search for dark matter at the lhc using missing transverse energy

Search for dark matter at the LHC using missing transverse energy

Sarah Alam Malikon behalf of the CMS collaboration

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Dark matter- There is strong evidence for the existence of dark matter - evidence from bullet cluster, gravitational lensing, rotation curves- probe dark matter via direct detection, indirect detection and colliders

Direct detection experiments

- aim to observe recoil of dark matter (DM) off nucleus.- excesses observed by several experiments, not confirmed by others

- low mass region not accessible to direct detection experiments

- limited by threshold effects, backgrounds, etc.

- less sensitive to spin-dependent couplings

- tremendous need for independent verification from non-astrophysical experiments

Colliders provide alternative, complementary way to search for dark matter

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Dark matter production

Direct searches Collider searches

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Phenomenology

Bai, Fox and Harnik, JHEP 1012:048 (2010)

In framework of effective theory, assume DM is a Dirac fermion and interaction is characterized by contact interaction,

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Phenomenology


heavy mediator can be integrated out

⇤ = M/pg�gq

SM

SM

DM

DM

Set mass of mediator (M) to very high value

g𝝌gq


6

Phenomenology


SM

SM

DM

DM


g𝝌gq

Nature of mediator will determine form of SM-DM couplings.

Consider two possibilities:(a) vector mediator(b) axial-vector mediator


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Phenomenology


SM

SM

DM

DM


g𝝌gq

(a) For vector mediator, effective operator

OV =(�̄�µ�)(q̄�µq)

⇤2spin-independent


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Phenomenology


SM

SM

DM

DM


g𝝌gq

(b) For axial-vector mediator, effective operator

spin-dependentOAV =(�̄�µ�5�)(q̄�µ�5q)

⇤2


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Dark matter production at the LHC

Dark matter pair production at LHC- radiation of a photon/jet from initial state- DM particles produce missing energy

monophoton +MET monojet +MET

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Search for dark matter in monojet and monophoton events at CMS

Simple and striking signatures

Highest pT monophoton event,pT(photon) = 384 GeV, MET = 407 GeV

A monojet event,pT(jet) = 331 GeV, MET = 359 GeV

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Search for dark matter in monojet events

Select topology

• Large missing energy, Met > 200 GeV

• One energetic jet, pT > 110 GeV, |η| < 2.4

• Allow one additional jet (if it has pT > 30 GeV)

• Veto event if it has more than 2 jets

Reject background• QCD- Δφ(j1,j2) > 2.5 -remove events with back to back jets• EWKlepton rejection -reject events with isolated electrons, muons -veto events with isolated tracks

Basic Selection and Event Cleaning

• Primary vertex

• cuts based on jet constituents (charged and

neutral hadron and electromagnetic energies ),

removes cosmics, instrumental backgrounds

Selecting monojet events

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Select topology







• Primary vertex





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• Topology selection gets rid of most ttbar, QCD multijet

• remaining backgrounds from QCD dijet and EWK

After selecting monojet topology

Jet Multiplicity1 2 3 4

Eve

nts

1

10

210

310

410

510

610

710 νν→Z

νl→W

tt

QCD

-l+l→Z

Data

DM-AVd m=1GeV

3δ2DADD M

CMS Preliminary

=7 TeVs at -1

L dt = 4.7 fb∫

NJet

≥

Select topology







• Primary vertex





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After full event selection

[GeV] TmissE

200 400 600 800 1000

Eve

nts

/ 2

5 G

eV

1

10

210

310

410

510

νν→Z

νl→W

tt

QCD

-l+l→Z

Data

DM-AVd m=1GeV

3δ2DADD M

CMS Preliminary

=7 TeVs at -1

L dt = 4.7 fb∫

MET+Lep

) [GeV/c]1

(JetT

p0 200 400 600 800 1000

Eve

nts

/ 2

5 G

eV

/c

1

10

210

310

410

νν→Z

νl→W

tt

QCD

-l+l→Z

Data

DM-AVd m=1GeV

3δ2DADD M

CMS Preliminary

=7 TeVs at -1

L dt = 4.7 fb∫

Jet1Pt

- Remaining backgrounds after full event selection from Z(vv) (~70%) and W+jets (~30%),

use data-driven methods to estimate these.

- Other backgrounds from QCD, ttbar Z+jets and single top negligible (~1%), taken from

MC.

- Optimize value of Met cut for best sensitivity to DM search, Met > 350 GeV

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Data-driven background estimation

µµ

€

N(Z→νν) =NZ

obs − NZbgd

AZ •εZ•R Z→νν

Z→ ll⎛

⎝ ⎜

⎞

⎠ ⎟

]2M [GeV/c60 70 80 90 100 110 120

2E

ven

ts /

2 G

eV

/c

10

20

30

40-l+l→Z

Data

CMS Preliminary

=7 TeVs at -1

L dt = 4.7 fb∫

<120T

60<mT

Zleplep m

• use sample of Z→µµ events to estimate Z→𝜈𝜈

• select two isolated muons, opposite sign charge

• invariant mass 60 - 120 GeV

• uncertainty of 10%, mostly statistical

W+Jetsa lost e (μ)‘lost’ e/μ

]2 [GeV/cTM0 50 100 150 200

2E

vent

s / 1

0 G

eV/c

1

10

210

310

410νl→W-l+l→Z

tttQCDData

CMS Preliminary

=7 TeVs at -1

L dt = 4.7 fb∫

TWlepnu m

• lepton from W decay is ‘lost’ because its not within detector acceptance or not reconstructed/isolated

