Cosmic particle colliders: simulations of self-interacting dark matter with anisotropic scattering

Abstract

We investigate how self-interacting dark matter (SIDM) with anisotropic scattering affects the evolution of isolated dark matter haloes as well as systems with two colliding haloes. For isolated haloes, we find that the evolution can be adequately captured by treating the scattering as isotropic, as long as the isotropic cross-section is appropriately matched to the underlying anisotropic model. We find that this matching should not be done using the momentum transfer cross-section, as has been done previously. Matching should instead be performed via a modified momentum transfer cross-section that takes into account that dark matter particles can be relabelled after they scatter, without altering the dynamics. However, using cross-sections that are matched to give the same behaviour in isolated haloes, we find that treating dark matter scattering as isotropic underpredicts the effects of anisotropic dark matter scattering when haloes collide. In particular, the DM-galaxy offset induced by SIDM in colliding galaxy clusters is larger when we simulate the underlying particle model, than if we use a matched isotropic model. On the other hand, well motivated particle models with anisotropic scattering typically have cross-sections with a strong velocity dependence, and we discover a previously unrecognised effect that suppresses DM-galaxy offsets in colliding clusters making it hard for these systems to provide competitive constraints on such particle models.

Dark Matter Haloes and Subhaloes

“…It is possible to incorporate this new term within the Monte Carlo approach of traditional N-body simulations by adding "collisions" between each simulation particle and its immediate neighbours in a probabilistic way that reflects the effective scattering rate given by the cross-section (e.g. [55][56][57][58][59]; see Appendix A of [58] for a detailed derivation). An alternative to the N-body approach is the "gravothermal fluid" approximation [60], which considers an SIDM dark matter halo as a self-gravitating, spherically symmetric, ideal gas with an effective thermal conductivity (related to the self-scattering cross-section, see e.g.…”

“…Large relative velocities between gas and dark matter inherited from the photon-baryon coupling before recombination can impede the growth of gravitational perturbations and also stop gas from accreting into the first haloes[256]. This process, however, is only thought to be relevant for the formation of the first stars 59. This mass threshold is smaller at higher redshifts, see e.g.Fig.…”

Direct detection and complementary constraints for sub-GeV dark matter

“…For the case of a light mediator as studied here, this can be substantially different from a contact-like (isotropic) DM scattering, which is usually assumed in N -body simulations (see however Refs. [125][126][127] for first studies including angular dependent and frequent scatterings). On the other hand, bounds on the self-interaction rate have been reported that are significantly stronger than the fiducial maximal value(s) of σ T that we adopt in our analysis [123,[128][129][130].…”

A Testable Conspiracy: Simulating Baryonic Effects on Self-interacting Dark Matter Halos

“…Cluster mergers have been used by many studies to constrain SIDM (see e.g., Randall et al 2008;Dawson et al 2012;Kahlhoefer et al 2014;Massey et al 2015;Schaller et al 2015;Kim et al 2017;Robertson et al 2017), with typical limits of m 1 cm g 2 1  s -on the self-interaction cross-section. However, recent work by Kim et al (2017) showed that constraints based on the displacement of the stellar and DM centroids are overly stringent, weakening previous constraints.…”