Constraints on Dark Matter Annihilation in Clusters of Galaxies with the Fermi Large Area Telescope

Nearby clusters and groups of galaxies are potentially bright sources of high-energy gamma-ray emission resulting from the pair-annihilation of dark matter particles. However, no significant gamma-ray emission has been detected so far from clusters in the first 11 months of observations with the Fermi Large Area Telescope. We interpret this non-detection in terms of constraints on dark matter particle properties. In particular for leptonic annihilation final states and particle masses greater than ~200 GeV, gamma-ray emission from inverse Compton scattering of CMB photons is expected to dominate the dark matter annihilation signal from clusters, and our gamma-ray limits exclude large regions of the parameter space that would give a good fit to the recent anomalous Pamela and Fermi-LAT electron-positron measurements. We also present constraints on the annihilation of more standard dark matter candidates, such as the lightest neutralino of supersymmetric models. The constraints are particularly strong when including the fact that clusters are known to contain substructure at least on galaxy scales, increasing the expected gamma-ray flux by a factor of ~5 over a smooth-halo assumption. We also explore the effect of uncertainties in cluster dark matter density profiles, finding a systematic uncertainty in the constraints of roughly a factor of two, but similar overall conclusions. In this work, we focus on deriving limits on dark matter models; a more general consideration of the Fermi-LAT data on clusters and clusters as gamma-ray sources is forthcoming.Comment: accepted to JCAP, Corresponding authors: T.E. Jeltema and S. Profumo, minor revisions to be consistent with accepted versio

Deuteron NMR relaxation, spectra, and evidence for the order-disorder phase transition in (ND4)2PtCl6

Deuteron NMR relaxation and spectra were studied at the resonance frequency of 46 MHz in polycrystalline (ND(4))(2)PtCl(6) between 300-5 K. The relaxation rate maximum near 50 K is about 53% smaller than the calculated maximum related to 120 degrees rotations about the threefold symmetry axes of the ammonium ion. The difference is explained by assuming for a N-D vector a total of 24 equilibrium directions, which in groups of six deviate from the nearest Pt-N vector by a certain angle Theta. So-called limited jumps between the directions of each group take place much more frequently than the large-angle rotations, thus rendering a fraction of the deuteron quadrupole coupling ineffective in relaxation. A motional model is presented, which takes into account both these motions simultaneously. A comparison with experimental data leads to Theta=26.0 degrees , in reasonable agreement with earlier neutron diffraction data. A sharp decrease found in the relaxation rate at the order-disorder phase transition temperature of 27.2 K is related to the fact that one of the six equilibrium directions becomes preferred. This leads to a formation of ordered domains, in which the active motion driving the relaxation is 120 degrees rotations. Two components in the spectra found below 55 K are related to domains (broad) and transition regions between domains (narrow). Reasons for the nonexponentiality observed below 20 K are discussed, the most likely explanation being that limited jumps dominate within transition regions and make the corresponding deuterons relax faster than those in domains