Dan HooperDan HooperParticle Astrophysics CenterParticle Astrophysics CenterFermi National LaboratoryFermi National Laboratory

dhooper@fnal.govdhooper@fnal.gov

Massachusetts Institute of Technology March 13, 2006

What do we know about dark matter?

What do we know about dark matter?

Ask An Astrophysicist:

A Great Deal!

The Existence of Dark Matter

•Galaxy and cluster rotation curves have pointed to the presence of large quantities of non-luminous matter for many decades (compelling since the 1970’s)

•White dwarfs, brown dwarfs, Jupiter-like planets, neutron stars, black holes, etc?

The Dark Matter Density

WMAP best-fit CDM model (for a flat Universe):

•73 % Dark energy (= 0.73)

•27 % Matter (Mh2 = 0.27)

h2 0.0076

•Big Bang nucleosynthesis combined with cosmic microwave background determine Bh2 0.024

•But, we also know M ~ 0.3, so most of the matter in the Universe is non-baryonic dark matter!

Fields and Sarkar, 2004

Baryonic Abundance

CDM

J. Tonry et al, 2003D. Spergel et al., ApJ, 2003

SDSS (and 2dF), 2005 M. Tegmark et al, 2004

A Cosmological Concordance Model

“The world is full of obvious thing which nobody by any chance ever observes.”

-Sherlock Holmes

What do we know about dark matter?

Ask An Astrophysicist:

A Great Deal!

What do we know about dark matter?

Ask An Astrophysicist:

A Great Deal!

Ask A Particle Physicist:Next to Nothing

(but we have some good guesses)

Axions, Neutralinos, Gravitinos, Axinos, Kaluza-Klein States, Heavy Fourth Generation Neutrinos, Mirror Particles, Stable States in Little Higgs Theories, WIMPzillas, Cryptons, Sterile Neutrinos, Sneutrinos, Light Scalars, Q-Balls, D-Matter, SuperWIMPS, Brane World Dark Matter,…

A virtual zoo of dark matter candidates have been proposed over the years. 100’s of viable candidates.

Weakly Interacting Massive Particles (WIMPs) are a particularly attractive class of dark matter candidates.

The Particle Nature of Dark Matter

•Freeze-out process: Stable particle X in thermal equilibrium in early Universe; later annihilation suppressed by Hubble expansion

•Freeze-out occurs at a temperature:

The Thermal Abundance of a WIMP

Automatically generates observed relic density!!!

•Relic abundance of a WIMP is naturally in Ω h2 ~ 0.01-1 range

•Strong motivation for WIMP dark matter

•Other types of dark matter only generate observed dark matter density if fixed by hand (axions, thermal gravitinos, WIMPzillas, etc.)

•Exception: SuperWIMP scenario

Focus on WIMP candidates for dark matter

The Thermal Abundance of a WIMP

Introduces new bosons for fermions and vice versa

Elegant extension of the Standard Model Natural solution to hierarchy problem

(stabilizes quadradic divergences to Higgs mass)

Restores unification of couplings Necessary in most string theories Likely to be discoverd at Tevatron and/or

LHC

Supersymmetry

To obtain sufficent proton stability, R-parity must be introduced

R-parity ensures that the Lightest Supersymmetric Particle (LSP) is stable

Possible supersymmetric dark matter candidate if LSP is not electrically charged or strongly interacting

The identity of the LSP depends of the mechanism supersymmetry breaking

Supersymmetry

Introduce minimal new particle content New bosons: squarks, sleptons New fermions: photino, zino, neutral higgsinos (neutralinos) charged wino and higgsino (charginos) and gluino Of these, only the lightest neutralino and

sneutrino are electrically neutral and non-strongly interacting (gravitino, axino are also possibilities in models beyond the MSSM)

Only the lightest neutralino naturally generates the observed dark matter density

The lightest neutralino: the most natural SUSY dark matter candidate

The Minimal Supersymmetric Standard Model (MSSM)

The Lightest Neutralino

•Neutralinos are Majorana particles (their own antiparticles)•Properties of the lightest neutralino can vary wildly depending on its composition•Tree-level annihilation to heavy fermions, higgs or gauge bosons, any of which can dominate•Magnitudes of the low velocity annihilation cross section and the elastic scattering cross section with nucleons can vary by many orders of magnitude •Very rich phenomenology

Bino Wino Higgsinos

Axions, Neutralinos, Gravitinos, Axinos, Kaluza-Klein States, Heavy Fourth Generation Neutrinos, Mirror Particles, Stable States in Little Higgs Theories, WIMPzillas, Cryptons, Sterile Neutrinos, Sneutrinos, Light Scalars, Q-Balls, D-Matter, SuperWIMPS, Brane World Dark Matter, etc…

