Annihilating Dark Matter Nicole Bell The University of Melbourne with John Beacom (Ohio State) Gianfranco Bertone (Paris, Inst. Astrophys.) and Gregory Mack (Ohio State) Dark 2007, Sydney, 27th July 2007
Outline • Galactic Positrons & Light Dark Matter • General Bound on the Dark Matter Annihilation Rate
0.511 MeV Gamma-Ray Emission Line The SPI camera on INTEGRAL satellite has measured the Galactic 511 keV gamma ray emission line: • 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.
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 INVISIBLY? Tree level process must be accompanied by the radiative correction Detectable photons emitted via “Internal Bremsstrahlung” processs.
Bremsstrahlung cross section Bremsstrahlung spectrum (per 511 keV gamma): 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
Assumptions: • 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)
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: e.g. implies • 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 Upper panel: – Annihilation flux superimposed on atmospheric neutrino background Lower panel: (Signal+Background)/Background
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 annihilation. Beacom, Bell and Mack, Phys. Rev. Lett. (in press).