rescaled!)

  • Cuts to reject SM background
    • 4 jets with pT > 50GeV
    • 1 jets with pT > 100GeV
    • ETmiss > max(0.2Meff,100GeV)
    • Meff > 400GeV
    • 2 leptons: same family & opposite signs, pT > 10GeV

Detector simulation (500fb-1)

fast simulation

Spin correlations suppressed!



  • Standard Model is very successful but fails to address several crucial issues. Models of physics beyond the Standard Model at TeV mass scale have been ongoing for at least 25 years. SUSY is a good candidate!
  • The final word can only come from experiments capable of probing TeV mass scale. LHC can potentially start answering some of our questions on physics BSM at some point next year.
  • ATLAS and CMS:
    • Observe squarks and gluinosbelow~2.5 TeV
    • Accurately measure squark, slepton and neutralino masses using cascade decays (provided chains are sufficiently long and rates are favourable)
    • Determine spin of neutralinos
  • SUSY discovery is possible in other models which I have not covered here:
    • Gauge Mediated Supersymmetry Breaking (GMSB)
    • Anomaly Mediated Supersymmetry Breaking (AMSB)
    • R-Parity Violation
searching for susy
A key signature for SUSY is large missing transverse energy associated with the non-interacting Lightest SUSY Particle (LSP) that is stable under the assumption of R-parity conservation

Ingredients for good missing-ET resolution are good hadronic calorimeters and with “hermetic” coverage

Note that LSP is candidate for dark matter

Searching for SUSY
  • Need calorimeter coverage up to |η| ~ 5, otherwise high-pT jets outside acceptance give large fake missing ET signature
msugra parameters
mSUGRA parameters:
  • m0 – common scalar mass
  • m1/2 – common gaugino mass
  • tan β – ratio of the vacuum expectation values of the two Higgs doublets in the model
  • A0 – common trilinear coupling
  • sgn μ – higgsino mass parameter

R-Parity: Conservation/Violation

  • R=+1 for Standard Model particles
  • R= -1 for SUSY particles
  • Two main SUSY scenarios: (RPV/RPC)
    • RP-Conserving
    • RP-Violating