* Astronomy

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RE: Dark Matter Annihilation

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Title: Annihilating Asymmetric Dark Matter
Author: Nicole F. Bell, Shunsaku Horiuchi, Ian M. Shoemaker

The relic abundance of particle and antiparticle dark matter (DM) need not be vastly different in thermal asymmetric dark matter (ADM) models. By considering the effect of a primordial asymmetry on the thermal Boltzmann evolution of coupled DM and anti-DM, we derive the requisite annihilation cross section. This is used in conjunction with CMB and Fermi-LAT gamma-ray data to impose a limit on the number density of anti-DM particles surviving thermal freeze-out. When the extended gamma-ray emission from the Galactic Center is reanalyzed in a thermal ADM framework, we find that annihilation into tau leptons prefer anti-DM number densities 1-4% that of DM while the b-quark channel prefers 50-100%.

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Title: Annihilation Signals from Asymmetric Dark Matter
Author: Edward Hardy, Robert Lasenby, James Unwin

In the simplest models of asymmetric dark matter (ADM) annihilation signals are not expected, since the DM is non-self-conjugate and the relic density of anti-DM is negligible. We investigate a new class of models in which a symmetric DM component, in the `low-mass' 1-10 GeV regime favoured for linking the DM and baryon asymmetries, is repopulated through decays. We find that, in models without significant velocity dependence of the annihilation cross section, observational constraints generally force these decays to be (cosmologically) slow. These late decays can give rise to gamma-ray signal morphologies differing from usual annihilation profiles. A distinctive feature of such models is that signals may be absent from dwarf spheroidal galaxies.

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Title: Asymmetric dark matter annihilation as a test of non-standard cosmologies
Authors: Graciela B. Gelmini, Ji-Haeng Huh, Thomas Rehagen

We show that the relic abundance of the minority component of asymmetric dark matter can be very sensitive to the expansion rate of the Universe and the temperature of transition between a non-standard pre-Big Bang Nucleosynthesis cosmological phase and the standard radiation dominated phase, if chemical decoupling happens before this transition. In particular, because the annihilation cross section of asymmetric dark matter is typically larger than that of symmetric dark matter in the standard cosmology, the decrease in relic density of the minority component in non-standard cosmologies with respect to the majority component may be compensated by the increase in annihilation cross section, so that the annihilation rate at present of asymmetric dark matter, contrary to general belief, could be larger than that of symmetric dark matter in the standard cosmology.

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Title: A new way to explain the 511keV signal from the center of the Galaxy and some dark matter experiments
Authors: J. Va'vra

The first gamma-ray line originating from outside the solar system that was ever detected is the 511keV emission from the center of our Galaxy. The accepted explanation of this signal is the annihilation of electrons and positrons. However, despite 30 years of intense theoretical and observational investigation, the main sources of positrons have not been identified up to now. In this paper we propose an alternative explanation: the observed signal is due to atomic transitions to small hydrogen atom, where electron is captured by proton on a small tight orbit around proton. This model may also be relevant to some dark matter searching experiments capable of observing a very small signal. We propose a detector to improve their detection-reach even further down to smaller signals equivalent to a single electron and a single photon of a few eV energy. This detector may be able to detect a prevailing direction of the small hydrogen (or a light-mass WIMP interacting with shell-electrons). We describe the status of the experimental search to find the small hydrogen atom, and propose a method how to discover it in the lab directly.

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Title: Annihilation of singlet fermionic dark matter into two photons
Authors: Mohammad Mahdi Ettefaghi, Reza Moazzemi

We consider an extension of the standard model in which a singlet fermionic particle, to serve as cold dark matter, and a singlet Higgs are added. We perform a reanalysis on the free parameters. In particular, demanding a correct relic abundance of dark matter, we derive and plot the coupling of the singlet fermion with the singlet Higgs, g_s, versus the dark matter mass. We analytically compute the pair annihilation cross section of singlet fermionic dark matter into two photons. The thermally averaged of this cross section is calculated for wide range of energies and plotted versus dark matter mass using g_s consistent with the relic abundance condition. We also compare our results with the Fermi-Lat observations.

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Title: Search for Neutrinos from Annihilating Dark Matter in the Direction of the Galactic Center with the 40-String IceCube Neutrino Observatory
Authors: The IceCube collaboration: R. Abbasi, Y. Abdou, M. Ackermann, J. Adams, J. A. Aguilar, M. Ahlers, D. Altmann, K. Andeen, J. Auffenberg, X. Bai, M. Baker, S. W. Barwick, V. Baum, R. Bay, K. Beattie, J. J. Beatty, S. Bechet, J. Becker Tjus, K.-H. Becker, M. Bell, M. L. Benabderrahmane, S. BenZvi, J. Berdermann, P. Berghaus, D. Berley, E. Bernardini, D. Bertrand, D. Z. Besson, D. Bindig, M. Bissok, E. Blaufuss, J. Blumenthal, D. J. Boersma, C. Bohm, D. Bose, S. Böser, O. Botner, L. Brayeur, A. M. Brown, R. Bruijn, J. Brunner, S. Buitink, M. Carson, J. Casey, M. Casier, D. Chirkin, B. Christy, F. Clevermann, S. Cohen, D. F. Cowen, A. H. Cruz Silva, M. Danninger, J. Daughhetee, J. C. Davis, C. De Clercq, F. Descamps, P. Desiati, G. de Vries Uiterweerd, T. DeYoung, J. C. Díaz-Vélez, et al. (211 additional authors not shown)

A search for muon neutrinos from dark matter annihilations in the Galactic Center region has been performed with the 40-string configuration of the IceCube Neutrino Observatory using data collected in 367 days of live-time starting in April 2008. The observed fluxes were consistent with the atmospheric background expectations. Upper limits on the self-annihilation cross-section are obtained for dark matter particle masses ranging from 100 GeV to 10 TeV. In the case of decaying dark matter, lower limits on the lifetime have been determined for masses between 200 GeV and 20 TeV.

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Title: Constraints on Dark Matter Annihilation in Clusters of Galaxies from Diffuse Radio Emission
Authors: Emma Storm, Tesla E. Jeltema, Stefano Profumo, Lawrence Rudnick

Annihilation of dark matter can result in the production of stable Standard Model particles including electrons and positrons that, in the presence of magnetic fields, lose energy via synchrotron radiation, observable as radio emission. Galaxy clusters are excellent targets to search for or to constrain the rate of dark matter annihilation, as they are both massive and dark matter dominated. In this study, we place limits on dark matter annihilation in a sample of nearby clusters using upper limits on the diffuse radio emission, low levels of observed diffuse emission, or detections of radio mini-haloes. We find that the strongest limits on the annihilation cross section are better than limits derived from the non-detection of clusters in the gamma-ray band by a factor of approximately 3 or more when the same annihilation channel and subtructure model, but different best-case clusters, are compared. The limits on the cross section depend on the assumed amount of substructure, varying by as much as 2 orders of magnitude for increasingly optimistic substructure models as compared to a smooth NFW profile. In our most optimistic case, using the results of the Phoenix Project (Gao et al. 2012b) we find that the derived limits reach below the thermal relic cross section of 3x10^-26 cm^3 s^-1 for dark matter masses as large as 400 GeV, for the bbar annihilation channel. We discuss uncertainties due to the limited available data on the magnetic field structure of individual clusters. We also report the discovery of diffuse radio emission from the central 30-40 kpc regions of the groups M49 and NGC4636.

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