The Particle Dark Matter Candidate Zoo

Some old saying about eggs and baskets comes to mind…

All Standard Model Particles Propagate in Bulk Kaluza-Klein (KK) Towers Appear, M ≈ R-1 ~ TeV Extra-Dimensional Momentum Conservation KK-Number

Conservation Realistic Models Must Be Orbifolded, KK-Number Violated,

but KK-Parity Conserved Lightest KK Particle (LKP) Stable

B(1) Most Natural Choice For LKP Excellent Dark Matter Candidate

Universal Extra Dimensions

t-channel KK fermion exchange diagrams dominate Unlike neutralinos, annihilations are NOT helicity suppressed

light fermions final states frequent Annihilations mostly to (large hypercharge) fermion pairs 20% taus, 20% muons, 20% electrons, 35% quarks, 3%

neutrinos, 2% higgs bosons Yields observed dark matter density for m ~ 900 GeV Coannihilations could accomodate 550-3000 GeV EWPO constrain m > 700 GeV

(Flacke, Hooper, March-Russell, 2005)

Kaluza Klein Dark Matter

•If mDM~ mEW (along with associated particles), discovery likely at LHC and/or Tevatron

•Strong constraints from LEP data

How To Search For A WIMP: Colliders

•Direct Detection

- Momentum transfer to detector through elastic scattering

•Indirect Detection - Observation of annihilation products (, e+, p, , etc.)

How To Search For A WIMP: Astrophysics

•Matter and anti-matter generated equally in dark matter annihilations (unlike other processes)

•Cosmic positron, anti-proton and anti-deuteron spectrum may contain signatures of particle dark matter

•Upcoming experiments (PAMELA, AMS-02) will measure the cosmic anti-matter spectrum with much greater precision, and at much higher energies

Indirect Detection: Anti-Matter

•Anti-protons/anti-deuterons -Energy loss length of tens of kpc (samples entire dark matter halo) -Depends critically on understanding of halo profile, galactic magnetic fields, and radiation (diffusion parameters and background difficult)

-For anti-deuterons, very low background and very low rate (single event discovery?)

•Positrons -Few kpc (or less) energy loss length (samples only local volume)

-Less dependent on understanding of halo profile, diffusion parameters

-Possible hints for dark matter present in existing data(?)

Indirect Detection: Anti-Matter

Indirect Detection: Positrons

•Positrons produced through a range of dark matter annihilation channels: (decays of heavy quarks, heavy leptons, gauge bosons, etc.)

•Positrons move under influence of galactic magnetic fields

•Energy losses through inverse compton and synchotron scattering with starlight, CMB

Indirect Detection: Positrons

•Determine positron spectrum at Earth by solving diffusion equation:

Diffusion Constant Energy Loss Rate Source TermInputs: •Diffusion constant

•Energy loss rate

•Annihilation cross section/modes

•Halo profile (inhomogeneities?)

•Boundary conditions

•Dark matter mass

Indirect Detection: Positrons

•Observed spectrum depends on dark matter particle properties:

Indirect Detection: Positrons

Supersymmetric (neutralino) origin of positron excess?

1) Spectrum can fit HEAT data2) Large annihilation rate required (non-thermal cross section and/or

large degree of inhomogeneities)

Indirect Detection: Positrons

Kaluza-Klein Dark Matter

-Hard annihilation modes preferred (20% to each of e+e-, +-, +-) still good fit to data

-As with neutralinos, ultimately requires a large quantity of local inhomogeneities to normalize to HEAT data

Hooper and G. Kribs PRD (hep-ph/0406026)

Indirect Detection: Positrons

The Annihilation Rate (Normalization)

-If a thermal relic is considered, a large degree of local inhomogeneity (boost factor) is required in dark matter halo

-Might local clumps of dark matter accommodate this?

Two mass scales:

-Sum of small mass (~10-1 - 10-6 M) clumps Small boost (2-10, whereas ~ 50 or more is required)

-A single large mass clump (~104 - 108 M) Unlikely at 10-4 level

Hooper, J. Taylor and J. Silk, PRD (hep-ph/0312076)H. Zhao, J. Taylor, J. Silk and Hooper (hep-ph/0508215)

Indirect Detection: Positrons

Where does this leave us?•Future cosmic positron experiments hold great promise

•PAMELA satellite, planned to be launched in 2006

•AMS-02, planned for deployment onboard the ISS (???)

