1. Gamma-ray From Annihilation of Dark Matter Particles Eiichiro Komatsu University of Texas at Austin AMS Meeting@CERN, April 23, 2007 Eiichiro Komatsu University of Texas at Austin AMS Meeting@CERN, April 23, 2007 K. Ahn & EK, PRD, 71, 021303R (2005); 72, 061301R (2005) S. Ando & EK, PRD, 73, 023521 (2006) S. Ando, EK, T. Narumoto & T. Totani, MNRAS, 376, 1635 (2007) S. Ando, EK, T. Narumoto & T. Totani, PRD, 75, 063519 (2007)

2. What Is Out There? WMAP 94GHz

3. What Is Out There?

4. 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  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 Relativistic Jets

5. Blazars  Blazars = A population of AGNs whose relativistic jets are directed towards us.  Inverse Compton scattering of relativistic particles in jets off photons -> gamma-rays, detected up to TeV  How many are there?  EGRET found ~60 blazars (out of ~100 identified sources)  GLAST is expected to find thousands of blazars.  GLAST’s point source sensitivity (>0.1GeV) is 2 x 10 -9 cm -2 s -1  AMS-2’s equivalent (>0.1GeV) point source sensitivity is about 10 times larger, ~ 10 -8 cm -2 s -1 (G. Lamanna 2002)  Blazars = A population of AGNs whose relativistic jets are directed towards us.  Inverse Compton scattering of relativistic particles in jets off photons -> gamma-rays, detected up to TeV  How many are there?  EGRET found ~60 blazars (out of ~100 identified sources)  GLAST is expected to find thousands of blazars.  GLAST’s point source sensitivity (>0.1GeV) is 2 x 10 -9 cm -2 s -1  AMS-2’s equivalent (>0.1GeV) point source sensitivity is about 10 times larger, ~ 10 -8 cm -2 s -1 (G. Lamanna 2002)

6. Blazar Luminosity Function Update  Luminosity-Dependent Density Evolution (LDDE) model fits the EGRET counts very well. This model has been derived from  X-ray AGN observations, including the soft X-ray background  Correlation between blazars and radio sources  LDDE predicts that GLAST should detect ~3000 blazars in 2 years.  This implies that AMS-2 would detect a few hundred blazars.  Luminosity-Dependent Density Evolution (LDDE) model fits the EGRET counts very well. This model has been derived from  X-ray AGN observations, including the soft X-ray background  Correlation between blazars and radio sources  LDDE predicts that GLAST should detect ~3000 blazars in 2 years.  This implies that AMS-2 would detect a few hundred blazars. Narumoto & Totani, ApJ, 643, 81 (2006) LDDE

7. Redshift distribution of blazars that would be detected by GLAST LDDE1: The best-fitting model, which accounts for ~1/4 of the gamma-ray background. LDDE2: A more aggressive model that accounts for 100% of the gamma-ray background. It is assumed that blazars are brighter than 10 41 erg/s at 0.1 GeV. Ando et al. (2007)

8.  -ray Background diffuse background  Un-resolved Blazars that are below the point-source sensitivity will contribute to the diffuse background.  EGRET has measured the diffuse background above the Galactic plane.  LDDE predicts that only ~1/4 of the diffuse light is due to blazars!  AMS-2 will do MUCH better than EGRET in the diffuse background diffuse background  Un-resolved Blazars that are below the point-source sensitivity will contribute to the diffuse background.  EGRET has measured the diffuse background above the Galactic plane.  LDDE predicts that only ~1/4 of the diffuse light is due to blazars!  AMS-2 will do MUCH better than EGRET in the diffuse background (G. Lamanna 2002) Ando et al. (2007)

