prospects for detecting dark matter in light of the wmap...

January 29, 2008. CCAPP, Ohio State University 1

Prospects for detecting dark matter in light of the WMAP

Haze

Gabrijela ZaharijasArgonne National Laboratory


Outline:

Short intro to dark matter, evidence and properties. How do we look for it: direct and indirect searches. High energy gamma rays (indirect) searches: techniques

and observational targets Galactic Center - how good of a target it really is? Hint of DM discovery – WMAP Haze Is it true or not? Test through high energy gamma ray

observations. Work in progress: more on the DM detection in the

Galactic center.


What is the Universe made of? ● Ordinary Matter: ∼5%

–Ordinary Matter inside stars (shinning): ≤1% –Ordinary Matter outside stars: ∼4%

● Dark Matter: ∼25% ● Dark Energy: ∼70%

We do not know much about 95% of the Universe....


Dark matter is the name given to the invisible mass whose gravity governs the observed motions of stars and gas.

Evidence for Dark Matter is overwhelming:

● Gravitational lensing,● Temperature of hot gas in galaxy clusters,● Galactic rotational curves,● Structure formation and cosmic microwave background

Note: dark matter has been observed at all scales, from galaxies and clusters of galaxies to cosmological scales.

But, could it be that we just do not we just do not understand gravityunderstand gravity?

A collision between two galaxy clusters, in the constellation of Carina. A classic shaped bow shock wave is visible in the gas of the smaller Bullet cluster (pink on right). The distribution of mass (green contours) was determined through gravitational lensing.

MOND people are looking

for ways around that...

NASA, 2006.The clear separation of dark matter and gas clouds is claimed to be a direct evidence that dark matter exists.

So, what do we know about dark matter?

● Its amount,● It is electrically neutral, ● It interacts with ordinary matter only weakly ● And ... it has to be massive (typically ≥GeV): from the hierarchical order the structures have formed.

What might dark matter be made of?

● Turns-out explanations with ordinary objects (MACHOS) or standard model particles (neutrinos) are not working. ● Weakly Interacting Massive Particles (WIMPS): very general category, some particle which is massive and interacts weakly - predicted in many beyond-the-standard-model theories....


How is dark matter distributed?

N-body simulations predict a UNIVERSAL profile:

They agree with observations in galaxies and galaxy clusters, for large r, ρ∝r-3.At the centers of objects, ρ∝r-γ (a cusp), is not observationally constrained... typical values from simulations give =1 (γ NFW) to =1.5 (γ Moore)A core (not a cusp) at the center is plausible, as well.


How to detect particle dark matter?

Direct detection: measure elastic recoil of nuclei in a detector when they collide with dark matter particle.

Indirect Searches: observe the radiation produced in dark matter annihilation.


Indirect Searches

WIMPs with electroweak scale masses annihilating into the most commonly assumed final states (such as gauge bosons or heavy quarks) transfer most of their energy ( 70%) into ∼ neutrinos, and smaller

fractions into gamma rays (13-20%), electron-positron pairs (10-13%) and proton- antiproton pairs (2-5%)

High-energy gamma-ray detection

gamma ray flux ∝

∗ DM density2 (integral over the line of site)∗ annihilation cross section ∗ number of gammas produced per annihilation ∗mDM-2

Typical values:● DM density: usually assumed some of the universal profiles● cross section: standardly taken the value needed to produce the correct DM abundance in the Big Bang model● DM mass: electroweak scale mass, 50-800 GeV, typically assumed


Where do we look for gamma rays?

Annihilation signal ∝ DM density2 → we look in the places where the density of DM is density of DM is high, and possibly the astrophysical backgrounds lowhigh, and possibly the astrophysical backgrounds low.

For example: ● the center of our Galaxy ● dwarf satellite galaxies of the Milky Way (Ursa Minor, Willman I) ● Spikes around intermediate mass black holes● Micro halos – through their proper motion

The Galactic Center is close to us, and has high DM density, but it is a crowded place...DM observations limited by high backgrounds

Few inner light years of the Galactic center are rich with astrophysically active objects, for example:● 3 million solar masses black hole SgrA*● few pc away, Super Nova remnant SgrA East All surrounded by hot ionized gas...

X ray image of central 60 ly. White dot is a suspected flare from a comet sized object falling into the black hole.

