Gamma-ray From Annihilation of Dark Matter ParticlesGamma-ray From Annihilation of Dark Matter Particles

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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)

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What Is Out There?

WMAP 94GHz

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What Is Out There?

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Deciphering Gamma-ray Sky Deciphering Gamma-ray Sky

 Astrophysical: Galactic vs Extra-galacticAstrophysical

 Galactic origin (diffuse)

 E.g., Decay of neutral pions produced by cosmic-rays int eracting 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 Gala xy

 Extra-galactic Origin

 Dark matter annihilation in the other galaxies

 Astrophysical: Galactic vs Extra-galacticAstrophysical

 Galactic origin (diffuse)

 E.g., Decay of neutral pions produced by cosmic-rays int eracting 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 Gala xy

 Extra-galactic Origin

 Dark matter annihilation in the other galaxies

Relativistic Jets

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Blazars 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 ti mes 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 ti mes larger, ~ 10^-8 cm^-2 s^-1 (G. Lamanna 2002)

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Blazar Luminosity Function Update

Luminosity-Dependent Density Evolution (LDDE) model fits the EGRET counts very well. This model has been de rived 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 de rived 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

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Redshift distribution of blazars that would be detected by GLAST Redshift distribution of blazars that would be detected by GLAST

•LDDE1: The best-fitting model, which accounts for

~1/4 of the gamma-ray ba ckground.

•LDDE2: A more aggressi ve model that accounts fo r 100% of the gamma-ray background.

•It is assumed that blazar s are brighter than 10⁴¹ er g/s at 0.1 GeV.

Ando et al. (2007)

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-ray Background

 Un-resolved Blazars that are be low the point-source sensitivity will contribute to the diffuse badiffuse ba

ckground ckground.

 EGRET has measured the diffu se background above the Galact ic plane.

 LDDE predicts that only ~1/4 of the diffuse light is due to bl azars!

 AMS-2 will do MUCH better th an EGRET in the diffuse backg round

 Un-resolved Blazars that are be low the point-source sensitivity will contribute to the diffuse badiffuse ba

ckground ckground.

 EGRET has measured the diffu se background above the Galact ic plane.

 LDDE predicts that only ~1/4 of the diffuse light is due to bl azars!

 AMS-2 will do MUCH better th an EGRET in the diffuse backg round

(G. Lamanna 2002)

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Dark matter (WIMP) annihilation

 WIMP dark matter

annihilates into gamma- ray photons.

 The dominant mode: jets

Branching ratios for line emission (two gamma & gamma+Z⁰) 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+Z⁰) are small.

 WIMP mass is likely around GeV–TeV, if

WIMP is neutralino-like.

 Can GLAST or AMS-2 see this?

GeV-γ

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DM Annihilation in MW DM Annihilation in MW

Diemand, Khlen & Madau, ApJ, 657, 262 (200 7)

•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)

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Why MW? There are many more dark matter halos out

there!

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?”

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Gamma-ray Anisotropy From Dark Matter

Annihilation

Gamma-ray Anisotropy From Dark Matter

Annihilation

 Dark matter halos trace the large-scale structure Dark matter halos trace the large-scale structure of the universe.

of the universe.

 The distribution of gamma-rays from these sources must be inhomogeneous, with a well defined must

angular power spectrum 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 Dark matter halos trace the large-scale structure of the universe.

of the universe.

 The distribution of gamma-rays from these sources must be inhomogeneous, with a well defined must

angular power spectrum 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)

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

WMAP 94GHz

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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 SkyAnisotropy in Gamma-ray Sky))

= (= (MEAN INTENSITYMEAN INTENSITY) x ) 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 dar k 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(Anisotropy in Gamma-ray Sky))

= (MEAN INTENSITY= (MEAN INTENSITY) x ) 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 dar k matter annihilation?

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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,

3. Substructure inside of each halo.

θ (= π / l)

Dark matter halo

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A Few Equations A Few Equations

Gamma-ray intensity:

Spherical harmonic expansion:

Limber’s equation:

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Astrophysical Background: Ani sotropy from Blazars

 Blazars also trace the large-scale structure.

 The observed anisotropy may be described as the sum of bl azars and dark matter annihilation.

 Again, three ingredients are necessary:

1. Luminosity function of blazars,

3. “Bias” of blazars: the extent to which blazars trace the und erlying matter distribution.

 This turns out to be unimportant (next slide)

 Is the blazar power spectrum different sufficiently fro m the dark matter annihilation power spectrum?

 Blazars also trace the large-scale structure.

 The observed anisotropy may be described as the sum of bl azars and dark matter annihilation.

 Again, three ingredients are necessary:

1. Luminosity function of blazars,

3. “Bias” of blazars: the extent to which blazars trace the und erlying matter distribution.

