Topics on Dark Matter Annihilation Eiichiro Komatsu University

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 Ge. V 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); Kamionkowski & 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 parameters

Direct Annihilation to e+ e- Hooper & Silk (2005) • Positrons lose their energy as they propagate through the magnetized interstellar medium.

Decays of Ws, Zs m. DM=100 Ge. V 300 Ge. V Hooper & Silk (2005) • No sharp features exist in this case because positrons are secondary products. 600 Ge. V

(Hooper ar. Xiv: 0710. 2062) 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 particles around us? • Possible, but not very likely (theoretically)… • AMS-02’s data will contribute to this subject significantly.

Hooper & Silk (2005) AMS-02 Forecast • Blue: AMS-02; Red: PAMELA

Hooper & Silk (2005) Somewhat More Realistic • These values are below the current data.

Microwave Sky WMAP 94 GHz

Gamma-ray Sky

Deciphering Gamma-ray Sky • Astrophysical: Astrophysical Galactic vs Extra-galactic – Galactic origin (diffuse) • E. g. , Decay of neutral pions produced by cosmic-rays interacting with the interstellar 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 94 GHz

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

Finkbeiner (2004) “WMAP Haze” • The WMAP haze is consistent with synchrotron from relativistic electrons that are produced by DM annihilation.

Finkbeiner (2004); Hooper, Finkbeiner & Dobler (2007) 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~3 x 10 -26 cm 3/s (in agreement with the present-day cosmic DM density) • The required DM particle mass is m. DM~100 Ge. V to ~Te. V (in agreement with neutralinos)

v: A Closer Look Required for cosmic DM density b W Z e • Both mass and v of DM inferred from the haze are reasonable… Neutralinos?

How About Gamma-rays? • The haze is produced by relativistic electrons, which could be the product of DM annihilation. • How about other products, like gammarays? Here

Hooper, Zaharijas, Finkbeiner & Dobler (2007) Predicted Signals from G. C. • Errors: GLAST forecast • Data Points: HESS measurements of G. C.

Hooper, Zaharijas, Finkbeiner & Dobler (2007) Bit Away from G. C. (0. 3 to 0. 5 deg) • Much lower astrophysical background.

Extra-galactic Background Ando et al. (2007) • Is the “bump” at ~10 Ge. V real? • The issue: the amplitude of the bump depends on how you remove the Galactic contribution…

De Boer et al. take this very seriously • De Boer et al. (ar. Xiv: 0705. 0094) report on a “new determination” of the extragalactic 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.

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)

Summary • Positron Excess – This classic problem is still there. – DM explanation is possible -- AMS-02’s contribution 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 AMS-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!