9. Dark matter (WIMP) annihilation  WIMP dark matter annihilates into gamma- ray photons.  The dominant mode: jets  Branching ratios for line emission (two gamma & gamma+Z 0 ) are small.  WIMP mass is likely around GeV–TeV, if WIMP is neutralino-like.  Can GLAST or AMS-2 see this?  WIMP dark matter annihilates into gamma- ray photons.  The dominant mode: jets  Branching ratios for line emission (two gamma & gamma+Z 0 ) are small.  WIMP mass is likely around GeV–TeV, if WIMP is neutralino-like.  Can GLAST or AMS-2 see this? GeV-γ Ando et al. (2007)

11. DM Annihilation in MW Diemand, Khlen & Madau, ApJ, 657, 262 (2007) Simulated map of gamma-ray flux by Diemand et al., as seen from 8kpc away from the center. Challenging for AMS-2 (Jacholkowska et al. 2006)

13. Why MW? There are many more dark matter halos out there!  WIMP dark matter particles are annihilating everywhere.  Why focus only on MW? There are so many dark matter halos in the universe.  We can’t see them individually, but we can see them as the background light.  We might have seen this already in the background light: the real question is, “how can we tell, for sure, that the signal is indeed coming from dark matter?”  WIMP dark matter particles are annihilating everywhere.  Why focus only on MW? There are so many dark matter halos in the universe.  We can’t see them individually, but we can see them as the background light.  We might have seen this already in the background light: the real question is, “how can we tell, for sure, that the signal is indeed coming from dark matter?”

14. Gamma-ray Anisotropy From Dark Matter Annihilation  Dark matter halos trace the large-scale structure of the universe. must angular power spectrum  The distribution of gamma-rays from these sources must be inhomogeneous, with a well defined angular power spectrum.  If dark matter annihilation contributes >30%, it should be detectable by GLAST in anisotropy.  A smoking gun for dark matter annihilation.  It would be very interesting to study if AMS-2 would be able to detect anisotropy signal --- remember, the mean intensity will be measured by AMS-2 very well!  Dark matter halos trace the large-scale structure of the universe. must angular power spectrum  The distribution of gamma-rays from these sources must be inhomogeneous, with a well defined angular power spectrum.  If dark matter annihilation contributes >30%, it should be detectable by GLAST in anisotropy.  A smoking gun for dark matter annihilation.  It would be very interesting to study if AMS-2 would be able to detect anisotropy signal --- remember, the mean intensity will be measured by AMS-2 very well! Ando & EK (2006); Ando, EK, Narumoto & Totani (2007)

15. “HST” for charged particles, and “WMAP” for gamma-rays? WMAP 94GHz

16. Why Anisotropy?  The shape of the power spectrum is determined by the structure formation, which is well known.  Schematically, we have: (Anisotropy in Gamma-ray Sky) = (MEAN INTENSITY) x   The mean intensity depends on particle physics: annihilation cross-section and dark matter mass.   The fluctuation power, , depends on structure formation.  The hardest part is the prediction for the mean intensity. However… Remember that the mean intensity has been measured already!  The prediction for anisotropy is robust. All we need is a fraction of the mean intensity that is due to DM annihilation.  Blazars account for ~1/4 of the mean intensity. What about dark matter annihilation?  The shape of the power spectrum is determined by the structure formation, which is well known.  Schematically, we have: (Anisotropy in Gamma-ray Sky) = (MEAN INTENSITY) x   The mean intensity depends on particle physics: annihilation cross-section and dark matter mass.   The fluctuation power, , depends on structure formation.  The hardest part is the prediction for the mean intensity. However… Remember that the mean intensity has been measured already!  The prediction for anisotropy is robust. All we need is a fraction of the mean intensity that is due to DM annihilation.  Blazars account for ~1/4 of the mean intensity. What about dark matter annihilation?

