Phase-space structure in the local dark matter distribution and its signature in direct detection experiments

Abstract

We study predictions for dark matter (DM) phase‐space structure near the Sun based on high‐resolution simulations of six galaxy haloes taken from the Aquarius project. The local DM density distribution is predicted to be remarkably smooth; the density at the Sun differs from the mean over a best‐fitting ellipsoidal equidensity contour by less than 15 per cent at the 99.9 per cent confidence level. The local velocity distribution is also very smooth, but it differs systematically from a (multivariate) Gaussian distribution. This is not due to the presence of individual clumps or streams, but to broad features in the velocity modulus and energy distributions that are stable in both space and time and reflect the detailed assembly history of each halo. These features have a significant impact on the signals predicted for weakly interacting massive particle and axion searches. For example, weakly interacting massive particles recoil rates can deviate by ∼10 per cent from those expected from the best‐fitting multivariate Gaussian models. The axion spectra in our simulations typically peak at lower frequencies than in the case of multivariate Gaussian velocity distributions. Also in this case, the spectra show significant imprints of the formation of the halo. This implies that once direct DM detection has become routine, features in the detector signal will allow us to study the DM assembly history of the Milky Way. A new field, ‘DM astronomy’, will then emerge.

Readout strategies for directional dark matter detection beyond the neutrino background

“…[27]) as an isotropic Maxwell-Boltzmann distribution, corresponding to an isotropic halo with a 1/r 2 density profile and a flat rotation curve. Simulated halos have velocity distributions which deviate systematically from the Maxwell-Boltzmann distribution [31][32][33][34][35][36][37]. However, these deviations do not have a large effect on the discovery potential of directional detection experiments [38,39].…”

Dependence of direct detection signals on the WIMP velocity distribution

“…High resolution dark matter only simulations of the formation of Milky Way like dark matter halos, find speed distributions which deviate systematically from a multivariate Gaussian (the simplest anisotropic generalisation of the Maxwellian distribution) [30,38,32,34]. There are more low speed particles, and the peak in the distribution is lower.…”

“…Historically analytic halo models have been used to calculate the WIMP signals and analyse data. In the past few years velocity distributions from high resolution simulations of the formation of Milky Way like halos [30,31,32,33,34,35] have been used to calculate some of the WIMP signals [36,37,38,32,39,34,35,40,41].…”

Indirect detection of self-interacting asymmetric dark matter

“…We note that simulations support the assumption of a locally Gaussian velocity distribution even for cold dark matter [40], and the isothermal approximation is better for self-interacting dark matter [41], [42]. The velocity distribution for two non-relativistic S particles is…”