Dark matter at lhc
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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

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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

  • 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)

  • 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

Background is cocktail of different SM processes

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

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)

  • 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

  • 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

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

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

  • With 500 pb-1 @ 7TeV:

  • Could exclude up to

    • - msquark ~ 700 GeV

    • - mgluino ~ 600 GeV

  • Improve limits of Tevatron

Tony Weidberg

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 for

Tony Weidberg

llq End Points

M(lqq) end point gives mass difference

Tony Weidberg

SUSY Higgs

  • BR( )

    can be large.

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

M(b bar)

SUSY Mass determination

  • Can use measured end points in global fit  SUSY masses:

  • Results for 1 fb-1 SU3:

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

  • Number of bunches increasing

  • 36 colliding bunches Lmax ~ 10 31 cm-2 s-1

Tony Weidberg


  • 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

  • Fit resolution in x/y in slices of SET.

  • Data and MC in excellent agreement.

QCD Jets

  • Spectrum for di-jet mass agrees well with LO QCD calculations.

  • Extends beyond 2 TeV!

Tony Weidberg

Dijet Mass Spectrum

  • Already allowed best limits on q* production

  • Mq*> 1.26 TeV.

Tony Weidberg

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

  • Very similar story to muon channel

  • Data driven background estimates  very clean signal after MET>25 GeV cut

Tony Weidberg

Z ee and Z  m m

Tony Weidberg


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

Tony Weidberg


  • 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

  • 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

  • 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

Backup Slides

Fitted SUSY Masses

  • Fits for SU3 1 fb-1 and SU4 0.5 fb-1

Tony Weidberg

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

7 TeV vs 14 TeV

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 (1)

Determining Masses of Invisible Particles (2)

  • Vary fraction of MET assigned to two LSPs & find lower bound (upper bounds) subject to constraints

Example Fits

MY=200 GeV

250 GeV

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