17. A Simple Route to the Angular Power Spectrum  To compute the power spectrum of anisotropy from dark matter annihilation, we need three ingredients: 1.Number of halos as a function of mass, 2.Clustering of dark matter halos, and 3.Substructure inside of each halo.  To compute the power spectrum of anisotropy from dark matter annihilation, we need three ingredients: 1.Number of halos as a function of mass, 2.Clustering of dark matter halos, and 3.Substructure inside of each halo. θ (= π / l) Dark matter halo

18. A Few Equations Gamma-ray intensity: Spherical harmonic expansion: Limber’s equation:

19. Astrophysical Background: Anisotropy from Blazars  Blazars also trace the large-scale structure.  The observed anisotropy may be described as the sum of blazars and dark matter annihilation.  Again, three ingredients are necessary: 1.Luminosity function of blazars, 2.Clustering of dark matter halos, and 3.“Bias” of blazars: the extent to which blazars trace the underlying matter distribution.  This turns out to be unimportant (next slide)  Is the blazar power spectrum different sufficiently from the dark matter annihilation power spectrum?  Blazars also trace the large-scale structure.  The observed anisotropy may be described as the sum of blazars and dark matter annihilation.  Again, three ingredients are necessary: 1.Luminosity function of blazars, 2.Clustering of dark matter halos, and 3.“Bias” of blazars: the extent to which blazars trace the underlying matter distribution.  This turns out to be unimportant (next slide)  Is the blazar power spectrum different sufficiently from the dark matter annihilation power spectrum?

20. Predicted Angular Power Spectrum Ando, Komatsu, Narumoto & Totani (2007)  At 10 GeV for 2-yr observations of GLAST  Blazars (red curves) easily discriminated from the DM signal --- the blazar power spectrum is nearly Poissonian.  The error blows up at small angular scales due to angular resolution (~0.1 deg) & blazar contribution.  At 10 GeV for 2-yr observations of GLAST  Blazars (red curves) easily discriminated from the DM signal --- the blazar power spectrum is nearly Poissonian.  The error blows up at small angular scales due to angular resolution (~0.1 deg) & blazar contribution. 39% DM61% DM 80% DM97% DM

21. What If Substructures Were Disrupted … 39% DM61% DM 97% DM80% DM S/N goes down as more subhalos are disrupted in massive parent halos. In this particular example, the number of subhalos per halo is proportinal to M 0.7, where M is the parent halo mass. If no disruption occurred, the number of subhalos per halo should be proportional to M. S/N goes down as more subhalos are disrupted in massive parent halos. In this particular example, the number of subhalos per halo is proportinal to M 0.7, where M is the parent halo mass. If no disruption occurred, the number of subhalos per halo should be proportional to M.

22. “ No Substructure ” or “ Smooth Halo ” Limit 39% DM61% DM 97% DM80% DM Our Best Estimate: “If dark matter annihilation contributes > 30% of the mean intensity, GLAST should be able to detect anisotropy.” A similar analysis can be done for AMS-2. Our Best Estimate: “If dark matter annihilation contributes > 30% of the mean intensity, GLAST should be able to detect anisotropy.” A similar analysis can be done for AMS-2.

23. Positron-electron Annihilation in the Galactic Center Jean et al. (2003); Knoedlseder et al. (2005);Weidenspointner et al. (2006)  INTEGRAL/SPI has detected a significant line emission at 511 keV from the G.C.  Extended over the bulge -- inconsistent with a point source!  Flux ~ 10 -3 ph cm -2 s -1  Continuum emission indicates that more than 90% of annihilation takes place in positronium.  INTEGRAL/SPI has detected a significant line emission at 511 keV from the G.C.  Extended over the bulge -- inconsistent with a point source!  Flux ~ 10 -3 ph cm -2 s -1  Continuum emission indicates that more than 90% of annihilation takes place in positronium.

