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Dark Matter at LHC. Introduction Hierarchy problem & motivation for dark matter. SUSY. Alternative BSM LHC & ATLAS Performance Outlook & Summary. Introduction. Hierarchy problem Why does the SM Higgs remain light?

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dark matter at lhc
Dark Matter at LHC
  • Introduction
    • Hierarchy problem & motivation for dark matter.
    • SUSY.
    • Alternative BSM
  • LHC & ATLAS Performance
  • Outlook & Summary

Tony Weidberg

  • Hierarchy problem
    • Why does the SM Higgs remain light?
    • Expect radiative corrections  mass to highest scale in the theory (eg M_planck).
    • Requires improbable fine tuning
  • Solution requires new physics @ TeV scale.
    • eg SUSY
    • If we assume R parity conservation  LSP is stable  candidate for dark matter.
    • Other BSM theories also provide dark matter candidates
      • Eg UED: lightest particle has negative KK parity and would therefore be stable  dark matter candidate.

Tony Weidberg

  • If squarks or gluinos <~ 1 TeV  large s  high rates at LHC.
  • Cascade decays to LSP.
  • Assume R parity  LSP stable  Missing transverse energy (MET) in detector
  • Very generic SUSY search:
    • Multi jets + MET in excess of SM background
  • Details are model dependent
  • Can’t explore 105 dimensional parameter space of MSSM so need some unified model.
  • Look at SUGRA as an example.
  • 5 parameters, m0, m1/2, tanb, A0, sign(m).
  • LSP is dark matter candidate but don’t want too much!
    • Restricts regions in parameter space so that there is efficient annihilation of LSP.
  • SU1 m0=70 GeV m1/2=350 GeV tanb=10
    • Coannihilation: near degenerate
  • SU2 m0=3550 GeV m1/2 = 300 GeV tanb=10
    • high higgsino 
  • SU3 m0=100 GeV m1/2=300 GeV tanb=6
    • Bulk: LSP annihilation exchange of sleptons.
  • SU4: low mass point close to TeV limits.
  • SU6 m0=320 GeV, m1/2=375 GeV tanb=50
    • enhances annihilation.
  • SU8.1 m0=210 GeV m1/2=360 GeV tanb=40.
    • Coannhiliation with small
  • SU9 m0=300 GeV, m1/2=425 GeV tanb=20
    • Enhanced Higgs production.
useful definitions
Useful Definitions
  • MET:
  • Requires good 4p calorimeters + muons
  • Infer presence of LSP from large MET
  • Can’t reconstruct mass event by event because of MET is only in transverse plane.
  • Define Transverse Mass
  • For 2 body decays  end point at mass of parent (eg MW).
definitions 2
Definitions (2)
  • Effective Mass
  • Meff discriminates between SUSY and SM.
  • Peak in Meff gives first crude estimate of SUSY mass scale.
  • More complicated variables required for mass determinations in events with two invisible particles eg “stranverse” mass
  • Cosmics, beam halo, beam gas.
  • Fake MET from QCD jets
  • Real MET from SM backgrounds eg
    • W -> l n
    • t tabr, t b W, Wl n
    • Z + jets, Z nn
  • Reliable estimates essential for all backgrounds before SUSY discovery can be claimed!

Time difference between scintillators on two sides ATLAS

sm backgrounds
SM Backgrounds

Background is cocktail of different SM processes

S/B high at large Meff but still need data driven estimates.

qcd background 1
Need data driven approach.

Use pT balance in photon jet events

Photon well measured  resolution due to jets.

Gaussian fits give s vs pt

QCD Background (1)

Resolutions vs photon pT

qcd background 2
QCD Background (2)
  • Estimate non-Gaussian tails from 3 jet events
    • Badly measured jet direction close to MET
  • Combine Gaussian & non-Gaussian tails  Jet Transfer Function (JTF)
  • Estimate QCD background from data & JTF
w ttbar backgrounds
W & ttbar Backgrounds
  • Use MT<100 GeV to define control region



Count events in control region  predict SM background in signal region

More sophisticated variations to allow for signal contamination of control region.

no lepton mode
No Lepton Mode

Background is cocktail of different SM processes

Z nn irreducible

W/top from lost leptons

S/B high at large Meff but still need data driven estimates.

reach in sugra space
Reach in SUGRA space

500 pb-1 @ 7 TeV

Aim 1000 pb-1 by end 2011

Gluino 0.5 TeV

Squark 0.5 TeV

jet PT > [100,40,40,40] GeV

ETmiss > 80 GeV

reach for squarks gluinos
Reach for squarks & gluinos
  • With 500 pb-1 @ 7TeV:
  • Could exclude up to
    • - msquark ~ 700 GeV
    • - mgluino ~ 600 GeV
  • Improve limits of Tevatron

Tony Weidberg

mass fitting
Mass Fitting
  • Can’t easily determine mass because don’t know how much MET carried away by each LSP.
  • Can determine mass differences by fitting end points of spectra.
  • eg squark decay chain:
end point analysis
End Point Analysis
  • End point for

Tony Weidberg

llq end points
llq End Points

M(lqq) end point gives mass difference

Tony Weidberg

susy higgs
SUSY Higgs
  • BR( )

can be large.

