Axion minicluster streams in the solar neighbourhood

A consequence of QCD axion dark matter being born after inflation is the emergence of ultra-small-scale substructures known as miniclusters. Although miniclusters merge to form minihalos, this intrinsic granularity is expected to remain imprinted on small scales in our galaxy, leading to potentially damning consequences for the campaign to detect axions directly on Earth. This picture, however, is modified when one takes into account the fact that encounters with stars will tidally strip mass from the miniclusters, creating pc-long tidal streams that act to refill the dark matter distribution. Here we ask whether or not this stripping rescues experimental prospects from the worst-case scenario in which the majority of axions remain bound up in unobservably small miniclusters. We find that the density sampled by the Earth on mpc-scales will be, on average, around 70-90% of the known local DM density, and at a typical point in the solar neighbourhood, we expect most of the dark matter to be comprised of debris from $\mathcal{O}(10^2$-$10^3)$ overlapping streams. If haloscopes can measure the axion signal with high-enough frequency resolution, then these streams are revealed in the form of an intrinsically spiky lineshape, in stark contrast with the standard assumption of a smooth, featureless Maxwellian distribution -- a unique prediction that constitutes a way for experiments to distinguish between pre and post-inflationary axion cosmologies.Comment: 5 + 8 page

A Dark Matter Hurricane: Measuring the S1 Stream with Dark Matter Detectors

The recently discovered S1 stream passes through the Solar neighbourhood on a low inclination, counter-rotating orbit. The progenitor of S1 is a dwarf galaxy with a total mass comparable to the present-day Fornax dwarf spheroidal, so the stream is expected to have a significant DM component. We compute the effects of the S1 stream on WIMP and axion detectors as a function of the density of its unmeasured dark component. In WIMP detectors the S1 stream supplies more high energy nuclear recoils so will marginally improve DM detection prospects. We find that even if S1 comprises less than 10% of the local density, multi-ton xenon WIMP detectors can distinguish the S1 stream from the bulk halo in the relatively narrow mass range between 5 and 25 GeV. In directional WIMP detectors such as CYGNUS, S1 increases DM detection prospects more substantially since it enhances the anisotropy of the WIMP signal. Finally, we show that axion haloscopes possess by far the greatest potential sensitivity to the S1 stream. Once the axion mass has been discovered, the distinctive velocity distribution of S1 can easily be extracted from the axion power spectrum.Comment: 21 pages, 11 figure

Detecting WIMPs, neutrinos and axions in the next generation of dark matter experiment

The first direct detection of dark matter is anticipated in coming years by one of a range of experimental strategies. Because the identity of dark matter remains unknown, the strategy that will be successful in this one cannot say. However beneath this fundamental particle physics uncertainty lies another uncertainty with regard to the structure of the dark matter halo of the Milky Way that must be confronted when interpreting data from terrestrial experiments. However these astrophysical uncertainties might only be resolved with the very same experiments; in fact, directly detecting dark matter represents the only way to probe the ultralocal structure of the halo. This thesis explores the impact of astrophysical uncertainties on the particle physics goals of dark matter detection but also the extent to which we might in the future be able to resolve those uncertainties. The discussion is framed around the detection of three types of particle, two of which are dark matter candidates: weakly interacting massive particles (WIMPs), neutrinos and axions. In the case of WIMPs I consider how upcoming directionally sensitive experiments can be used to probe the full 3-dimensional velocity distribution to learn about dark matter substructure. A range of model dependent and independent statistical approaches are tested under various astrophysical benchmarks. I also explore prospects for WIMP direct detection when faced with the ultimate neutrino background, as expected in the next generation of experiment. In this eventuality the uncertainties in the neutrino flux are essential in predicting the WIMP models inaccessible due to the background. However the same is true of astrophysical uncertainties. Once astrophysical uncertainties are accounted for the neutrino floor limit is raised in cross section by up to an order of magnitude and the accuracy of any potential WIMP particle measurement is greatly increased. Addressing these concerns, I demonstrate how one should go about subtracting the neutrino background. This involves a return to directional detection. I find that even non-ideal circumstances, the neutrino and WIMP signals can be distinguished and the neutrino floor overcome. Finally in the context of axions, I discuss the prospects for microwave cavity haloscope experiments to perform "axion astronomy". Haloscopes measure the direct conversion of axions into photons and hence can make potentially much finer measurements of the dark matter halo compared with WIMPs. I develop a technique to extract astrophysical parameters, such as the halo velocity dispersion and laboratory velocity, as well as learn about properties of substructure from tidal streams and axion miniclusters. I show that a level of precision can be achieved in relatively short duration haloscope experiments that can match or improve upon that of astronomical observations