Torsten Bringmann

New positron spectral features from supersymmetric dark matter: A way to explain the PAMELA data?

Department of Physics, Stockholm University, AlbaNova University Center, SE - 106 91 Stockholm, Sweden(Dated: August 27, 2008)The space-borne antimatter experiment PAMELA has recently reported a surprising rise in thepositron to electron ratio at high energies. It has also recently been found that electromagneticradiative corrections in some cases may boost the gamma-ray yield from supersymmetric darkmatter annihilations in the galactic halo by up to three or four orders of magnitude, providingdistinct spectral signatures for indirect dark matter searches to look for. Here, we investigatewhether 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 gammarays; selecting models with a small mass difference between the neutralino and sleptons, like inthe 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 positronratio measured by the PAMELA satellite. To do so, however, very large “boost factors” have tobe invoked that are not expected in current models of halo structure. If the predicted cutoff wouldalso 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 spectralfeature as an effect of dark matter annihilation. At the end of the paper, we briefly comment onthe impact of radiative corrections on other annihilation channels, in particular antiprotons andneutrinos.

Pre-launch estimates for GLAST sensitivity to dark matter annihilation signals

We investigate the sensitivity of the Gamma-ray Large Area Space Telescope (GLAST) for indirectly detecting weakly interacting massive particles (WIMPs) through the γ-ray signal that their pair ann ...

Particle models and the small-scale structure of dark matter

The kinetic decoupling of weakly interacting massive particles (WIMPs) in the early universe sets a scale that can directly be translated into a small-scale cutoff in the spectrum of matter density fluctuations. The formalism presented here allows a precise description of the decoupling process and thus the determination of this scale to a high accuracy from the details of the underlying WIMP microphysics. With decoupling temperatures of several MeV to a few GeV, the smallest protohalos to be formed range between 10-11 and almost 10-3 solar masses?a somewhat smaller range than what was found earlier using order-of-magnitude estimates for the decoupling temperature; for a given WIMP model, the actual cutoff mass is typically about a factor of 10 greater than derived in that way, though in some cases the difference may be as large as a factor of several hundreds. Observational consequences and prospects to probe this small-scale cutoff, which would provide a fascinating new window into the particle nature of dark matter, are discussed.

Gamma rays from Kaluza-Klein dark matter

A TeV gamma-ray signal from the direction of the Galactic center (GC) has been detected by the HESS experiment. Here, we investigate whether Kaluza-Klein (KK) dark matter annihilations near the GC can be the explanation. Including the contributions from internal bremsstrahlung as well as subsequent decays of quarks and tau leptons, we find a very flat gamma-ray spectrum which drops abruptly at the dark matter particle mass. For a KK mass of about 1 TeV, this gives a good fit to the HESS data below 1 TeV. A similar model, with gauge coupling roughly 3 times as large and a particle mass of about 10 TeV, would give both the correct relic density and a photon spectrum that fits the complete range of data.

Gamma-ray and Radio Constraints of High Positron Rate Dark Matter Models Annihilating into New Light Particles

The possibility of explaining the positron and electron excess recently found by the PAMELA and ATIC collaborations in terms of dark matter (DM) annihilation has attracted considerable attention. M ...

Gamma Rays from Heavy Neutralino Dark Matter

We consider the gamma-ray spectrum from neutralino dark matter annihilations and show that internal bremsstrahlung of pair final states gives a previously neglected source of photons at energies near the mass of the neutralino. For masses larger than about 1 TeV, and for present day detector resolutions, this results in a characteristic signal that may dominate not only over the continuous spectrum from W fragmentation, but also over the gammagamma and gammaZ line signals which are known to give large rates for heavy neutralinos. Observational prospects thus seem promising.