24. INTEGRAL/SPI Spectrum  Ortho-positronium continuum is clearly seen (blue line)  Best-fit positronium fraction = (96 +- 4)%  Where do these positrons come from?  Ortho-positronium continuum is clearly seen (blue line)  Best-fit positronium fraction = (96 +- 4)%  Where do these positrons come from? Churazov et al. (2005)

25. Light Dark Matter Annihilation  Light (~MeV) dark matter particles can produce non- relativistic positrons, which would produce line emission at 511keV. The required (S-wave) annihilation cross section (~a few x 10 -26 cm 3 s -1 ) is indeed reasonable!  Boehm et al., PRL, 92, 101301 (2004)  Hooper et al., PRL, 93, 161302 (2004)  The fact that we see a line sets an upper limit on the positron initial energy of ~3 MeV.  Beacom & Yuksel, PRL, 97, 071102 (2006)  Continuum gamma-ray is also produced via the “internal bremsstrahlung”, XX -> e + e -   Beamcom, Bell & Bertone, PRL, 94, 171301 (2005)  How about the extra-galactic background light?  Light (~MeV) dark matter particles can produce non- relativistic positrons, which would produce line emission at 511keV. The required (S-wave) annihilation cross section (~a few x 10 -26 cm 3 s -1 ) is indeed reasonable!  Boehm et al., PRL, 92, 101301 (2004)  Hooper et al., PRL, 93, 161302 (2004)  The fact that we see a line sets an upper limit on the positron initial energy of ~3 MeV.  Beacom & Yuksel, PRL, 97, 071102 (2006)  Continuum gamma-ray is also produced via the “internal bremsstrahlung”, XX -> e + e -   Beamcom, Bell & Bertone, PRL, 94, 171301 (2005)  How about the extra-galactic background light?

26. AGNs, Supernovae, and Dark Matter Annihilation…  The extra-galactic background in 1-20MeV region is a superposition of AGNs, SNe, and possibly DM annihilation.  SNe cannot explain the background.  AGNs cut off at ~1MeV.  ~20 MeV DM fits the data very well.  The extra-galactic background in 1-20MeV region is a superposition of AGNs, SNe, and possibly DM annihilation.  SNe cannot explain the background.  AGNs cut off at ~1MeV.  ~20 MeV DM fits the data very well. Ahn & EK, PRD, 71, 021303R; 71, 121301R; 72, 061301R (05) COMPTEL SMM HEAO-1 AGNs SNe DM

27. Implications for AMS-2?  Gamma-rays from DM annihilation of MeV dark matter, or possible positron excess, are out of reach.  Too low an energy for AMS-2 to measure…  Gamma-rays from DM annihilation of MeV dark matter, or possible positron excess, are out of reach.  Too low an energy for AMS-2 to measure…

28. Summary  Convincing evidence for gamma-rays from DM will have a huge impact on particle physics and cosmology. The extra-galactic gamma-ray background  The Galactic Center may not be the best place to look. The extra-galactic gamma-ray background, which has been measured by EGRET and will be measured more precisely by AMS-2 and GLAST, may hold the key. the power spectrum of cosmic gamma-ray anisotropy is a very powerful probe  The mean intensity is not enough: the power spectrum of cosmic gamma-ray anisotropy is a very powerful probe.  If >30% of the mean intensity comes from dark matter annihilation (at 10 GeV), GLAST will detect it in two years.  Prospects for detecting it in AMS-2 data remain to be seen.  A possibility of MeV dark matter is very intriguing.  But, it is out of reach for AMS-2…  Convincing evidence for gamma-rays from DM will have a huge impact on particle physics and cosmology. The extra-galactic gamma-ray background  The Galactic Center may not be the best place to look. The extra-galactic gamma-ray background, which has been measured by EGRET and will be measured more precisely by AMS-2 and GLAST, may hold the key. the power spectrum of cosmic gamma-ray anisotropy is a very powerful probe  The mean intensity is not enough: the power spectrum of cosmic gamma-ray anisotropy is a very powerful probe.  If >30% of the mean intensity comes from dark matter annihilation (at 10 GeV), GLAST will detect it in two years.  Prospects for detecting it in AMS-2 data remain to be seen.  A possibility of MeV dark matter is very intriguing.  But, it is out of reach for AMS-2…