dark matter role of lhc

8/8/2019 Dark Matter Role of Lhc

http://slidepdf.com/reader/full/dark-matter-role-of-lhc 1/23

Seminar 290E Presentation Benjamin Hooberman

Understanding Dark Matter:

The Role of the LHC




DISCLAIMER

• NONE of the material presented in this talk is my

original work. (I am not even a member of an

LHC group). Plots and material have been taken

from the references listed at the end of this talk.




The Current Situation

• What we know about dark matter:

– It’s there (and it’s neutral & stable)

– Hot WIMP DM (ie. neutrinos), MACHOs

ruled out→ no remaining SM candidate!

– Some remaining candidates: cold WIMP

DM, axions (I’ll focus on cold WIMP DM,

specifically in context of cMSSM)

– Constraints from astrophysical experiments

• What we don’t know about dark matter:

–

Still no direct detection* – WHAT IS IT?

– Detailed properties:

masses, couplings, etc.

LHC

*unless you believe DAMA

CHANDRA: The Bullet Cluster




Cosmic Microwave

Background Experiments

• CMB: the left-over radiation from the big bang• In early universe, DM provides “seeds” for density fluctuations in

photon-baryon fluid, reflected in CMB spectrum observed today

• Very accurate measurement of relic density: ΩCDMh2=0.110±0.006

•

No sensitivity to mass, interaction cross section, identity of DM




Direct Detection Experiments

•Seek direct detection of interaction between DM particle & detector

• No observation yet→ constrain cross section as a function of mass

• No sensitivity to relic density

• Detection would confirm presence of DM, but still wouldn’t know what it is




Indirect DM Searches

• Look for γ -ray annihilation products of WIMP DM: χχ→γγ /γ Z• (Also possible: detect ν’s from χχ→ννX conversion in the sun)

• DM already detected? Further experiments (ie. GLAST) neededto rule out alternative possible sources

EGRET: excess of 1-10 GeV γ ‘s from

Galactic Center→ ~80 GeV WIMP?

CANGAROO, VERITAS, HESS:

0.2-10 TeV γ ‘s from GC




The Role of the LHC

• If DM is made of WIMPs, they will be produced at LHC in abundance• Indirect DM detection using missing ET signatures

• Unique role of LHC: multiple measurements allow understanding of

underlying theory, determination of identity of DM




WIMP DM Signatures at LHC

• Consequences of R-parity conservation – SUSY particles produced in pairs

– Stability of LSP→ DM candidate (usually lightest neutralino)

• Squark/gluino pair production with cascade decays→

high pT jets+(possible)isolated leptons+ missing ET

χ0s escape

undetected:

missing ET

g-pair production

with subsequent

cascade decays

~

0

1χ




LHC DM Program (assuming cMSSM)

• Discover SUSY using jets+leptons+missing ET signature

• Identify SUSY LSP (usually ) as DM candidate particle

• Measure properties of , properties of particles coupling to

(I will review phenomenology and few key measurements)

• Use measurements to predict neutralino relic densityΩχh2

• Ωχh2= ΩCDMh2→ major break-thru in understanding DM

• Relevant measurements depend on physics mechanism which

determinesΩχh2: depends on point in cMSSM parameter space

cMSSM Parameters:

m0, m1/2, tanβ, A0, sign(µ):

SUSY masses, couplings

multiple

measurements

of SUSY

observables

Ωχh2

0

1χ 0

1χ 0

1χ

)n(nvσ3Hndt

dn2

χ (eq)

2

χ χ

χ −><−−=




cMSSM and Ωχh2

regions compatible with

observed DM relic density

m1/2

m 0

• Bulk region compatible withobserved relic density butdisfavored by MH, b→sγ

• Moving up in m0-m1/2 plane

increases sparticle masses,Ωχh2 increases

• Additional annihilationmechanism needed toreduceΩχh

2

• 3 possible regions whichprovide such mechanisms,represent various physicsprocess contributing to Ωχh

2

cMSSM Parameter

Space Projection




Bulk Region and Ωχh2



m1/2

m 0

• Ωχh2 depends on rate of

χχ→l+l- via slepton

exchange

• Annihilation rate depends

on slepton mass

cMSSM Parameter

Space Projection




• Search for decay chain

• Endpoint in dilepton mass distribution

depends on

• Endpoint in dilepton+jet mass distribution

depends on

• Two more quantities: min value of

dilepton+jet mass, max value of single

lepton+jet mass

• These 4 quantities can

be used to solve for

• Use these 4 parameters

to determine cMSSM

parameters

(Selected) Bulk Region Measurements

0

1

0

2 χ ~

lqlll

~

qχ ~

qq~ mm ±±

→→→

)M(χ ),M(χ ),l~

M(1

0

2

0

±

)q~M(),M(χ ),M(χ ),l~

M( 1

0

2

0

±

)q~M(),M(χ ),M(χ ),l~

M( 1

0

2

0

±

Dilepton Mass (GeV) Dilepton+Jet Mass (GeV)

Choose jet giving

minimum M(llj)

opposite sign, same

flavor lepton pair

(e+e-/µ+µ-/τ+τ-)




Co-Annihilation Region and Ωχh2



m1/2

m 0

•Stau mass nearly degenerate withneutralino mass

• enhanced, reduces Ωχh2

• Annihilation rate depends on:

• Excess staus removed via

cMSSM Parameter

Space Projection

τγχ τ~ →

)M(χ )τ~M(∆M 0

1−=

τττ~τ~ →




(Selected) Co-Annihilation

Region Measurements

• Search for decay chain

• Similar to bulk region decay chain,

but only τ-pair excess because:

• provides soft τ

• ττ only excess with soft τ: smoking

gun for co-annihilation region

• PT distribution of soft τ provides

information on

0

1

0

2 χ ~τqτττ~qχ ~qq~mm ±±

→→→

τ+τ- pair

1)ττχ ττ~BR(χ 0

1

0

2 ≈→→

)M(χ )τ~M(∆M 0

1−=

soft τ

0

1χ ττ~→




Focus Point Region and Ωχh2



m1/2

m 0

• Neutralino acquires large

Higgsino component

• Decay to vector boson pairs

enhanced, reduces Ωχh2

cMSSM Parameter

Space Projection




(Selected) Focus Point

Region Measurements•

Phenomenology much more difficult toprobe at LHC

– heavy sfermions

– suppressed

• Might probe Ωχh2, might just observe

excess of SUSY events, might not even

observe SUSY

• Search for decays,

M(ll) endpoint related to:

• gluino → neutralino decays enhanced,

search for decays with top pair. M(tt)

endpoint related to:

0

1

0

2 χ llll~

χ mm ±± →→

0 10000 20000 30000 40000 50000 60000 70000 80000 900000 10000 20000 30000 40000 50000 60000 70000 80000 900000

50

100

150

200

250

300

0

1

0

3 χ llχ m±→

0

1

0

2 χ llχ m±→

M(ll) GeV

300 400 500 600 700 800 900 1000 11000

500

1000

1500

2000

2500

3000

3500

4000

Minv totale

ttχ g~ 0

3→

ttχ g~0

2→

ttχ g~ 0

4→

ttχ g~0

1→

2,3)(n )M(χ )M(χ 0

1

0

n =−

2,3,4),1(n )M(χ )g~M( 0

n =−

M(ll) (GeV)

M(tt) (GeV)

ATLAS

300 fb-1

(~5 years data)

No SM BKG

0

1

0

n χ llχ m±→




Rapid Annihilation Funnel and Ωχh2



m1/2

m 0

•Coincidentally, M(A) ≈ 2M(χ)

• Decay via s-channel A/H

resonance enhanced,

reducesΩχh2

• Annihilation cross section

depends on R=M(χ)/M(A), Γ (A)

cMSSM Parameter

Space Projection




• Large squark/gluino masses:

discovery at LHC not guaranteed

• Can also search for decay chain

(same as co-annihilation region), but

τ is harder: larger

• Large tanβ, so A is observable for

MA<700-750 GeV

• Measure M(A), check if M(A)≈2M(χ)

– A→µµ (extremely precise, not alwaysavailable depending on MA, tanβ)

– A→ττ (precision limited to few %)

(Selected) Rapid Annihiliation

Funnel Region Measurements

τ+τ- pair

hard τ

0

1

0

2 χ ~

τqτττ~

qχ ~

qq~ mm ±±

→→→

)M(χ )τ~M(∆M 0

1−=




Complementarity: Hadron & Lepton Colliders

• Hadron Colliders: Discovery Machines

– Large center-of-mass energy

– Large cross section for production of

new states• Lepton Colliders: Precision Probes

– Tunable center-of-mass energy

– Known initial state energy/angular

momentum – Less QCD backgrounds/uncertainties

Mass of Dark Matter

C o s m i c A b u n d a n c e

Schematic Representation Only

(no units)




Distinguishing UED from SUSY

• Universal Extra Dimensions: analtenative BSM scenario to SUSY

• Also introduces new (Kaluza-Klein)partners to existing SM particles

• Provides natural DM candidate

(usually KK partner of U(1) hyper-charge boson)

• Signature similar to SUSY(jets+leptons+missing ET)

• Crucial difference: KK-partners have

same spin, SUSY partners have ±½spin as SM particles

• Difficult to distinguish at LHC,straightforward at lepton collider

SUSY

UED

s (GeV)

σ ( e + e - →

X X ) ( f b )

DM candidatesmuon

KK muon




Ωχh2 Determination

LHC Bulk Region

5 GeV erroron ττ edge

0.5 GeV error

on ττ edge

LHC+ILC Focus Point Region

~20% uncertainty ~10% uncertainty

LHC precision

severely limited

Need ILC for precision

measurementLHC

LHC+ILC 0.5 TeV

LHC+ILC 1.0 TeV

Ωχh2

Ωχh2 Ωχh

2

P r o b .

D e n s i t y d P / d x




Conclusions

• LHC will play critical role in understanding DM

• Results to be compared with results from astrophysical

experiments

• Agreement between collider-based experiment &

astrophysical experiments required to confirm presence

and determine identity of DM→→→→ major break-thru in

understanding DM.

• Precision of LHC measurements to be improved upon

(perhaps…) with future lepton collider




References

• M. Battaglia, I. Hinchliffe, D. Tovey. Cold Dark Matter and theLHC. arXiv:hep-ph/0406147

• D. Toback. Measuring the Dark Matter Relic Density at the

LHC. ICHEP 2008.

•

T. Lari. Focus Point Studies at LHC. Euro-GDR 2007.• A. Belyaev. Exploring SUSY Focus Point Region at the LHC.

Soton-HEP Seminar.

• G. Gelmini. Search for Dark Matter. ICHEP 2008.

• H. Murayama. The Next Twenty Years in Particle Physics. UM

Physics Dept. Colloquium 2004.

• Nojiri, Polisello, Tovey. Constraining Dark Matter in the MSSM

at the LHC. arXiv:hep-ph/0512204v1

dark matter role of lhc

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