Indirect Detection: Positrons

With a “HEAT sized” signal:•Dramatic signal for either PAMELA or AMS-02

•Clear, easily identifiable signature of dark matter

Hooper and J. Silk, PRD (hep-ph/0409104)

Indirect Detection: Positrons

With a smaller signal:•More difficult for PAMELA or AMS-02

•Still one of the most promising dark matter search techniques

Hooper and J. Silk, PRD (hep-ph/0409104)

Indirect Detection: Positrons

Hooper and J. Silk, PRD (hep-ph/0409104)

Value for thermal abundance

•AMS-02 can detect a thermal (s-wave) relic up to ~200 GeV, for any boost factor, and all likely annihilation modes

•For modest boost factor of ~ 5, AMS-02 can detect dark matter as heavy as ~1 TeV

•PAMELA, with modest boost factors, can reach masses of ~250 GeV

•Non-thermal scenarios (AMSB, etc), can be easily tested

Prospects for Neutralino Dark Matter:

Indirect Detection: Positrons

Prospects For Kaluza-Klein Dark Matter:•For KKDM, AMS-02 can exclude thermal mass range for modest boost factors

•Coannihilation scenarios with large masses (m > 1 TeV) may remain untested

•Other DM models with annihilations to charged leptons will be highly constrained, especially for lower masses

Hooper and J. Silk, PRD (hep-ph/0409104)

Value for thermal abundanceValue for thermal abundanceCross section for KKDM

Lower limit from EWPO

•WIMPs elastically scatter with massive bodies (Sun)

•Captured at a rate ~ 1018 s-1 (p/10-8 pb) (100 GeV/m)2

•Over billions of years, annihilation/capture rates equilibrate

•Annihilation products absorbed, except for neutrinos

Indirect Detection: Neutrinos

The IceCube Neutrino Telescope

•Full cubic kilometer instrumented volume

•Technology proven with predecessor, AMANDA

•First string of detectors deployed in 2004/2005, 8 more strings deployed in 2005/2006 (80 in total)•Sensitive to muon neutrinos above ~ 100 GeV

•Similar physics reach to KM3 in Mediterranean Sea

Indirect Detection: Neutrinos

•Neutrino flux depends on the capture rate, which is in turn tied to the elastic scattering cross section

•Direct detection limits impact rates anticipated in neutrino telescopes

Indirect Detection: Neutrinos

•WIMPs become captured in the Sun through spin-independent and spin-dependent scattering

•Direct detection constraints on spin-dependent scattering are still very weak

Indirect Detection: Neutrinos

Spin-Independent Spin-Dependent

What Kind of Neutralino Has a Large Spin-Dependent Coupling?

Indirect Detection: Neutrinos

qq

q q

Z

Always Small |fH1|2 - |fH2|2

Substantial Higgsino Component Needed

What Kind of Neutralino Has a Large Spin-Dependent Couplings?

Indirect Detection: Neutrinos

Large Rate At IceCube/KM3

F. Halzen and Hooper (hep-ph/0510048)

Large Rate in IceCube/KM3

Kaluza-Klein Dark Matter

Indirect Detection: Neutrinos

•Spin-dependent scattering naturally dominates

•Annihilations to +- (20%), (3%) lead to large number of neutrinos

•Spin-independent cross section well beyond reach of direct detection

B(1) B(1)

q q

q(1)

SD ≈ 2 x 10-6 pb (TeV/m)4 (0.1/rq)2

Kaluza-Klein Dark Matter

Indirect Detection: Neutrinos

•Low masses, quasi-degenerate KK quarks, lead to large rates

•Relic density and EWPO considerations lead to prediction of 0.5-30 events/yr in km-scale telescopes (IceCube, KM3)

Hooper and G. Kribs, PRD (hep-ph/0208261), F. Halzen and Hooper (hep-ph/0510048)

Advantages of Gamma-Rays

Indirect Detection: Gamma-Rays

•Propagate undeflected (point sources possible)

•Propagate without energy loss (spectral information)

•Distinctive spectral features (lines), provide potential “smoking gun”

•Wide range of experimental technology (ACTs, satellite-based)

Disadvantages of Gamma-Rays•Flux depends critically on poorly known inner halo profiles

predictions dramatically vary from model to model

•Astrophysical backgrounds

The Galactic Center Region

Indirect Detection: Gamma-Rays

•Likely to be the brightest source of dark matter annihilation radiation

•Detected in ~TeV gamma-rays by three ACTs: Cangaroo-II, Whipple and HESS

•Possible evidence for dark matter?

