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ABSTRACT: The axion is one of the leading particle candidates for the universe's dark matter component. Despite possessing very small couplings, the axion's interaction with photons can be utilized to search for it using a microwave cavity detector. The Axion Dark Matter eXperiment (ADMX) uses such a detector to search for axions in our galactic halo. ADMX has recently added a new, high resolution channel to search for axions in discrete flows. ADMX's medium resolution channel searches for axions in the thermalized component of the halo. We review the motivation for the axion and its properties which make it a good dark matter candidate. We also review the arguments for the existence of discrete flows in galactic halos. A flow of discrete axions with small velocity dispersion will appear as a very narrow peak in the output of a microwave cavity detector. A high resolution search can detect such a peak with large signal to noise. We have performed such a search. The details of the high resolution axion search and analysis procedure are presented. In this search, no axion signal was found in the mass range 1.98-2.17 micro-eV. We place upper limits on the density of axions in local discrete flows based on this result.
An important, open research topic today is to understand the relevance that dark matter halo substructure may have for dark matter searches. In the standard cosmological model, halo substructure or subhalos are predicted to be largely abundant inside larger halos, for example, galaxies such as ours, and are thought to form first and later merge to form larger structures. Dwarf satellite galaxies—the most massive exponents of halo substructure in our own galaxy—are already known to be excellent targets for dark matter searches, and indeed, they are constantly scrutinized by current gamma-ray experiments in the search for dark matter signals. Lighter subhalos not massive enough to have a visible counterpart of stars and gas may be good targets as well, given their typical abundances and distances. In addition, the clumpy distribution of subhalos residing in larger halos may boost the dark matter signals considerably. In an era in which gamma-ray experiments possess, for the first time, the exciting potential to put to test the preferred dark matter particle theories, a profound knowledge of dark matter astrophysical targets and scenarios is mandatory should we aim for accurate predictions of dark matter-induced fluxes for investing significant telescope observing time on selected targets and for deriving robust conclusions from our dark matter search efforts. In this regard, a precise characterization of the statistical and structural properties of subhalos becomes critical. In this Special Issue, we aim to summarize where we stand today on our knowledge of the different aspects of the dark matter halo substructure; to identify what are the remaining big questions, and how we could address these; and, by doing so, to find new avenues for research.
We discuss the possibility of GLAST detecting gamma-rays from the annihilation of neutralino dark matter in the Galactic halo. We have used 'Via Lactea', currently the highest resolution simulation of cold dark matter substructure, to quantify the contribution of subhalos to the annihilation signal. We present a simulated allsky map of the expected gamma-ray counts from dark matter annihilation, assuming standard values of particle mass and cross section. In this case GLAST should be able to detect the Galactic center and several individual subhalos. One of the most exciting discoveries that the Gamma-ray Large Area Space Telescope (GLAST) could make, is the detection of gamma-rays from the annihilation of dark matter (DM). Such a measurement would directly address one of the major physics problems of our time: the nature of the DM particle. Whether or not GLAST will actually detect a DM annihilation signal depends on both unknown particle physics and unknown astrophysics theory. Particle physics uncertainties include the type of particle (axion, neutralino, Kaluza-Klein particle, etc.), its mass, and its interaction cross section. From the astrophysical side it appears that DM is not smoothly distributed throughout the Galaxy halo, but instead exhibits abundant clumpy substructure, in the form of thousands of so-called subhalos. The observability of DM annihilation radiation originating in Galactic DM subhalos depends on their abundance, distribution, and internal properties. Numerical simulations have been used in the past to estimate the annihilation flux from DM substructure, but since the subhalo properties, especially their central density profile, which determines their annihilation luminosity, are very sensitive to numerical resolution, it makes sense to re-examine their contribution with higher resolution simulations.
The unambiguous detection of dark matter annihilation in our Galaxy would unravel one of the most outstanding puzzles in particle physics and cosmology. Recent observations have motivated models in which the annihilation rate is boosted by the Sommerfeld effect, a nonperturbative enhancement arising from a long-range attractive force. We applied the Sommerfeld correction to Via Lactea II, a high-resolution N-body simulation of a Milky Way-sized galaxy, to investigate the phase-space structure of the galactic halo. We found that the annihilation luminosity from kinematically cold substructure could be enhanced by orders of magnitude relative to previous calculations, leading to the prediction of gamma-ray fluxes from as many as several hundred dark clumps that should be detectable by the Fermi satellite.
The objective of the workshop series “The Identification of Dark Matter” is to assess critically the status of work attempting to identify what constitutes dark matter; in particular, to consider what techniques are currently being used, how successful they are, and what new techniques are likely to improve the prospects for identifying dark matter candidates in the future. This proceedings volume includes reviews on major particle astrophysics topics in the field of dark matter, as well as short contributed papers.
Based on a Simons Symposium held in 2018, the proceedings in this volume focus on the theoretical, numerical, and observational quest for dark matter in the universe. Present ground-based and satellite searches have so far severely constrained the long-proposed theoretical models for dark matter. Nevertheless, there is continuously growing astrophysical and cosmological evidence for its existence. To address present and future developments in the field, novel ideas, theories, and approaches are called for. The symposium gathered together a new generation of experts pursuing innovative, more complex theories of dark matter than previously considered.This is being done hand in hand with experts in numerical astrophysical simulations and observational techniques—all paramount for deciphering the nature of dark matter. The proceedings volume provides coverage of the most advanced stage of understanding dark matter in various new frameworks. The collection will be useful for graduate students, postdocs, and investigators interested in cutting-edge research on one of the biggest mysteries of our universe.
The axion is a hypothetical elementary particle that came out of a solution to the Strong-CP problem in Quantum Chromodynamics (QCD). The properties of the axion also make it a highly compelling candidate for dark matter. Axion haloscopes are instruments that search for axion dark matter in the local Milky Way halo by searching for the conversion of an axion into photons via the inverse Primakoff effect. The Axion Dark Matter eXperiment (ADMX) is by far the most sensitive axion haloscope, being the first to exclude benchmark models for the QCD axion and continuing to take data at high sensitivity. This thesis reports on a run by ADMX which excluded dark matter axions in the galactic halo over the mass range 2.81--3.31[mu]eV. This unprecedented sensitivity in this mass range is achieved by deploying an ultra low-noise Josephson parametric amplifier as the first-stage signal amplifier.
We review the status of two ongoing large-scale searches for axions which may constitute the dark matter of our Milky Way halo. The experiments are based on the microwave cavity technique proposed by Sikivie, and marks a ''second-generation'' to the original experiments performed by the Rochester-Brookhaven-Fermilab collaboration, and the University of Florida group.