Annihilating Dark Matter

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Annihilating Dark Matter
Annihilating Dark Matter
Nicole Bell
The University of Melbourne
John Beacom (Ohio State) Gianfranco Bertone (Paris, Inst. Astrophys.)
and Gregory Mack (Ohio State)
Dark 2007, Sydney, 27th July 2007
 Galactic Positrons & Light Dark Matter
 General Bound on the Dark Matter
Annihilation Rate
Galactic Positrons
0.511 MeV Gamma-Ray Emission Line
The SPI camera on INTEGRAL satellite has measured
the Galactic 511 keV gamma ray emission line:
 511  9.9 42..71 10 4 photons cm -2 s 1
Flux originates from near the Galactic center
Emission region: 2-D gaussian with FWHM ~ 9 degrees.
INTEGRAL sees no evidence for discrete sources. Only
weak evidence for a disk component.
Knodlseder et al,
A&A, 441, 513, 2005.
SPI / INTEGRAL 511 keV observation (astro-ph/0309442)
Possible Positron Sources
Astrophysical sources?
E.g. Compact objects, massive stars, supernovae, GRBs,
cosmic rays.
However: difficult to explain the intensity and emission region.
Something Exotic?
A mechanism associated with dark matter concentration at the
Galactic center might be more natural.
Annihilation of dark matter?
Light (MeV) Dark Matter
Boehm, Hooper, Silk, Casse and Paul, PRL 92, 101301 (2004)
 DM particle  with mass 1-100 MeV
Light mass allows the DM to annihilate only into e+e- pairs.
(annihilation to photons or neutrinos postulated not to occur.)
Positrons produced with energy = m
Loose energy by primarily by ionization
Form positronium
Positronium annihilates to produce 511 keV photons.
Can positrons really be produced
Tree level process
  e  e 
must be accompanied by the radiative correction
  e  e 
Detectable photons emitted via “Internal Bremsstrahlung” processs.
Bremsstrahlung cross section
d Br
 1   s'     s'  
  total 
ln  2   1 1    
 E   me     s  
s  4m2
s'  4m (m  E )
 total  tree level cross section
Bremsstrahlung spectrum (per 511 keV gamma):
dN Br  f
1 d Br
   (1  f )  
2  total dE
f=positronium fraction
Galactic Diffuse Gamma Ray Background (EGRET)
(Strong, Moskalenko and Reimer. astro-ph/0406254)
Diffuse Gamma ray background has mild variation across a large region.
Bremsstrahlung Spectrum & COMPTEL/EGRET constraints
 Maximum permitted
contribution of internal
bremsstrahlung photons
to gamma ray background
Positron diffusion length is small
Positrons brought to rest quickly
All positrons annihilate at rest
If any of these assumptions are violated, a higher
dark matter annihilation rate is required to obtain
the same 511keV intensity  stronger constraint.
Also note that we don’t need to make any assumptions about
the dark matter density profile, as we tie the bremsstrahlung
flux directly to the 511 keV flux.
Annihilation in Flight
Beacom and Yuksel, astro-ph/0512411
If positrons produced at mildly relativistic energies,
higher energy gamma rays will be produced due to
in-flight annihilation.
 Positrons must be injected with E > 3 MeV
Conclusions - Positrons
ANY mechanism which produces energetic positrons will be
accompanied by gamma rays from internal bremsstrahlung.
ANY scenario (including astrophysical production) in which
the Galactic positrons are created with energy > 20 MeV will
violate COMPTEL/EGRET constraints.
Beacom, Bell and Bertone, Phys. Rev. Lett. 94, 171301 (2005)
General Bound on the Dark
Matter Annihilation Rate
Dark Matter Annihilation
Self-annihilation cross-section is a fundamental property of
dark matter
For thermal relics it is specified by the DM relic density:
More generally, it controls the DM annihilation rate in
galaxies today  can affect galaxy halo density profiles
Two general constraints on dark matter disappearance:
Constraint 1. - Unitarity Bound
Unitarity sets a general upper bound on the cross-section:
(in the low-velocity limit, where the cross-section is s-wave dominated)
In galaxies today:
L. Hui
Most restrictive for high masses.
Constraint 2. Kaplinghat-Knox-Turner Model
Phys. Rev. Lett. 85. 3335 (2000)
Large dark matter annihilation rate
flattens galaxy cores
invoked to resolve conflict between
predicted (sharp cusps) and observed
(flat cores) halo density profiles.
KKT model requires cross-sections ~107 times larger than the
natural scale for a thermal relic:
Reinterpret this type of model as upper bound on
Annihilation to Standard Model
final states:
All final states except neutrinos produce gamma rays,
Bound the total cross-section from the neutrino signal limit
i.e. by assuming Br(“invisible”) = 100%
Diffuse Neutrino Signal:
Annihilation to neutrinos:
Diffuse flux (Ullio, et al.):
The factor  accounts for the increase in density due to the
clustering of dark matter in halos. (=1 corresponds to all
matter being at the average density in the Universe today)
Atmospheric neutrinos are the background
Conservative detection criteria: Signal 100% as large as
angle averaged atmospheric neutrino background.
Diffuse Annihilation Signal
m  10,10 ,10 GeV
Upper panel: –
Annihilation flux
superimposed on
atmospheric neutrino
Lower panel:
Upper bounds on the dark matter
total annihilation cross-section
Annihilation effect in Galaxy Halos?
 Annihilation flattens halo cusps to a core density of:
 Our bound implies that for all m > 0.1 GeV:
Only affects the very inner region of typical galaxies.
e.g. In the Milky Way, this density occurs only at radii < 1 pc for
an NFW profile (and maybe not at all for less steep profiles).
Dark matter annihilation cannot have a macroscopic effect
on galactic halos.
Conclusions – Annihilation rate
Dark matter total annihilation cross-section (i.e. disappearance
rate) can be bounded by least detectable annihilation products
(i.e. neutrino appearance rate.)
Neutrino bound much stronger than Unitarity for m<10TeV.
Dark Matter halos cannot be significantly modified by
Beacom, Bell and Mack, Phys. Rev. Lett. (in press).
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