120 research outputs found
Indirect Dark Matter Signatures in the Cosmic Dark Ages I. Generalizing the Bound on s-wave Dark Matter Annihilation from Planck
Recent measurements of the cosmic microwave background (CMB) anisotropies by
Planck provide a sensitive probe of dark matter annihilation during the cosmic
dark ages, and specifically constrain the annihilation parameter
. Using new results (Paper II)
for the ionization produced by particles injected at arbitrary energies, we
calculate and provide values for photons and pairs
injected at keV-TeV energies; the value for any dark matter
model can be obtained straightforwardly by weighting these results by the
spectrum of annihilation products. This result allows the sensitive and robust
constraints on dark matter annihilation presented by the Planck Collaboration
to be applied to arbitrary dark matter models with -wave annihilation. We
demonstrate the validity of this approach using principal component analysis.
As an example, we integrate over the spectrum of annihilation products for a
range of Standard Model final states to determine the CMB bounds on these
models as a function of dark matter mass, and demonstrate that the new limits
generically exclude models proposed to explain the observed high-energy rise in
the cosmic ray positron fraction. We make our results publicly available at
http://nebel.rc.fas.harvard.edu/epsilon.Comment: 14 pages, 4 figures, supplemental data / tools available at
http://nebel.rc.fas.harvard.edu/epsilon. Accompanying paper to "Indirect Dark
Matter Signatures in the Cosmic Dark Ages II. Ionization, Heating and Photon
Production from Arbitrary Energy Injections". v2 adds references, extra
example in Fig. 4, and small updates from accompanying paper. This version to
be submitted to Phys Rev
Too Hot, Too Cold or Just Right? Implications of a 21-cm Signal for Dark Matter Annihilation and Decay
Measurements of the temperature of the baryons at the end of the cosmic dark
ages can potentially set very precise constraints on energy injection from
exotic sources, such as annihilation or decay of the dark matter. However,
additional effects that lower the gas temperature can substantially weaken the
expected constraints on exotic energy injection, whereas additional radiation
backgrounds can conceal the effect of an increased gas temperature in
measurements of the 21-cm hyperfine transition of neutral hydrogen. Motivated
in part by recent claims of a detection of 21-cm absorption from a redshift of
17 by the EDGES experiment, we derive the constraints on dark matter
annihilation and decay that can be placed in the presence of extra radiation
backgrounds or effects that modify the gas temperature, such as dark
matter-baryon scattering and early baryon-photon decoupling. We find that if
the EDGES observation is confirmed, then constraints on light dark matter
decaying or annihilating to electrons will in most scenarios be stronger than
existing state-of-the-art limits from the cosmic microwave background,
potentially by several orders of magnitude. More generally, our results allow
mapping any future measurement of the global 21-cm signal into constraints on
dark matter annihilation and decay, within the broad range of scenarios we
consider.Comment: 22 pages with appendices, 12 figures, comments welcome; v2:
references added with comments, typos corrected, minor change to millicharged
DM limit
Two Emission Mechanisms in the Fermi Bubbles: A Possible Signal of Annihilating Dark Matter
We study the variation of the spectrum of the Fermi Bubbles with Galactic
latitude. Far from the Galactic plane (|b| > 30 degrees), the observed
gamma-ray emission is nearly invariant with latitude, and is consistent with
arising from inverse Compton scattering of the interstellar radiation field by
cosmic-ray electrons with an approximately power-law spectrum. The same
electrons in the presence of microgauss-scale magnetic fields can also generate
the the observed microwave "haze". At lower latitudes (b < 20 degrees), in
contrast, the spectrum of the emission correlated with the Bubbles possesses a
pronounced spectral feature peaking at 1-4 GeV (in E^2 dN/dE) which cannot be
generated by any realistic spectrum of electrons. Instead, we conclude that a
second (non-inverse-Compton) emission mechanism must be responsible for the
bulk of the low-energy, low-latitude emission. This second component is
spectrally similar to the excess GeV emission previously reported from the
Galactic Center (GC), and also appears spatially consistent with a luminosity
per volume falling approximately as r^-2.4, where r is the distance from the
GC. We argue that the spectral feature visible in the low-latitude Bubbles is
the extended counterpart of the GC excess, now detected out to at least 2-3 kpc
from the GC. The spectrum and angular distribution of the signal is consistent
with that predicted from ~10 GeV dark matter particles annihilating to leptons,
or from ~50 GeV dark matter particles annihilating to quarks, following a
distribution similar to the canonical Navarro-Frenk-White (NFW) profile. We
also consider millisecond pulsars as a possible astrophysical explanation for
the signal, as observed millisecond pulsars possess a spectral cutoff at
approximately the required energy. Any such scenario would require a large
population of unresolved millisecond pulsars extending at least 2-3 kpc from
the GC.Comment: 26 pages, 20 figure
Multi-Step Cascade Annihilations of Dark Matter and the Galactic Center Excess
If dark matter is embedded in a non-trivial dark sector, it may annihilate
and decay to lighter dark-sector states which subsequently decay to the
Standard Model. Such scenarios - with annihilation followed by cascading
dark-sector decays - can explain the apparent excess GeV gamma-rays identified
in the central Milky Way, while evading bounds from dark matter direct
detection experiments. Each 'step' in the cascade will modify the observable
signatures of dark matter annihilation and decay, shifting the resulting
photons and other final state particles to lower energies and broadening their
spectra. We explore, in a model-independent way, the effect of multi-step
dark-sector cascades on the preferred regions of parameter space to explain the
GeV excess. We find that the broadening effects of multi-step cascades can
admit final states dominated by particles that would usually produce too
sharply peaked photon spectra; in general, if the cascades are hierarchical
(each particle decays to substantially lighter particles), the preferred mass
range for the dark matter is in all cases 20-150 GeV. Decay chains that have
nearly-degenerate steps, where the products are close to half the mass of the
progenitor, can admit much higher DM masses. We map out the region of
mass/cross-section parameter space where cascades (degenerate, hierarchical or
a combination) can fit the signal, for a range of final states. In the current
work, we study multi-step cascades in the context of explaining the GeV excess,
but many aspects of our results are general and can be extended to other
applications.Comment: 18 pages, 15 figures, 2 tables; comments welcome. Updated to
published versio
- β¦