The Cangaroo-II Observation

Indirect Detection: Gamma-Rays

•Consistent with WIMP in ~1-4 TeV mass range

•Roughly consistent with Whipple/Veritas

Hooper, Perez, Silk, Ferrer and Sarkar, JCAP, astro-ph/0404205

The Cangaroo-II Observation

Indirect Detection: Gamma-Rays

•Consistent with WIMP in ~1-4 TeV mass range

•Roughly consistent with Whipple/Veritas

The HESS Obsevation•Superior telescope

•Inconsistent with Cangaroo-II

•Extends at least to ~10 TeV

•WIMP of ~10-40 TeV mass needed

D. Horns, PLB, astro-ph/0408192

Can A Neutralino Be As Heavy As 10-40 TeV?

Indirect Detection: Gamma-Rays

•Very heavy neutralinos tend to overclose the Universe

•Largest annihilation cross sections (lowest relic abundance) are found for Wino-like or Higgsino-like neutralinos

h2~0.1 for ~1 TeV Higgsino, or ~3 TeV Wino

•Significantly larger masses are possible only if coannihilations are carefully arranged (for example, S. Profumo, hep-ph/0508628)

Can A Neutralino Be As Heavy As 10-40 TeV?

Indirect Detection: Gamma-Rays

•Electroweak precision observables indicate the presence of a light higgs boson (near the EW scale)•Large contributions to the higgs mass come from particle loops:

•In unbroken SUSY, boson and fermion loops exactly cancel

• If mSUSY >> mHiggs , extreme fine tuning required

•mSUSY below ~1 TeV is strongly preferred

Messenger Sector Dark Matter

Indirect Detection: Gamma-Rays

•In Gauge Mediated SUSY Breaking (GMSB) models, SUSY is broken in ~100 TeV sector

•LSP is a light gravitino (1-10 eV), poor DM candidate

•Lightest messenger particle is naturally stable, multi-TeV scalar neutrino is a viable dark matter candidate

Dimopolous, Giudice and Pomarol, PLB (hep-ph/9607225)

Han and Hemfling, PLB (hep-ph/9708264)

Han, Marfatia, Zhang, PRD (hep-ph/9906508)

Hooper and J. March-Russell, PLB (hep-ph/0412048)

Messenger Sector Dark Matter

Indirect Detection: Gamma-Rays

Hooper and J. March-Russell, PLB (hep-ph/0412048)

•Gamma-ray spectrum (marginally) consistent with HESS data

•Normalization requires highly cuspy, compressed, or spiked halo profile

•With further HESS observation ofregion, dark matter hypothesis shouldbe conclusively tested

•Source appears increasingly likely to be of an astrophysical origin

Astrophysical Origin of Galactic Center Source?

Indirect Detection: Gamma-Rays

•A region rich in extreme astrophysical objects•Particle acceleration associated with supermassive black hole?Aharonian and Neronov (astro-ph/0408303), Atoyan and Dermer (astro-ph/0410243)•Nearby Supernova Remnant to close to rule out

•If this source is of an astrophysicalnature, it would represent a extremelychallenging background for future dark matter searches to overcome (GLAST, AMS, etc.)

Hooper, Perez, Silk, Ferrer and Sarkar, JCAP, astro-ph/0404205

Dwarf Spheriodal Galaxies

Indirect Detection: Gamma-Rays

•Several very high mass-to-light dwarf galaxies in Milky Way(Draco, Sagittarius, etc.)

•Little is known for certain about the halo profiles of such objects

•For example, draco mass estimates range from 107 to 1010 solar masses

broad range of predictions for annihilation rate/gamma-ray flux

•May provide several very bright sources of dark matter annihilation radiation… or very, very little

•Detection of Draco by CACTUS experiment??? (Bergstrom & Hooper, hep-ph/0512317; Profumo & Kamionkowski, astro-ph/0601249)

•Very exciting prospects exist for direct, indirect and collider searches for dark matter

•Cosmic anti-matter searches will be sensitive to thermally produced (s-wave) WIMPs up to hundreds of GeV (PAMELA) or ~1 TeV (AMS-02)

•Kilometer scale neutrino telescopes (IceCube, KM3) will be capable of detecting mixed gaugino-higgsino neutralinos, and many KKDM models

•Gamma-ray astronomy is improving rapidly, but it is difficult to predict the prospects for dark matter detection given the astrophysical uncertainties; Dwarf spheriodals are among the most promising sources

Summary

The Cork Is Still In the Champagne Bottle…

•In addition to indirect searches…

•Direct detection experiments (CDMS) have reached ~10-7 pb level, with 1-2 orders of magnitude expected in near future (many of the most attractive SUSY models)

•Collider searches (LHC, Tevatron) are exceedingly likely to discover Supersymmetry or whatever other new physics is associated with the electroweak scale

…But Maybe Not For Long