1,374 research outputs found
Spectral cutoffs in indirect dark matter searches
Indirect searches for dark matter annihilation or decay products in the
cosmic-ray spectrum are plagued by the question of how to disentangle a dark
matter signal from the omnipresent astrophysical background. One of the
practically background-free smoking-gun signatures for dark matter would be the
observation of a sharp cutoff or a pronounced bump in the gamma-ray energy
spectrum. Such features are generically produced in many dark matter models by
internal Bremsstrahlung, and they can be treated in a similar manner as the
traditionally looked-for gamma-ray lines. Here, we discuss prospects for seeing
such features with present and future Atmospheric Cherenkov Telescopes.Comment: 4 pages, 2 figures, 1 table; conference proceedings for TAUP 2011,
Munich 5-9 Se
Aspects of production and kinetic decoupling of non-thermal dark matter
We reconsider non-thermal production of WIMP dark matter in a systematic way
and using a numerical code for accurate computations of dark matter relic
densities. Candidates with large pair annihilation rates are favored,
suggesting a connection with the anomalies in the lepton cosmic-ray flux
detected by Pamela and Fermi. Focussing on supersymmetric models we will
consider the impact of non-thermal production on the preferred mass scale for
dark matter neutralinos. We have also developed a new formalism to solve the
Boltzmann's equation for a system of coannihilating species without assuming
kinetic equilibrium and applied it to the case of pure Winos.Comment: Proceedings for the conference TAUP 201
Constraints on small-scale cosmological perturbations from gamma-ray searches for dark matter
Events like inflation or phase transitions can produce large density
perturbations on very small scales in the early Universe. Probes of small
scales are therefore useful for e.g. discriminating between inflationary
models. Until recently, the only such constraint came from non-observation of
primordial black holes (PBHs), associated with the largest perturbations.
Moderate-amplitude perturbations can collapse shortly after matter-radiation
equality to form ultracompact minihalos (UCMHs) of dark matter, in far greater
abundance than PBHs. If dark matter self-annihilates, UCMHs become excellent
targets for indirect detection. Here we discuss the gamma-ray fluxes expected
from UCMHs, the prospects of observing them with gamma-ray telescopes, and
limits upon the primordial power spectrum derived from their non-observation by
the Fermi Large Area Space Telescope.Comment: 4 pages, 3 figures. To appear in J Phys Conf Series (Proceedings of
TAUP 2011, Munich
Thermal decoupling and the smallest subhalo mass in dark matter models with Sommerfeld-enhanced annihilation rates
We consider dark matter consisting of weakly interacting massive particles
(WIMPs) and revisit in detail its thermal evolution in the early universe, with
a particular focus on models where the annihilation rate is enhanced by the
Sommerfeld effect. After chemical decoupling, or freeze-out, dark matter no
longer annihilates but is still kept in local thermal equilibrium due to
scattering events with the much more abundant standard model particles. During
kinetic decoupling, even these processes stop to be effective, which eventually
sets the scale for a small-scale cutoff in the matter density fluctuations.
Afterwards, the WIMP temperature decreases more quickly than the heat bath
temperature, which causes dark matter to reenter an era of annihilation if the
cross-section is enhanced by the Sommerfeld effect. Here, we give a detailed
and self-consistent description of these effects. As an application, we
consider the phenomenology of simple leptophilic models that have been
discussed in the literature and find that the relic abundance can be affected
by as much two orders of magnitude or more. We also compute the mass of the
smallest dark matter subhalos in these models and find it to be in the range of
about 10^{-10} to 10 solar masses; even much larger cutoff values are possible
if the WIMPs couple to force carriers lighter than about 100 MeV. We point out
that a precise determination of the cutoff mass allows to infer new limits on
the model parameters, in particular from gamma-ray observations of galaxy
clusters, that are highly complementary to existing constraints from g-2 or
beam dump experiments.Comment: minor changes to match published versio
Is dark matter with long-range interactions a solution to all small-scale problems of \Lambda CDM cosmology?
The cold dark matter (DM) paradigm describes the large-scale structure of the
universe remarkably well. However, there exists some tension with the observed
abundances and internal density structures of both field dwarf galaxies and
galactic satellites. Here, we demonstrate that a simple class of DM models may
offer a viable solution to all of these problems simultaneously. Their key
phenomenological properties are velocity-dependent self-interactions mediated
by a light vector messenger and thermal production with much later kinetic
decoupling than in the standard case.Comment: revtex4; 6 pages, 3 figures; minor changes to match published versio
New Positron Spectral Features from Supersymmetric Dark Matter - a Way to Explain the PAMELA Data?