  • Clean signal for b bar because SM suppressed by MET cut.

M(b bar)

susy mass determination
SUSY Mass determination
  • Can use measured end points in global fit  SUSY masses:
  • Results for 1 fb-1 SU3:
mass determinations
Mass Determinations
  • More sophisticated tools being developed.
  • Use event by event information
  • M3c variable: lower and upper bounds for mass of LSP by varying fraction of Etmiss given to two LSPs
    • event by event subject to constraints:
    • Momentum conservation
    • MET
    • Mass differences (already measured from end point analysis).
  • Might get much higher precision on mass LSP
  • E.g. Barr, Pinder & Serna, arXiv:0811.2138v1 claim precision ~ 1 GeV for 100 fb-1 for SPS1a.

Tony Weidberg

lhc performance
LHC Performance
  • Number of bunches increasing
  • 36 colliding bunches Lmax ~ 10 31 cm-2 s-1

Tony Weidberg

atlas cms
  • Will show results from ATLAS
  • Similar quality results from CMS
    • ICHEP 2010: http://indico.cern.ch/contributionDisplay.py?contribId=75&confId=73513
  • SUSY search needs
    • good missing Et
    • Jets
    • Leptons
    • b-tagging
    • tau id

Large tail at high MET removed by cleaning cuts

Rates agree with MC

Tony Weidberg

met resolution
MET Resolution
  • Fit resolution in x/y in slices of SET.
  • Data and MC in excellent agreement.
qcd jets
QCD Jets
  • Spectrum for di-jet mass agrees well with LO QCD calculations.
  • Extends beyond 2 TeV!

Tony Weidberg

dijet mass spectrum
Dijet Mass Spectrum
  • Already allowed best limits on q* production
  • Mq*> 1.26 TeV.

Tony Weidberg

w m n
W m n
  • Low MET background dominated
    • Fit shape of QCD background to control region at low MET
    • Very clean signal after MET>25 GeV cut.

Tony Weidberg

w e n
W e n
  • Very similar story to muon channel
  • Data driven background estimates  very clean signal after MET>25 GeV cut

Tony Weidberg

b tagging
  • B lifetime  separate b jets from light quarks using displaced vertices.

Tony Weidberg

b tagging1
  • Use signed transverse impact parameter of 3 tracks in jet to define jet probability for light quark jets.
  • Clear signal for b jets at low jet probability.

Tony Weidberg

top physics
Top Physics
  • Signals in e/m + jets, reasonable S/B in lepton+4 jets after b-tagging.
  • Better S/B but smaller BR for di-lepton channels

Very early look at a SUSY search …

  • After SUSY cuts:
  • jet PT > [70,30,30,30] GeV
  • ETmiss > 40 GeV
  • Df(jet,ETmiss) [0.2,0.2,0.2]
  • ETmiss/Meff cut > 0.2
outlook summary
Outlook & Summary
  • Large cross section for strongly interacting particles at LHC  high rates for squarks and gluons.
  • Significant improvement in Tevatron limits after 2010/11 run.
  • Large mass reach for SUSY discovery after upgrade to 14 TeV and full luminosity.
  • Can do much more than just discover SUSY: can determine many parameters, eventually pin down mass of dark matter candidate.
  • Alternative BSM that can also provide dark matter candidates will also be observable through MET.
  • LHC ramp up going well.
  • ATLAS & CMS working well and producing physics results
fitted susy masses
Fitted SUSY Masses
  • Fits for SU3 1 fb-1 and SU4 0.5 fb-1

Tony Weidberg

msugra parameters
mSUGRA Parameters
  • SU3 1 fb-1
  • M0 and M1/2 well determined.
  • Some constraint on tan beta
  • Little constraint on A0
  • Sign (mu) not fixed.

Tony Weidberg

more info
More Info
  • LHC: Steve Myers talk at ICHEP
    • http://indico.cern.ch/contributionDisplay.py?contribId=73&confId=73513
  • ATLAS MC studies CSC book
    • http://cdsweb.cern.ch/record/1125884?ln=en
  • ATLAS tau-id see note and event display
    • https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2010-086/ATLAS-CONF-2010-086.pdf
determining masses of invisible particles 2
Determining Masses of Invisible Particles (2)
  • Vary fraction of MET assigned to two LSPs & find lower bound (upper bounds) subject to constraints
example fits
Example Fits

MY=200 GeV

250 GeV