Particle Models and the Small-Scale Structure of Dark Matter

TL;DR

This paper develops a precise formalism to determine the small-scale cutoff in matter density fluctuations caused by WIMP kinetic decoupling, refining predictions of protohalo masses and exploring observational implications for dark matter microphysics.

Contribution

It introduces a high-accuracy formalism for calculating the decoupling scale from WIMP microphysics, improving previous estimates of protohalo mass ranges.

Findings

Smallest protohalos range from 10^{-11} to 10^{-3} solar masses.

Actual cutoff mass is typically about 10 times larger than previous estimates.

Decoupling temperatures are between several MeV and a few GeV.

Abstract

The kinetic decoupling of weakly interacting massive particles (WIMPs) in the early universe sets a scale that can directly be translated into a small-scale cutoff in the spectrum of matter density fluctuations. The formalism presented here allows a precise description of the decoupling process and thus the determination of this scale to a high accuracy from the details of the underlying WIMP microphysics. With decoupling temperatures of several MeV to a few GeV, the smallest protohalos to be formed range between 10^{-11} and almost 10^{-3} solar masses -- a somewhat smaller range than what was found earlier using order-of-magnitude estimates for the decoupling temperature; for a given WIMP model, the actual cutoff mass is typically about a factor of 10 greater than derived in that way, though in some cases the difference may be as large as a factor of several 100. Observational consequences and prospects to probe this small-scale cutoff, which would provide a fascinating new window into the particle nature of dark matter, are discussed