The space-borne antimatter experiment PAMELA has recently reported a
surprising rise in the positron to electron ratio at high energies. It has also
recently been found that electromagnetic radiative corrections in some cases
may boost the gamma-ray yield from supersymmetric dark matter annihilations in
the galactic halo by up to three or four orders of magnitude, providing
distinct spectral signatures for indirect dark matter searches to look for.
Here, we investigate whether the same type of corrections can also lead to
sizeable enhancements in the positron yield. We find that this is indeed the
case, albeit for a smaller region of parameter space than for gamma rays;
selecting models with a small mass difference between the neutralino and
sleptons, like in the stau coannihilation region in mSUGRA, the effect becomes
more pronounced. The resulting, rather hard positron spectrum with a relatively
sharp cutoff may potentially fit the rising positron ratio measured by the
PAMELA satellite. To do so, however, very large "boost factors" have to be
invoked that are not expected in current models of halo structure. If the
predicted cutoff would also be confirmed by later PAMELA data or upcoming
experiments, one could either assume non-thermal production in the early
universe or non-standard halo formation to explain such a spectral feature as
an effect of dark matter annihilation. At the end of the paper, we briefly
comment on the impact of radiative corrections on other annihilation channels,
in particular antiprotons and neutrinos.Comment: corrected axis labels in Fig. 3; matches the published version (PRD,
in press
A Tentative Gamma-Ray Line from Dark Matter Annihilation at the Fermi Large Area Telescope
The observation of a gamma-ray line in the cosmic-ray fluxes would be a
smoking-gun signature for dark matter annihilation or decay in the Universe. We
present an improved search for such signatures in the data of the Fermi Large
Area Telescope (LAT), concentrating on energies between 20 and 300 GeV. Besides
updating to 43 months of data, we use a new data-driven technique to select
optimized target regions depending on the profile of the Galactic dark matter
halo. In regions close to the Galactic center, we find a 4.6 sigma indication
for a gamma-ray line at 130 GeV. When taking into account the look-elsewhere
effect the significance of the observed excess is 3.2 sigma. If interpreted in
terms of dark matter particles annihilating into a photon pair, the
observations imply a dark matter mass of 129.8\pm2.4^{+7}_{-13} GeV and a
partial annihilation cross-section of = 1.27\pm0.32^{+0.18}_{-0.28}
x 10^-27 cm^3 s^-1 when using the Einasto dark matter profile. The evidence for
the signal is based on about 50 photons; it will take a few years of additional
data to clarify its existence.Comment: 23 pages, 9 figures, 3 tables; extended discussion; matches published
versio
Sommerfeld Enhancement of DM Annihilation: Resonance Structure, Freeze-Out and CMB Spectral Bound
In the last few years there has been some interest in WIMP Dark Matter models
featuring a velocity dependent cross section through the Sommerfeld enhancement
mechanism, which is a nonrelativistic effect due to massive bosons in the dark
sector. In the first part of this article, we find analytic expressions for the
boost factor for three different model potentials, the Coulomb potential, the
spherical well and the spherical cone well and compare with the numerical
solution of the Yukawa potential. We find that the resonance pattern of all the
potentials can be cast into the same universal form. In the second part of the
article we perform a detailed computation of the Dark Matter relic density for
models having Sommerfeld enhancement by solving the Boltzmann equation
numerically. We calculate the expected distortions of the CMB blackbody
spectrum from WIMP annihilations and compare these to the bounds set by FIRAS.
We conclude that only a small part of the parameter space can be ruled out by
the FIRAS observations.Comment: 15 pages, 15 figures, version accepted by JCA
Antiproton constraints on dark matter annihilations from internal electroweak bremsstrahlung
If the dark matter particle is a Majorana fermion, annihilations into two
fermions and one gauge boson could have, for some choices of the parameters of
the model, a non-negligible cross-section. Using a toy model of leptophilic
dark matter, we calculate the constraints on the annihilation cross-section
into two electrons and one weak gauge boson from the PAMELA measurements of the
cosmic antiproton-to-proton flux ratio. Furthermore, we calculate the maximal
astrophysical boost factor allowed in the Milky Way under the assumption that
the leptophilic dark matter particle is the dominant component of dark matter
in our Universe. These constraints constitute very conservative estimates on
the boost factor for more realistic models where the dark matter particle also
couples to quarks and weak gauge bosons, such as the lightest neutralino which
we also analyze for some concrete benchmark points. The limits on the
astrophysical boost factors presented here could be used to evaluate the
prospects to detect a gamma-ray signal from dark matter annihilations at
currently operating IACTs as well as in the projected CTA.Comment: 32 pages; 13 figure
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