Dan Hooper Particle Astrophysics Center Fermi National Laboratory [email protected]

Dan HooperDan HooperParticle Astrophysics CenterParticle Astrophysics CenterFermi National LaboratoryFermi National Laboratory

[email protected]@fnal.gov

University of Kansas April 17, 2006

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 (conclusive evidence since the 1970’s)

Vera Rubin

Fritz Zwicky

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 (conclusive evidence since the 1970’s)

Vera Rubin

Fritz Zwicky

In the new age of precision cosmology, In the new age of precision cosmology, we now know much more! we now know much more!

The Density of our Universe

The anisotropies in the cosmic microwave background (CMB) have been studied to reveal the curvature and density of our Universe: tot

1 (about 10-29 grams/cm3)

The Composition of Our Universe

•In addition to matter, general relativity allows for a cosmological term, (vacuum energy/dark energy)

•Quantum field theory would suggest that ~ 1060, 10120, or 0

•So, we had expected to measure = 0


•In addition to matter, general relativity allows for a cosmological term, (vacuum energy/dark energy)

•Quantum field theory would suggest that ~ 1060, 10120, or 0

•So, we had expected to measure = 0

•Our expectations turned out to be wrong!Our expectations turned out to be wrong!


•Compare expansion history of our Universe to the CMB anisotropies and cluster masses

Flat, all matter Universe

Best fit to data


•Compare expansion history of our Universe to the CMB anisotropies and cluster masses

•In addition to matter, our Universe contains a great deal of dark energy (~ 0.72)

Flat, all matter Universe

Best fit to data

What’s The Matter?

•So ~30% of our Universe’s density is in the form of matter (mostly dark matter, as seen from galaxy rotation curves, clusters, etc.)

•So what kind of matter is it?

•First guess: Baryons (white dwarfs, brown dwarfs, neutron stars, jupiter-like planets, black holes, etc.)

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

B ~ 0.05

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

Fields and Sarkar, 2004

Baryon Abundance

•Observations of the large scale structure of our Universe can be compared to computer simulations

•Simulation results depend primarily on whether the dark matter is hot (relativistic) or cold (non-relativistic) when structures were formed

•Most of the Universe’s matter must be Cold Dark Matter

Cold Dark Matter and Structure Formation

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

-Sherlock Holmes



A Great Deal!



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

•Stable particle, X, in thermal equilibrium in early Universe (freely created and annihilated, roughly as plentiful as ordinary types of matter)

•As Universe cools, number density of X becomes Boltzman suppressed

•But expansion of the Universe makes finding X’s to annihilate with difficult, suppressing the annihilation rate

The Thermal Abundance of a WIMP

•Expansion leads to a thermal freeze-out of X particles

•For a particle with weak scale interactions, freeze-out occurs at a temperature, T~MX/20

•With weak scale interactions, freeze out leads to a density of X particles of ~1


•Expansion leads to a thermal freeze-out of X particles

•For a particle with weak scale interactions, freeze-out occurs at a temperature, T~MX/20

•With weak scale interactions, freeze out leads to a density of X particles of ~1


Automatically generates observed relic density!!!Automatically generates observed relic density!!!

Elegant extension of the Standard Model

For each fermion in nature, a corresponding boson must also exist (and vice versa)

New spectrum of “superpartner” particles yet to be discovered

Supersymmetry

Not introduced for dark matter

Why Supersymmetry?

Not introduced for dark matter Higgs mass stability

Why Supersymmetry?

•Electroweak precision observables indicate the presence of a light Higgs boson (around ~100 GeV)

•Large contributions to the Higgs mass come from particle loops:

•Without SUSY, ~ MGUT or ~ MPlanck ultra-heavy Higgs

•With TeV scale SUSY, boson and fermion loops nearly cancel light Higgs

Supersymmetry and the Mass of the Higgs Boson

Not introduced for dark matter Higgs mass stability Grand Unification

Why Supersymmetry?

