The Dark Matter at the End of the Galaxy

TL;DR

This paper proposes a new dark matter velocity distribution function based on Lambda-CDM cosmology, which impacts direct detection rate predictions, especially near the escape velocity.

Contribution

It introduces a novel velocity distribution ansatz motivated by cosmological models and confirms its validity with N-body simulations, affecting detection rate calculations.

Findings

The new distribution has fewer high velocity particles than Maxwell-Boltzmann.

N-body simulations support the proposed velocity distribution.

Detection rates are significantly affected near the escape velocity threshold.

Abstract

Dark matter density profiles based upon Lambda-CDM cosmology motivate an ansatz velocity distribution function with fewer high velocity particles than the Maxwell-Boltzmann distribution or proposed variants. The high velocity tail of the distribution is determined by the outer slope of the dark matter halo, the large radius behavior of the Galactic dark matter density. N-body simulations of Galactic halos reproduce the high velocity behavior of this ansatz. Predictions for direct detection rates are dramatically affected for models where the threshold scattering velocity is within 30% of the escape velocity.