 This turns out to be unimportant (next slide)

 Is the blazar power spectrum different sufficiently fro m the dark matter annihilation power spectrum?

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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% DM 61% DM

80% DM 97% DM

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What If Substructures Were Disrupted…

39% DM 61% DM

97% DM 80% 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 p er halo should be proport ional 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 p er halo should be proport ional to M.

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“No Substructure”

or “Smooth Halo” Limit

“No Substructure”

or “Smooth Halo” Limit

39% DM 61% DM

97% DM 80% DM

Our Best Estimate:

“If dark matter annihilation

contributes > 30% of the mean intensity, GLAST should be able to detect

anisotropy.”

• A similar analysis ca n 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 ca n be done for AMS- 2.

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Positron-electron Annihilation i n the Galactic Center

Jean et al. (2003); Knoedlseder et al. (2005);Weidenspointner et al. (2006)

 INTEGRAL/SPI has detected a s ignificant line emission at 511 k eV from the G.C.

 Extended over the bulge -- inconsi stent with a point source!

 Flux ~ 10^-3 ph cm^-2 s^-1

 Continuum emission indicates th at more than 90% of annihilation takes place in positronium.

 INTEGRAL/SPI has detected a s ignificant line emission at 511 k eV from the G.C.

 Extended over the bulge -- inconsi stent with a point source!

 Flux ~ 10^-3 ph cm^-2 s^-1

 Continuum emission indicates th at more than 90% of annihilation takes place in positronium.

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INTEGRAL/SPI Spectrum INTEGRAL/SPI Spectrum

 Ortho-positronium c ontinuum is clearly s een (blue line)

 Best-fit positronium fraction = (96 +- 4)%

 Where do these posit rons come from?

 Ortho-positronium c ontinuum is clearly s een (blue line)

 Best-fit positronium fraction = (96 +- 4)%

 Where do these posit rons come from?

Churazov et al. (2005)

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Light Dark Matter Annihilation Light Dark Matter Annihilation

 Light (~MeV) dark matter particles can produce non-rela tivistic positrons, which would produce line emission at 511keV. The required (S-wave) annihilation cross sec tion (~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 posi tron 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? How about the extra-galactic background light?

 Light (~MeV) dark matter particles can produce non-rela tivistic positrons, which would produce line emission at 511keV. The required (S-wave) annihilation cross sec tion (~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 posi tron 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? How about the extra-galactic background light?

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AGNs, Supernovae, and

Dark Matter Annihilation…

AGNs, Supernovae, and

Dark Matter Annihilation…

 The extra-galactic backgr ound in 1-20MeV region i s a superposition of AGN s, SNe, and possibly DM annihilation.

 SNe cannot explain the ba ckground.

 AGNs cut off at ~1MeV.

 ~20 MeV DM fits the da ta very well.

 The extra-galactic backgr ound in 1-20MeV region i s a superposition of AGN s, SNe, and possibly DM annihilation.

 SNe cannot explain the ba ckground.

 AGNs cut off at ~1MeV.

 ~20 MeV DM fits the da ta very well.

Ahn & EK, PRD, 71, 021303R; 71, 121301R; 72, 061301R (05)

COMPTEL SMM

HEAO-1

AGNs

SNe

DM

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Implications for AMS-2?

 Gamma-rays from DM annihilation of Me V dark matter, or possible positron excess, a re out of reach.

 Too low an energy for AMS-2 to measure…

 Gamma-rays from DM annihilation of Me V dark matter, or possible positron excess, a re out of reach.

 Too low an energy for AMS-2 to measure…

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Summary Summary

Convincing evidence for gamma-rays from DM will have a huge impact on particle physics and cosmology.

The Galactic Center may not be the best place to look. The extra-galactic gamma-The extra-galactic gamma- ray background

ray background, which has been measured by EGRET and will be measured more precisely by AMS-2 and GLAST, may hold the key.

The mean intensity is not enough: the power spectrum of cosmic gamma-ray anisotropy is a vethe power spectrum of cosmic gamma-ray anisotropy is a ve ry powerful probe

ry powerful probe.

If >30% of the mean intensity comes from dark matter annihilation (at 10 GeV), GLAST will detec t 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 Galactic Center may not be the best place to look. The extra-galactic gamma-The extra-galactic gamma- ray background

ray background, which has been measured by EGRET and will be measured more precisely by AMS-2 and GLAST, may hold the key.

The mean intensity is not enough: the power spectrum of cosmic gamma-ray anisotropy is a vethe power spectrum of cosmic gamma-ray anisotropy is a ve ry powerful probe

ry powerful probe.

If >30% of the mean intensity comes from dark matter annihilation (at 10 GeV), GLAST will detec t 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…