Gamma ray signal measured by HESS experiment, superimposed on a X-ray map of the GC. The location of SgrA* is indicated by a cross. Notice: the angular resolution of high energy Gamma experiments is quite low.

The HESS experiment measured a bright source in the Galactic center..

Its spectrum is well described by a power-law, dNγ/dEγ∝E−α, where = 2.25, over the αrange of 160 GeV to 20 TeV, and is therefore most likely of astrophysical origin.

If so, it represents a very bright background for DM detection with the GLAST detector if it extends also to lower energies (1-100 GeV). (G.Z., D. Hooper, Phys.Rev.D73:103501,2006)

GLAST sensitivity, without HESS source

GLAST sensitivity, WITH HESS source

Excluded by EGRET

Excluded by HESS


1. Ground based detectors (Atmospheric Cerenkov Telescopes): Eγ≥100 GeV; γs interact with matter very high in the atmosphere producing e+e- pairs, which start a cascade of showers of particles, producing Cherenkov radiation.

High energy gamma-ray detection techniques:


View of the 4 telescopes of H.E.S.S. (High Energy Stereoscopic System) in Namibia, South-West Africa.


2. Satellite experiments (EGRET, GLAST): measure gamma rays directly

GLAST: The launch is scheduled for the spring of 2008. EGRET: finished its mission in 2000.

Complementary to ground based telescopes because they measure lower energies!

Artistic rendering of the GLAST satellite.

GLAST

58

2008

Lots of progress today in the high energy gamma ray astronomy...

Dark matter experiments, around the world:

Bertone, G., arXiv:0710.5603 [astro-ph]


After all the search, are there any hints of dark matter discovery?


Yes, several...

We will first discuss the WMAP Haze, and compare it to the others afterward


The WMAP haze

The Wilkinson MicroWaveMicroWave Anisotropy Probe measured gamma rays in five channels, 20-90 GHz.

It measured cosmic microwave background making large contribution to our knowledge of cosmology.

Also, it provided the best measurement of standard Interstellar Medium emission mechanisms: including synchrotron and free-free emission (free e- scattered off of Hydrogen ions), thermal (vibrational) emission from dust grains and spinning (rotational) dust.


Surprisingly, these observations reveled an excess in the center of our Galaxy.

Such excess is radially symmetric and extends to about 20 deg (1-2 kpc) around the Galactic center: “the WMAP haze”. (D. P. Finkbeiner, Astrophys. J. 614, 186 (2004))

Conventional sources for this emission are unlikely. (D. P. Finkbeiner, astro-ph/0409027)

Dust is mapped by far-IR surveys, thermal bremsstrahlungtraced by the Hα recombination line in neutral H and synchrotron emission spatially correlates with the 408 MHz survey – WMAP measurement fits predictions reasonably well over the all sky, with the exception of the inner 20 deg of the Galaxy.


A possible explanation: A WIMP with an electroweak scale mass will produce relativistic electrons and positrons which, in the presence of the Galactic magnetic field, emit synchrotron photons in the frequency range of WMAP.

Moreover, it has been shown that if the dark matter density profile is moderately cusped ∝ r-1.2, and the standard value of the annihilation cross section (corresponding to thermally decoupled relic) assumed – the intensity of HAZE can be the intensity of HAZE can be explainedexplained. (Hooper, D. et al., astro-ph:0705.3655)

The dark matter halo profile determines the angular distribution of the resulting synchrotron emission.

NFW

22 GHz ρ∝ r-1.2

For the default and more extreme values of diffusion parameters.

For a wide range of masses and annihilation modes, the cross section required is within a factor of approximately two of the value required for a s-wave thermal relic, v 3 10σ ×∼ 26− cm3/s.

22/33 GHz

The WIMP’s mass and leading annihilation modes are important in determining the spectrum of the haze.

e+e-

W+W-

μ+μ-

ZZbb

τ+τ-

Other hints of DM discovery:● Cosmic ray Positron excess (HEAT): might be due to DM annihilations to e+e- – but DM models predict fluxes smaller than observed – enhancement needed. Will be verified soon with PAMELA.● 511 keV photon emission from the Galactic Bulge (INTEGRAL): due to e+e- annihilation, but DM should be very light (few MeV), and also have temperature dependent cross section in order not to overproduce population of e+e- today. Also, an explanation including excited states of ~500 GeV DM has been proposed. ● Diffuse Galactic gamma rays (EGRET): requires DM profile concentrated in two toroidal rings, too many e+ produced, could be explained only with astrophysical sources... ● Diffuse Extragalactic gamma rays (EGRET): requires cuspy DM profile throughout the Universe, but no cusp in the Milky Way.


