8 Oct 07 Feng 1

THE SEARCH FOR DARK MATTER

Jonathan Feng

University of California, Irvine8 October 2007

CSULB Colloquium

Graphic: N. Graf

8 Oct 07 Feng 2

WHAT IS THE UNIVERSE MADE OF?

An age old question, but…

Recently there have been remarkable advances in our understanding of the Universe on the largest scales

We live in interesting times: for the first time in history, we have a complete inventory of the Universe

8 Oct 07 Feng 3

• Remarkable agreement

Dark Matter: 23% ± 4% Dark Energy: 73% ± 4% [Baryons: 4% ± 0.4% Neutrinos: ~0.5%]

• Remarkable precision (~10%)

• Remarkable results

The Inventory

8 Oct 07 Feng 4

Historical PrecedentEratosthenes measured the size of the Earth in 200 B.C.

• Remarkable precision (~10%)

• Remarkable result

• But just the first step in centuries of exploration

Alexandria

Syene

8 Oct 07 Feng 5

Dark Matter Questions

• What particle forms dark matter?• What is its mass?• What is its spin?• What are its other quantum numbers and interactions?• Is dark matter composed of one particle species or many?• How and when was it produced?

• Why does DM have the observed value?

• How is dark matter distributed now?• What is its role in structure formation?• Is it absolutely stable?

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Known DM properties

DARK MATTER

• Not baryonic

DM: precise, unambiguous evidence

for new particles

• Not hot

• Not short-lived

8 Oct 07 Feng 7

Dark Matter Candidates

• The Wild, Wild West of particle physics: axions, warm gravitinos, neutralinos, Kaluza-Klein particles, Q balls, wimpzillas, self-interacting particles, self-annihilating particles, fuzzy dark matter, superWIMPs,…

• Masses and interaction strengths span many, many orders of magnitude

• But independent of cosmology, new particles are required to understand the weak force

8 Oct 07 Feng 8

Naturalness and the Weak Scale

mh ~ 100 GeV, ~ 1019 GeV cancellation of 1 part in 1034

At Mweak ~ 100 GeV we expect new particles:supersymmetry, extra dimensions, something!

Classical

= +

= −

Quantum

eL eR

8 Oct 07 Feng 9

THE “WIMP MIRACLE”

(1) Initially, new particle is in thermal equilibrium:

↔ f f

(2) Universe cools:

N = NEQ ~ e m/T

(3) s “freeze out”:

N ~ const

(1)

(2)

(3)

8 Oct 07 Feng 10

• The amount of dark matter left over is inversely proportional to the strength of annihilation:

DM ~ 1/Av

• What’s the constant of proportionality?

• Impose a natural relation:

k2/m2

DOE/NSF HEPAP Subpanel (2005)

Remarkable “coincidence”: cosmology alone also tells

us we should explore the weak scale

[ba

nd

wid

th f

rom

k =

0.5

– 2

, S

an

d P

wa

ve]

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STABILITY

• DM must be stable

• Problems ↕

Discrete symmetry↕

Stability

• In many theories, dark matter is easier to explain than no dark matter

New Particle States

Standard ModelParticles

Stable

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DARK MATTER CANDIDATES

There are two classes of candidates that explain the “coincidence” and naturally give the right amount of dark matter:

WIMPs: weakly-interacting massive particles

SuperWIMPs: superweakly-interacting

massive particles

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WIMPs from SupersymmetryThe classic WIMP: neutralinos predicted by supersymmetry

Goldberg (1983); Ellis et al. (1983)

Supersymmetry: extends rotations/boosts/translations, string theory, unification of forces,… For every known particle X, predicts a partner particle X̃

Neutralino ( ̃, Z̃, H̃u, H̃d )

Particle physics alone is lightest supersymmetric particle, stable, mass ~ 100 GeV. All the right properties for WIMP dark matter!

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DM = 23% ± 4% stringently constrains models

Fe

ng

, Ma

tche

v, Wilcze

k (20

03

)Focus

point

region

Co-annihilation

region

Bulk

regionYellow: pre-WMAPRed: post-WMAP

Too much

dark matter

Cosmology excludes many possibilities, favors certain regions

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WIMPs from Extra Dimensions

Garden hose

• Extra spatial dimensions could be curled up into small circles.

