1,721,046 research outputs found
Spin-Dependent WIMPs in DAMA?
No full textWe investigate whether the annual modulation observed in the DAMA experiment can be due to a weakly-interacting massive particle (WIMP) with an axial-vector (spin-dependent; SD) coupling to nuclei. We evaluate the SD WIMP-proton cross section under the assumption that such scattering accounts for the DAMA modulation, and we do the same for a SD WIMP-neutron cross section. We show that SD WIMP-proton scattering is ruled out in a model-independent fashion by null searches for energetic neutrinos from WIMP annihilation in the Sun, and that SD WIMP-neutron scattering is ruled out for WIMP masses greater than or similar to 20 GeV by the null result with the DAMA Xe detector. A SD WIMP with mass less than or similar to 20 GeV is still compatible, but only if the SD WIMP-neutron interaction is four orders of magnitude greater than the WIMP-proton interaction
Velocity distributions and annual-modulation signatures of weakly-interacting massive particles
Full text linkAn annual modulation in the event rate of the NaI detector of the DAMA collaboration has been used to infer the existence of particle dark matter in the Galactic halo. Bounds on the WIMP mass and WIMP-nucleon cross section have been derived. These analyses have assumed that the local dark-matter velocity distribution is either isotropic or has some bulk rotation. Here we consider the effects of possible structure in the WIMP velocity distribution on the annual-modulation amplitude. We show that if we allow for a locally anisotropic velocity dispersion tensor, the interpretation of direct detection experiments could be altered significantly. We also show that uncertainties in the velocity distribution function that arise from uncertainties in the radial density profile are less important if the velocity dispersion is assumed to be isotropic
A running spectral index in supersymmetric dark-matter models with quasistable charged particles
Full text linkWe show that charged particles decaying in the early Universe can induce a scale-dependent or running spectral index in the small-scale linear and nonlinear matter power spectrum and discuss examples of this effect in minimal supersymmetric models in which the lightest neutralino is a viable cold-dark-matter candidate. We find configurations in which the neutralino relic density is set by coannihilations with a long-lived stau and the late decay of staus partially suppresses the linear matter power spectrum. Nonlinear evolution on small scales then causes the modified linear power spectrum to evolve to a nonlinear power spectrum similar to (but different in detail) models parametrized by a constant running alpha(s)=dn(s)/dlnk by redshifts of 2 to 4. Thus, Lyman-alpha forest observations, which probe the matter power spectrum at these redshifts, might not discriminate between the two effects. However, a measurement of the angular power spectrum of primordial 21-cm radiation from redshift zapproximate to30-200 might distinguish between this charged-decay model and a primordial running spectral index. The direct production of a long-lived charged particle at future colliders is a dramatic prediction of this model
Violation of statistical isotropy and homogeneity in the 21-cm power spectrum
No full textMost inflationary models predict primordial perturbations to be statistically isotropic and homogeneous. Cosmic microwave background (CMB) observations, however, indicate a possible departure from statistical isotropy in the form of a dipolar power modulation at large angular scales. Alternative models of inflation, beyond the simplest single-field slow-roll models, can generate a small power asymmetry, consistent with these observations. Observations of clustering of quasars show, however, agreement with statistical isotropy at much smaller angular scales. Here, we propose to use off-diagonal components of the angular power spectrum of the 21-cm fluctuations during the dark ages to test this power asymmetry. We forecast results for the planned SKA radio array, a future radio array, and the cosmic-variance-limited case as a theoretical proof of principle. Our results show that the 21-cm line power spectrum will enable access to information at very small scales and at different redshift slices, thus improving upon the current CMB constraints by ∼2 orders of magnitude for a dipolar asymmetry and by ∼1-3 orders of magnitude for a quadrupolar asymmetry case
Tests for Primordial Non-Gaussianity
No full textWe investigate the relative sensitivities of several tests for deviations from Gaussianity in the primordial distribution of density perturbations. We consider models for non-Gaussianity that mimic that which comes from inflation as well as that which comes from topological defects. The tests we consider involve the cosmic microwave background (CMB), large-scale structure, high-redshift galaxies, and the abundances and properties of clusters. We find that the CMB is superior at finding non-Gaussianity in the primordial gravitational potential (as inflation would produce), while observations of high-redshift galaxies are much better suited to find non-Gaussianity that resembles that expected from topological defects. We derive a simple expression that relates the abundance of high-redshift objects in non-Gaussian models to the primordial skewness
Detecting the integrated Sachs-Wolfe effect with high-redshift 21-cm surveys
No full textWe investigate the possibility of detecting the integrated Sachs-Wolfe (ISW) effect by cross-correlating 21-cm surveys at high redshifts with galaxies in a way similar to the usual CMB-galaxy cross-correlation. The high-redshift 21-cm signal is dominated by CMB photons that travel freely without interacting with the intervening matter, and hence its late-time ISW signature should correlate extremely well with that of the CMB at its peak frequencies. Using the 21-cm temperature brightness instead of the CMB would thus be a further check of the detection of the ISW effect, measured by different instruments at different frequencies and suffering from different systematics. We also study the ISW effect on the photons that are scattered by HI clouds. We show that a detection of the unscattered photons is achievable with planned radio arrays, while one using scattered photons will require advanced radio interferometers, either an extended version of the planned Square Kilometre Array or futuristic experiments such as a lunar radio array
Dark-matter spike at the Galactic center?
Full text linkThe past growth of the central black hole (BH) might have enhanced the density of cold dark matter halo particles at the galactic center. We compute this effect in realistic growth models of the present (2-3) x 10(6) M-., BH from a low-mass seed BH, with special attention to dynamical modeling in a realistic galaxy environment with merger and orbital decay of a seed BH formed generally outside the exact center of the halo. An intriguing "very-dense spike" of dark matter has been claimed in models of Gondolo and Silk with a density high enough to contradict with experimental upper bounds of neutralino annihilation radiation. This "spike" disappears completely or is greatly weakened when we include important dynamical processes neglected in their idealized or restrictive picture with cold particles surrounding an at-the-center zero-seed adiabatically growing BH. For the seed BH to spiral in and settle to the center within a Hubble time by dynamical friction, the seed mass must be at least a significant fraction of the present BH. Any subsequent at-the-center growth of the BH and steepening of the central Keplerian potential well can squeeze the halo density distribution only mildly, whether the squeezing happens adiabatically or instantaneously
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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