Two-photon annihilation of Kaluza–Klein dark matter

We investigate the fermionic one-loop cross section for the two-photon annihilation of Kaluza–Klein (KK) dark matter particles in a model of universal extra dimensions (UED). This process gives a nearly mono-energetic gamma-ray line with energy equal to the KK dark matter particle mass. We find that the cross section is large enough that if a continuum signature is detected, the energy distribution of gamma-rays should end at the particle mass with a peak that is visible for an energy resolution of the detector at the per cent level. This would give an unmistakable signature of a dark matter origin of the gamma-rays, and a unique determination of the dark matter particle mass, which in the case studied should be around 800 GeV. Unlike the situation for supersymmetric models where the two-gamma peak may or may not be visible depending on parameters, this feature seems to be quite robust in UED models, and should be similar in other models where annihilation into fermions is not helicity suppressed. The observability of the signal still depends on largely unknown astrophysical parameters related to the structure of the dark matter halo. If the dark matter near the galactic centre is adiabatically contracted by the central star cluster, or if the dark matter halo has substructure surviving tidal effects, prospects for detection look promising.

Dark Matter signals from Draco and Willman 1: Prospects for MAGIC II and CTA

The next generation of ground-based Imaging Air Cherenkov Telescopes will play an important role in indirect dark matter searches. In this article, we consider two particularly promising candidate sources for dark matter annihilation signals, the nearby dwarf galaxies Draco and Willman 1, and study the prospects of detecting such a signal for the soon-operating MAGIC II telescope system as well as for the planned installation of CTA, taking special care of describing the experimental features that affect the detectional prospects. For the first time in such studies, we fully take into account the effect of internal bremsstrahlung, which has recently been shown to considerably enhance, in some cases, the gamma-ray flux in the high energies domain where Atmospheric Cherenkov Telescopes operate, thus leading to significantly harder annihilation spectra than traditionally considered. While the detection of the spectral features introduced by internal bremsstrahlung would constitute a smoking gun signature for dark matter annihilation, we find that for most models the overall flux still remains at a level that will be challenging to detect, unless one adopts somewhat favorable descriptions of the smooth dark matter distribution in the dwarfs.

Cosmological evolution of homogeneous universal extra dimensions

Recent observational achievements within cosmology and astrophysics have lead to a concordance model in which the energy content in our Universe is dominated by presumably fundamentally new and exotic ingredients – dark energy and dark matter. To reveal the nature of these ingredients is one of the greatest challenges in physics.The detection of a signal in gamma rays from dark matter annihilation would significantly contribute to revealing the nature of dark matter. This thesis presents derived imprints in gamma-ray spectra that could be expected from dark matter annihilation. In particular, dark matter particle candidates emerging in models with extra space dimensions, extending the standard model to be supersymmetric, and introducing an inert Higgs doublet are investigated. In all these scenarios dark matter annihilation induces sizeable and distinct signatures in their gamma-ray spectra. The predicted signals are in the form of monochromatic gamma-ray lines or a pronounced spectrum with a sharp cutoff at the dark matter particle’s mass. These signatures have no counterparts in the expected astrophysical background and are therefore well suited for dark matter searches.Furthermore, numerical simulations of galaxies are studied to learn how baryons, that is, stars and gas, affect the expected dark matter distribution inside disk galaxies such as the Milky Way. From regions of increased dark matter concentrations, annihilation signals are expected to be the strongest. Estimations of dark matter induced gamma-ray fluxes from such regions are presented.The types of dark matter signals presented in this thesis will be searched for with existing and future gamma-ray telescopes.Finally, a claimed detection of dark matter annihilation into gamma rays is discussed and found to be unconvincing.

High-energetic Cosmic Antiprotons from Kaluza-Klein Dark Matter

The lightest Kaluza–Klein particle (LKP) in models with universal extra dimensions is an interesting dark matter candidate that has recently received great attention. Here, we investigate the antiproton flux from LKP annihilations in the galactic halo. In our analysis we include different halo density profiles and allow for part of the dark matter to be concentrated in clumps rather than being distributed homogeneously. After re-analysing the observational bounds on the allowed amount of clumpiness, we find that LKP annihilations may well give a significant contribution to the antiproton flux at energies higher than about 10xa0GeV, while for energies above around 500xa0GeV the conventional background is expected to dominate again. The shortly upcoming PAMELA satellite will already be able to measure part of this high-energy window, while planned experiments like AMS-02 will have access to the full energy range of interest.