First-Principles Formalism for Simulating Self-Interacting Dark Matter

TL;DR

This paper develops a first-principles approach to accurately simulate self-interacting dark matter by deriving effective macroscopic equations from microscopic particle physics, challenging standard assumptions in galactic simulations.

Contribution

It introduces a novel first-principles formalism linking particle physics to simulation particles for self-interacting dark matter, especially in the short-mean-free-path regime.

Findings

Demonstrates that standard assumptions about phase-space evolution are not always valid.

Derives an effective force and interaction cross section from fundamental particle physics.

Provides a new framework for more accurate galactic dark matter simulations.

Abstract

It is plausible that the dark matter particles have non-gravitational interactions among themselves. If such self interactions are large enough, they could leave an imprint on the morphology of galaxies. These effects can be studied with numerical simulations, which serve as the primary tool to predict the non-linear evolution of galactic structure. A standard assumption is that the course-grained phase-space distribution of the macroscopic simulation particles follows the same evolution equation as that of the fundamental dark matter particles. This Letter tests this assumption directly for the case of frequent dark matter scatterings, demonstrating that this is not generically true. Specifically, we develop a first-principles map from a microscopic particle physics description of self-interacting dark matter to a representation of macroscopic simulation particles for theories in the short-mean-free-path regime. Using this procedure, we show the emergence of an effective force between the simulation particles and derive their interaction cross section, which depends on the one from fundamental particle physics. This work provides the first explicit map from particle physics to simulation, which will facilitate exploring the phenomenological implications for galactic dynamics.