• Particles moving in extra dimensions appear as a set of copies of normal particles.

mas

s

1/R

2/R

3/R

4/R

0

…

Servant, Tait (2002) Cheng, Feng, Matchev (2002)

DM

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WIMP Detection: No-Lose Theorem

f

fAnnihilation

Correct relic density Efficient annihilation then Efficient annihilation now Efficient scattering now

f

f

Scattering

Crossing

symmetry

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DIRECT DETECTION

• WIMP essentials:

v ~ 10-3 c

Kinetic energy ~ 100 keV

Local density ~ 1 / liter

• Detected by recoils off ultra-sensitive underground detectors

WIMPWIMP

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FUTURE DIRECT DETECTION

mSUGRA

FocusPoint

Region

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Indirect Detection

Dark Matter Madlibs!

Dark matter annihilates in ________________ to a place

__________ , which are detected by _____________ . some particles an experiment

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Dark Matter annihilates in the galactic center to a place

photons , which are detected by Cerenkov telescopes . some particles an experiment

HESS Telescope Array in Namibia

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Dark Matter annihilates in the center of the Sun to a place

neutrinos , which are detected by AMANDA, IceCube . some particles an experiment

(km -2 yr

-1) AM

AN

DA

in the Antarctic Ice

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IDENTIFYING WIMPS

If WIMPs contribute significantly to dark matter, we will see signals before the end of the decade:

Direct dark matter searches

Indirect dark matter searches

Tevatron at Fermilab

Large Hadron Collider at CERN (2008)

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What then?• Cosmology can’t

discover SUSY• Particle colliders

can’t discover DM

Lifetime > 10 7 s 1017 s ?

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THE EXAMPLE OF BBN

• Nuclear physics light element abundance predictions

• Compare to light element abundance observations

• Agreement we understand the universe back to

T ~ 1 MeVt ~ 1 sec

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DARK MATTER ANALOGUE

(1)

(2)

(3)

• Particle physics dark matter abundance prediction

• Compare to dark matter abundance observation

• How well can we do?

American Linear Collider Cosmology Subgroup

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Contributions to NeutralinoWIMP Annihilation

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WMAP(current)

Planck(~2010)

LHC (“best case scenario”)

ILC

LCC1

RELIC DENSITY DETERMINATIONS

% level comparison of predicted hep with observed cosmo

ALC

PG

Cosm

ology Subgroup

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IDENTIFYING DARK MATTERAre hep and cosmo identical?

Congratulations! You’ve

discovered the identity of dark

matter and extended our

understanding of the Universe to T=10 GeV, t=1 ns (Cf. BBN at

T=1 MeV, t=1 s)

Yes

Yes

Yes

Calculate the new

hep

Can you discover another particle

that contributes to DM?

Which is bigger?

No

hepcosmo

Does it account for the rest of

DM?

YesNo

Did you make a

mistake?

Does itdecay?

Can you identify a source of entropy

production?

NoYes

No

No

Yes

Can this be resolved with some wacky cosmology?

Yes

No

No

Are you sure?

Yes

Think about the cosmological

constant problem

No

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DIRECT DETECTION IMPLICATIONS

Current Sensitivity

Near Future

Future

Theoretical Predictions

Bae

r, Bala

zs, Belyaev, O

’Farrill (2003)

LHC + ILC m < 1 GeV, < 10%

Comparison tells us about local dark matter density and velocity profiles

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HESSCOLLIDERS ELIMINATE PARTICLE PHYSICS UNCERTAINTIES,

ALLOW ONE TO PROBE ASTROPHYSICAL DISTRIBUTIONS

ParticlePhysics

Astro-Physics

Halo profiles are poorly understood, controversial near the galactic center

INDIRECT DETECTION IMPLICATIONS

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SuperWIMP Dark Matter

• All of these signals rely on DM having weak force interactions. Is this required?

• Strictly speaking, no – the only required DM interactions are gravitational (much weaker than weak).

• But the relic density “coincidence” strongly prefers weak interactions.

Is there an exception to this rule?

