Topics on Dark Matter Annihilation

Eiichiro Komatsu

University of Texas at Austin

AMS Meeting@JSC

October 22, 2007

Three Topics

• The Positron Excess– DM Annihilation?

• DM Annihilation in the Galactic Center– New evidence? WMAP “Haze”

• Extra-galactic Gamma-rays from DM Annihilation– EGRET spectrum update

Positron Excess:AMS-01’s Result

• The positron excess at >10 GeV is still there.

• The origin of the excess is still unknown.

• DM Annihilation?

Positron Excess from DM Annihilation

• This is a classic subject.– Tylka (1989); Turner & Wilczek (1990); Kamionko

wski & Turner (1991); many others

• The main idea: positrons are produced by decays of the products of DM annihilation, such as– Gauge bosons, quarks, leptons– The dominant modes depend on the DM model pa

rameters

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Direct Annihilation to e+e- • Positrons

lose their energy as they propagate through the magnetized interstellar medium.

Hooper & Silk (2005)

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Decays of Ws, Zs• No sharp fe

atures exist in this case because positrons are secondary products.

Hooper & Silk (2005)

mDM=100 GeV

300 GeV

600 GeV

The Current State of Affairs

• Theoretical calculations show that the measured positron excess is “too large” (by a factor of ~50) to be produced by annihilation of smooth dark matter distribution around us (within a few kpc).– Are there (yet unknown) concentrations of DM part

icles around us?• Possible, but not very likely (theoretically)…

• AMS-02’s data will contribute to this subject significantly.

(Hooper arXiv:0710.2062)

AMS-02 Forecast

• Blue: AMS-02; Red: PAMELA

Hooper & Silk (2005)

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Somewhat More Realistic

• These values are below the current data.

Hooper & Silk (2005)

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Microwave SkyWMAP 94GHz

Gamma-ray Sky

Deciphering Gamma-ray Sky• AstrophysicalAstrophysical: Galactic vs Extra-galactic

– Galactic origin (diffuse)• E.g., Decay of neutral pions produced by cosmic-rays interacting with the i

nterstellar medium.

– Extra-galactic origin (discrete sources)• Active Galactic Nuclei (AGNs)• Blazars• Gamma-ray bursts

• Exotic: Galactic vs Extra-galactic– Galactic Origin

• Dark matter annihilation in the Galactic Center• Dark matter annihilation in the sub-halos within the Galaxy

– Extra-galactic Origin• Dark matter annihilation in the other galaxies

“HST” for charged particles, and “WMAP” for gamma-rays?

WMAP 94GHz

New Evidence From WMAP?

• Doug Finkbeiner at Harvard claims that there is a significant excess of synchrotron emission in the WMAP’s microwave data in the inner (1-2 kpc) region of the Galactic Center.– He calls this the WMAP “haze.”

Here

“WMAP Haze”

• The WMAP haze is consistent with synchrotron from relativistic electrons that are produced by DM annihilation.

Finkbeiner (2004)

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WMAP Haze: Implications for DM

• The inferred DM density profile in the Galactic Center is ~r-1.2 (in agreement with N-body simulations)

• The required annihilation cross section is v~3x10-26 cm3/s (in agreement with the present-day cosmic DM density)

• The required DM particle mass is mDM~100 GeV to ~TeV (in agreement with neutralinos)

Finkbeiner (2004); Hooper, Finkbeiner & Dobler (2007)

v: A Closer Look

• Both mass and v of DM inferred from the haze are reasonable… Neutralinos?

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bW

Z

e

Required for cosmic DM density

How About Gamma-rays?

• The haze is produced by relativistic electrons, which could be the product of DM annihilation.

• How about other products, like gamma-rays?

Here

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Predicted Signals from G.C.

• Errors: GLAST forecast• Data Points: HESS measurements of G.C.

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Hooper, Zaharijas, Finkbeiner & Dobler (2007)

Bit Away from G.C. (0.3 to 0.5deg)

• Much lower astrophysical background.

Hooper, Zaharijas, Finkbeiner & Dobler (2007)

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Extra-galactic Background

• Is the “bump” at ~10 GeV real?

• The issue: the amplitude of the bump depends on how you remove the Galactic contribution…

Ando et al. (2007)

De Boer et al. take this very seriously

• De Boer et al. (arXiv:0705.0094) report on a “new determination” of the extra-galactic diffuse gamma-ray background.

• A new component: they now include DM annihilation in the Galactic model, and subtract it for a better determination of the true background light.

De Boer et al. Result

• Inclusion of the Galactic DM annihilation does affect the background level somewhat, but (fortunately) not very much.

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De Boer et al.’s favorite model• This is the

limitation of having only one piece of information: energy spectrum.

• We need anisotropy data! (Ando&Komatsu 2006)

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Summary• Positron Excess

– This classic problem is still there.– DM explanation is possible -- AMS-02’s contributio

n will be very important.

• WMAP Haze– Needs to be confirmed by Planck (European CMB

mission to be launched next year)– If it is due to DM annihilation, GLAST (as well as A

MS-02?) would be able to see gamma-rays from G.C.

• Extra-galactic background– This topic is getting hotter than ever.– We need more information, like anisotropy!