Fine Structure of Dark Matter Halos and its Effect on Terrestrial Detection Experiments

Terrestrial dark matter detection experiments probe the velocity-space distribution of dark matter particles in the vicinity of the Earth. We present a novel method, to be used in conjunction with standard cosmological simulations of hierarchical clustering, that allows one to extract a truly local velocity-space distribution in exquisite detail. Preliminary results suggest a new picture for this distribution which is decidedly non-Maxwellian but instead is characterized by randomly positioned peaks in velocity space. We discuss the implications of these results for experiments to detect axions and weakly interacting massive particles.

Figure 1

Top panel: The $r-v_r$ diagram at the final time. Middle panel: The velocity distribution determined from the forward run with a large (solid line) and small (dashed line) volume at $x_0$. Bottom panel: The reconstructed velocity distribution from our reverse-run technique. The dots indicate the location of streams from the forward run when spherical symmetry is applied.

Figure 2

Top panel: Speed distribution from the forward run with $N^{1/2}$ particles (dotted line) and from our reverse-run technique (solid line). Lower panel: Distribution of angle between the local bulk motion and the crossing points.

Figure 3

Top-left panel: WIMP recoil energy spectra from the reverse run (solid line) and a numerical Maxwellian distribution designed to match the average properties of the reverse-run results (dotted line). Bottom-left panel: Seasonal modulation factor $S(E)$. Top-right panel: Axion energy spectra from the reverse run (solid line) and the same Maxwellian (dotted line). Bottom-right panel: Seasonal modulation factor $S(E)$.