Fine-grained dark matter substructure and axion haloscopes

Abstract: The velocity distribution of cold dark matter within galaxies is expected to exhibit ultra-fine-grained substructure as a result of the many foldings of an initially smooth phase-space sheet under gravity. The innumerable folds of this sheet in the inner regions of a galactic halo would appear on solar-system scales like an extremely large number of spatially overlapping streams of dark matter particles. Some of these streams may also receive amplified densities if dark matter undergoes enhanced clustering in the early Universe, as is the case for axions in the post-inflationary scenario. This ultra-fine-grained dark matter substructure is usually considered irrelevant and undetectable for most direct detection experiments, but for axion haloscopes, this may not be the case. We develop and explore several plausible models for the degree of fine-grained axion dark matter substructure so as to evaluate its impact on the detectability of axions. We find that not only is this substructure detectable in haloscope experiments, but that it may enhance the detectability of the axion if data is analysed with a high-enough frequency resolution. This conclusion motivates ongoing high-resolution analyses of axion data by haloscope collaborations, which have the potential to reveal evidence of the QCD axion even if their nominal experimental sensitivity does not reach the required level under the Standard Halo Model assumption.