Feng, Rajaraman, Takayama (2003)

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• Consider supersymmetry:

Gravitons gravitinos G̃ ̃

• What if the G̃ ̃is the lightest superpartner?

• WIMPs freeze out as usual

• But then all WIMPs decay to gravitinos (after ~ month!)

No-Lose Theorem: Loophole

Gravitinos naturally inherit the right density, but interact only gravitationally – they are “superWIMPs”

G̃ ̃WIMP≈

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SuperWIMP DetectionSuperWIMPs evade all particle dark matter searches.

“Dark Matter may be Undetectable”

But cosmology is complementary: Superweak interactions very late decays l ̃ → G̃ ̃l observable consequences:

• Galaxy formation

Cold dark matter (WIMPs) seeds structure formation. Simulations of galaxies indicate more central mass than observed – cold dark matter may be too cold.

SuperWIMPs are produced warm, could resolve this problem.Kaplinghat (2005)

Cembranos, Feng, Rajaraman, Takayama (2005)

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Big Bang Nucleosynthesis

• 4He, D, 3He, 7Li, … abundances determined by BBN at ~ 1 to 100 s

• Late decays may modify these predictions (may be good!)

CMB

• The CMB spectrum is black body

• Late decays may produce observable distortions

Fixsen

et al. (1996)

Particle D

ata G

roup ( 2004)

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SuperWIMPs are produced in late decays with large velocity (v ~ 0.1c – c) – are warm dark matter.

Structure Formation

Cem

branos, F

eng, Ra

jaraman, T

akayama (2005)

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Big Bang Nucleosynthesis

Late decays may modify light element abundances

Fields, S

arka

r, PD

G (2002

)

Feng, R

ajarama

n, Takayam

a (20

03)

Some SUSY parameter space excluded, much ok, and possibly even preferred (explains 7Li underabundance)

8 Oct 07 Feng 37

• Late decays may also distort the CMB spectrum

• For 105 s < < 107 s, get

“ distortions”:

=0: Planckian spectrum

0: Bose-Einstein spectrumHu, Silk (1993)

• Current bound: || < 9 x 10-5

Future (DIMES): || ~ 2 x 10-6

Cosmic Microwave Background

Feng, Rajaraman, Takayama (2003)

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IDENTIFYING SUPERWIMPS

Sleptontrap

Reservoir

If superWIMPs produced in decays

l ̃ → G̃ ̃l

Colliders will produce many 100 GeV, charged particles that live ~ a month.

These can be trapped, moved to a quiet environment for observation.

LHC, ILC can trap as many as ~10,000/yr in 10 kton trap.

Hamaguchi, Kuno, Nakaya, Nojiri (2004)

Feng, Smith (2004)

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What We Would Learn

• We are sensitive to gravity in a particle physics experiment!

• Measurement of mG̃

G̃. SuperWIMP contribution to dark matter

F. Supersymmetry breaking scale BBN, CMB, structure formation in the lab

• Measurement of and El mG̃̃ and M*

Measurement of G̃Newton on fundamental particle scale

Gravitino hascorrect spin, couplings to be graviton partner: quantitative confirmation of supergravity

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CONCLUSIONS

Dark Matter: rich interplay between cosmology and particle physics

Extraordinary progress, but many open questions

Both cosmology and particle physics new particles at 100 GeV: bright prospects

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“One day, all of these will be dark matter papers”

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8 Oct 07 Feng 43

Direct Detection: Future

Current Sensitivity

Near Future

Future

Theoretical Predictions

Ba

er, B

ala

zs, Be

lyae

v, O’F

arrill (2

00

3)

8 Oct 07 Feng 44

Entropy Production

BBN and CMB constrain B at different times. Late decays produce entropy and modify B.

Some SUSY parameter space excluded, much ok, and some even preferred (explains 7Li underabundance)

S/S

Feng, R

ajarama

n, Takayam

a (20

03)

8 Oct 07 Feng 45

Large Hadron Collider

M1/2 = 600 GeV

m l̃ = 219 GeV L = 100 fb-1/yr

Optimal 10 kton trap has 4 coverage, 1 m thick

10 to 104 sleptons trapped / year (~1%)

Feng, S

mith (200

4)