• use muon+jets control sample, require 50 < MT < 100

• correct for inefficiencies to estimate remaining W+jets background

• 11% uncertainty on background

W+jets from W→µνZ→νν from Z→µµ

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Monojet Results

No excess of events over expected SM backgrounds

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Search for dark matter in monophoton events

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Selecting monophoton events

[GeV]TE200 300 400 500 600 700

Even

ts /G

eV

-410

-310

-210

-110

1

10

210

[GeV]TE200 300 400 500 600 700

Even

ts /G

eV

-410

-310

-210

-110

1

10

210CMS Preliminary = 7 TeVs

-1 Ldt = 4.7 fb∫=1 TeV, n=3)

XPSM + ADD (MDATA Total uncertainty on Bkg

γνν →γ Zν e→W

MisID Photon (MJ)γ+jets, Wγ

BeamHalo

[GeV]Tγp

200 300 400 500 600 700

Even

ts /G

eV-410

-310

-210

-110

1

10

210

[GeV]Tγp

200 300 400 500 600 700

Even

ts /G

eV-410

-310

-210

-110

1

10

210CMS Preliminary = 7 TeVs

-1 Ldt = 4.7 fb∫=1 TeV, n=3)

XPSM + ADD (MDATA Total uncertainty on Bkg

γνν →γ Zν e→W

MisID Photon (MJ)γ+jets, Wγ

BeamHalo

Photon selection* pT > 145 GeV* Central region of detector, |η| < 1.4442* Shower shape in calorimeter consistent with photon

MET* MET > 130 GeV

Remove excessive hadronic activity* No jet with pT > 40 GeV and |η| < 3.0* No track with pT > 20 GeV with ΔR < 0.04 from photon

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Backgrounds to monophoton search

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Monophoton results

No excess of events over expected SM backgrounds

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Limit Setting - Monophoton

• Signal events generated using MADGRAPH interfaced with PYTHIA• Mediator mass set to 10 TeV• Obtain Acceptance x Efficiency using MC samples passed through monophoton

selection• Upper limits on Monophoton cross sections converted to lower limits on Λ• Lower limits on Λ then translated to limit on dark matter-nucleon cross section

as a function of dark matter mass

�SI = 9 µ2

⇡⇤4 �SD = 0.33µ2

⇡⇤4where µ =

m�mp

m� +mp

Spin dependent Spin independent


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Limit Setting - Monojet

• Signal events generated using MADGRAPH interfaced with PYTHIA• Mediator mass set to 40 TeV• Upper limits on Monojet cross sections translated into lower limits on Λ• Lower limits on Λ then translated to limit on dark matter-nucleon cross section

as a function of dark matter mass

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]2 Mass [GeV/cχ1 10 210 310

]2

-Nucl

eon C

ross

Sect

ion [cm

χ

-4610

-4410

-4210

-4010

-3810

-3610

-3410

-3210

-3010CMS Monojet, 90% CLCMS Monophoton, 90% CL

CDMSII 2011Picasso 2009COUPP 2011

CMS Preliminary=7 TeVs at

-1L dt = 4.7 fb∫

Spin Dependent

Dark matter spin dependent results

Limits represent the most stringent constraints by several orders of magnitude over entire 1 -1000 GeV mass range

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Dark matter spin independent results

]2 Mass [GeV/cχ1 10 210 310

]2

-Nucl

eon C

ross

Sect

ion [cm

χ

-4610

-4410

-4210

-4010

-3810

-3610

-3410

-3210

-3010CMS Monojet, 90% CLCMS Monophoton, 90% CL XENON-100 CoGeNT 2011CDMSII 2011 CDMSII 2010

CMS Preliminary=7 TeVs at

-1L dt = 4.7 fb∫

Spin Independent

Best constraints for low mass dark matter, below 3.5 GeV, a region as yet unexplored by direct detection experiments

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Conclusions

• Presented searches for dark matter in the monojet and monophoton channels at CMS using 4.7 fb-1 of data

• Observed data consistent with predictions from SM backgrounds

• Set limits on dark matter-nucleon scattering cross section as a function of dark matter mass

• For spin-dependent models, limits represent the most stringent constraints by several orders of magnitude over the entire 1- 1000 GeV mass range

• For spin-independent models, best constraints for low mass dark matter, below 3.5 GeV, a region as yet unexplored by direct detection experiments

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WIMP Mass [GeV]1 10 210 310

]2W

IMP-

Nuc

leon

Cro

ss S

ectio

n [c

m

-4510

-4410

-4310

-4210

-4110

-4010

-3910

-3810

-3710

-3610

-3510

-3410

-3310Spin Independent

CMS (90%CL)CMS (95%CL)XENON100CDMSII 2011CDMSII 2010CoGeNT 2011

CMS Preliminary = 7 TeVs

-1 Ldt = 4.7 fb∫

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WIMP Mass [GeV]1 10 210 310

]2W

IMP-

Nuc

leon

Cro

ss S

ectio

n [c

m

-4510

-4410

-4310

-4210

-4110

-4010

-3910

-3810

-3710

-3610

-3510

-3410

-3310

Spin DependentCMS (90%CL)CMS (95%CL)COUPP 2011PICASSO 2009CDMSII 2011

CMS Preliminary = 7 TeVs

-1 Ldt = 4.7 fb∫

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Conclusions

Presented searches for new physics in the monojet and monophoton channels using 4.7 fb-1 of data

Predictions for SM backgrounds consistent with observed data, no excess found

Proceeded to set limits on

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A Monojet event

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A monophoton event

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Phenomenology

In framework of effective theory, pair production of dark matter particles (assumed to be Dirac fermion) characterized by contact interaction, heavy mediator particle is integrated out

Cross section depends on mass and scale


Vector operator

Axial-vector operator

search for dark matter at the lhc using missing transverse energy

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