Self-Scattering for Dark Matter with an Excited State

TL;DR

This paper develops an analytic model for dark matter self-scattering including nearly-degenerate excited states, aiming to improve understanding of dark matter's role in galactic structures and indirect detection signals.

Contribution

It provides an accurate analytic approximation for elastic and inelastic s-wave cross sections in models with nearly-degenerate dark matter states, valid outside the perturbative regime at low velocities.

Findings

Derived an analytic approximation for scattering cross sections.

Applicable to low-velocity regimes relevant for galactic dynamics.

Facilitates incorporation of inelastic scattering in simulations.

Abstract

Self-interacting dark matter scenarios have recently attracted much attention, as a possible means to alleviate the tension between N-body simulations and observations of the dark matter distribution on galactic and sub-galactic scales. The presence of internal structure for the dark matter --- for example, a nearly-degenerate state in the spectrum that could decay, or be collisionally excited or de-excited --- has also been proposed as a possible means to address these discrepancies. Such internal structure can be a source of interesting signatures in direct and indirect dark matter searches, for example providing a novel explanation for the 3.5 keV line recently observed in galaxies and galaxy clusters. We analyze a simple model of dark matter self-scattering including a nearly-degenerate excited state, and develop an accurate analytic approximation for the elastic and inelastic s-wave cross sections, which is valid outside the perturbative regime provided the particle velocity is sufficiently low (this condition is also required for the s-wave to dominate over higher partial waves). We anticipate our results will be useful in incorporating inelastic self-scattering into N-body simulations, in order to study the quantitative impact of nearly-degenerate states in the dark matter spectrum on galactic structure and dynamics, and in computing the indirect signatures of multi-state dark matter.