Supersymmetry and Grand Unification

•If there is a Grand Unified Theory (GUT) in nature, then we expect the SM forces to become of equal strength at some high energy scale

•In the Standard Model, couplings become similar, but not equal

Supersymmetry and Grand Unification

•With Supersymmetry, the three forces can unify at a single scale

For the proton to be sufficiently stable, R-parity must be conserved

Evenness or oddness of superpartners is conserved

Consequence: the Lightest Supersymmetric Particle (LSP) is stable, and a potentially viable dark matter candidate

The identity of the LSP depends on the mechanism of supersymmetry breaking

Supersymmetry and Dark Matter

• Dark matter candidates must be electrically neutral, not colored

• Possibilities: photino Zino (neutral) higgsinos sneutrinos gravitino axino

The Lightest Supersymmetric Particle




Do not naturally generate the observed dark matter density





Ruled out by direct detection





Ruled out by direct detection

Mix to form 4 neutralinos

•Direct Detection

•Indirect Detection

•Colliders

How To Search For A WIMP

•Underground experiments hope to detect recoils of dark matter particles elastically scattering off of their detectors

•Prospects depend on the WIMP’s elastic scattering cross section with nuclei

•Leading experiments include CDMS (Minnesota), Edelweiss (France), and Zeplin (UK)

Direct Detection

•Elastic scattering can occur through Higgs and squark exchange diagrams:

Direct Detection

q q

h,H

q q

q~

•Cross section depends on numerous SUSY parameters: neutralino mass and composition, tan, squark masses and mixings, Higgs masses and mixings

SUSY Models

Direct Detection

•Current Status

CDMS

Zeplin, EdelweissDAMA

Supersymmetric

Models

Direct Detection

•Near-Future Prospects

CDMS


Supersymmetric

Models CDMS, Edelweiss Projections

Direct Detection

•Long-Term Prospects

CDMS


Supersymmetric

Models

Super-CDMS, Zeplin-Max

•Attempt to observe annihilation products of dark matter annihilating in halo, or elsewhere

•Prospects depend on both the characteristics of the dark matter particle and its distribution in the halo

•Gamma-rays, neutrinos, positrons, anti-protons and anti-deuterons each provide a potentially viable channel for the detection of dark matter

Indirect Detection

•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

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


•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


•Reduce systematics by studying the “positron fraction”

•When plotted this way, HEAT experiment observes a significant excess


Supersymmetric (neutralino) origin of positron excess?

-Spectrum generated by annihilating neutralinos can fit the HEAT data


Supersymmetric (neutralino) origin of positron excess?

-Spectrum generated by annihilating neutralinos can fit the HEAT data

-Normalization is another issue


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)


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 (???)


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)


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

•Still one of the most promising dark matter search techniques




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:

•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 are 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


•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


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

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


Spin-Independent Spin-Dependent

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


q q

Always Small |fH1|2 - |fH2|2

Substantial Higgsino Component Needed

qq

Z

q q

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


Large Rate At IceCube/KM3

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

Large Rate in IceCube/KM3

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


•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


•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


•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?


•Very heavy neutralinos tend to overclose the Universe

•Neutralinos heavier than a few TeV require fine tuning (through coannihilations) to evade too much relic density (S. Profumo, hep-ph/0508628)

•If superpartners are heavier than a few TeV, then the Higgs mass is no longer naturally light (one of the primary motivations for supersymmetry in the first place)

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


•Very heavy neutralinos tend to overclose the Universe

•Neutralinos heavier than a few TeV require fine tuning (through coannihilations) to evade too much relic density (S. Profumo, hep-ph/0508628)

•If superpartners are heavier than a few TeV, then the Higgs mass is no longer naturally light (one of the primary motivations for supersymmetry in the first place)

10-40 TeV Supersymmetry is extremely unattractive

Messenger Sector Dark Matter


•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


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?•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.) (Zaharijas and Hooper, astro-ph/0603540)

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

Dwarf Spheriodal Galaxies


•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)

•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

•Most promising channel is through neutralino-chargino production

For example,

•Tevatron searches for light squarks and gluinos are also interesting

•Tevatron SUSY searches only possible if superpartners are rather light

Supersymmetry At The Tevatron

•Squarks and gluinos will be produced prolificly at the LHC (probably discovered within first month of running)

•Squarks/gluinos decay to leptons+jets+missing energy (LSPs)

•Lightest neutralino mass to be measured to ~10% precision

•But is it dark matter?

•Calculated relic density should be

compared to CDM density

Supersymmetry At The LHC

Putting It All Together

•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

•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…

•Furthermore…

•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

Dan Hooper Particle Astrophysics Center Fermi National Laboratory [email protected]

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