Hooper, Dan, arxiv:0710.2062v1 [astro-ph]Requires only standard assumption.


How can we further test the nature of the Haze?

By looking for other DM annihilation products.

It turns out that detection of high energy gammas emitted from the Galactic Center, is the most promising way to confirm/reject the hypothesis of a DM origin of the Haze.


Signal measured by HESS, extrapolated to lower energies

Signal produced by “WMAP haze” DM annihilation; it would be distinguishable from the background in the GLAST experiment (5 year mission).

How would the signal of DM responsible for the WMAP Haze, look in the GLAST detector?

Hooper, D, GZ, Finkbeiner, D. and Dobler, G., arXiv:0709.3114 [astro-ph]

● ρ∝ r-1.2

● vσ *BF (DM mass) from finding of Hooper et al.


If instead, we consider an annulus around the galactic center, we can cut out the central bright astrophysical source, and greatly improve signal to background ratio.

The considered DM density profile is such that the signal due to the DM annihilation falls off slower than the leaking of central source signal in the annulus.

So, if WMAP haze is really due to the DM self-annihilation, GLAST should be able to detect it, by observing the Galactic Center (annulus) region.

Better signal/background and heavier DM detectable, in this region!

Galactic center Annulus region0.3-0.5o


GLAST will be able to detect DM annihilating to heavy quarks, gauge bosons or tau leptons over astrophysical backgrounds with 5 (3 )σ σ significance if they are lighter than approximately 320-500 GeV (500-750 GeV).

Only if the dark matter particles annihilate mostly to electrons or muons will GLAST be unable to identify the gamma ray spectrum associated with the WMAP Haze.


Caveats - astrophysical gamma ray backgrounds other than the HESS source are likely to exist in the inner degree surrounding the Galactic Center:

● Egret source : an unidentified EGRET source is known to be present approximately 0.2◦ from the Galactic Center. (angular window or energy cuts could be applied)

● Emission from the Galactic Ridge: gamma ray emission from the Galactic Ridge has been observed at TeV energies by HESS, in the b < 0.3 deg ∼

range, which correlates well with the molecular clouds. If it extends to lower energies, presents an additional background.

Detection prospects in the <0.5 deg annulus could be worse than shown, but important conclusions wouldn't change.


Summary – so far

● The WMAP haze can be explained with dark matter annihilation, when only standard values of the parameters are assumed.

● If the haze is due to dark matter self-annihilation, GLAST should be able to confirm it in the first couple of years of running (starting this spring)

● The annulus around the Galactic Center is probably a better place to look for a signal, as opposed to the usual choice of the Galactic center.


More on the DM annihilation detection in the annulus region:

● What is an optimal size of the annulus?● Prospects for future ACTs

●What is an optimal size of the annulus?

There are two types of background:● Galactic diffuse background (EGRET): correlated with the galactic plane – dominant for GLAST detector

● Cosmic ray backgrounds: experienced by ACTs; independent of galactic coordinates, due to the misidentification of showers initiated by gamma rays (e- or hadronic showers).

Results:

GLAST: region bmax > 7 deg has the biggest S/N.

ACTs: S/N flat to about 10, 20 deg.

Preliminary figure, work with P. Serpico.

Galactic diffuse background - GLAST

Cosmic ray background - ACTs

We considered the following geometry for the “annulus” region, suitable for diffuse background: bmax

Motivated by Inert Higgs Dark Matter Models: an added second Higgs doublet, with an imposed unbroken discrete Z2 symmetry that forbids its direct coupling to fermions; the standard model Higgs mass can be as high as about 500 GeV and still fulfill experimental precision tests; H0 can constitute DM if its mass is 10 80 GeV; −

Inert Higgs DM marginally observable. Boost factor of >~10, enough.

Detection of line signatures in the Annulus region:

Moore, 3σ

NFW, 3σ

Next generation ACTs: km2 arrays of ACT detectors.

AGIS and CTA experiments, in planning stages.

Should be sensitive to the detection in the annulus...

work with Buckley, J., White paper on gamma ray astronomy.


Summary

Annulus region around the Galactic center is a promising place to look for DM annihilation signal to gamma rays. Work on details in progress...

prospects for detecting dark matter in light of the wmap...

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