Dark Matter Velocity Distributions for Direct Detection: Astrophysical Uncertainties Are Smaller Than They Appear

Area of Science:

• Cosmology
• Astrophysics
• Particle Physics

Background:

• Direct detection experiments' sensitivity depends on local dark matter phase-space distribution.
• Cosmological hydrodynamical simulations model this distribution for Milky Way-like galaxies.
• Quantifying halo-to-halo variations in dark matter speeds is essential for astrophysical uncertainty estimation.

Purpose of the Study:

• To quantify the variation in local dark matter speeds using a large sample of simulated galaxies.
• To introduce a novel phase-space scaling procedure for extrapolating simulation results.
• To assess the impact of these variations on dark matter direct detection bounds.

Main Methods:

• Utilized nearly 100 Milky Way-like galaxies from the tng50 simulation.
• Applied a novel phase-space scaling procedure to reproduce the Milky Way's local standard-of-rest speed.
• Analyzed dark matter speed distributions and their deviation from the standard halo model.

Main Results:

• The ensemble of speed distributions is largely consistent with a truncated Maxwell-Boltzmann distribution.
• Individual distributions can deviate, particularly at high speeds.
• Dark matter-nucleon cross section limits vary by ~60% due to these distributions.

Conclusions:

• The astrophysical uncertainty in direct detection is at or below the systematic uncertainty of current ton-scale detectors.
• This uncertainty remains consistent even when selecting simulations with Milky Way-like merger histories.
• Provided tabulated speed distributions and fits for use in direct detection bound calculations.