Self-Interacting Dark Matter: From Particle Physics Theories to Structure Formation Simulations

Abstract

We develop a unified theory-to-simulation framework for self-interacting dark matter (SIDM), connecting microscopic dark-sector physics to nonlinear structure formation. We systematize particle-physics ingredients of representative SIDM models and map microscopic parameters and interactions onto structure-formation simulation inputs. These include internal-state populations and reaction networks, velocity- and angle-dependent cross sections, inelastic and dissipative energy exchange, decay rates and recoil kinematics, and modifications to cosmological initial conditions. The numerical framework builds on the efficient, scalable SIDM infrastructure introduced in our companion work for the moving-mesh code AREPO-2, providing substantial performance gains over AREPO-1. We implement isotropic and anisotropic self-scattering with constant or velocity-dependent cross sections, multi-state and inelastic reactions, and dissipative scattering, complemented by a dedicated dark-matter decay module. For strongly forward-peaked interactions, the anisotropic hybrid SIDM scheme combines frequent small-angle drag-diffusion with explicit rare large-angle scattering, achieving speed-ups of up to $\sim100\times$ relative to fully anisotropic Monte Carlo in the tested configurations. We validate the implementations through beam-lattice deflection experiments, thermalization and uniform-box decay tests, isolated-halo gravothermal core-collapse simulations, and cosmological box and zoom-in simulations. These tests recover analytical predictions, establish consistency between equivalent numerical treatments, and reproduce published inelastic and dissipative SIDM benchmarks. The resulting modular framework connects dark-sector models to nonlinear astrophysical predictions, enabling exploration of microscopic interactions beyond standard isotropic, elastic SIDM.