Dark Matter from Early Decays

TL;DR

The paper investigates dark matter that originates from early decays (DDM), focusing on how decays like $\mathrm{DDM} \rightarrow \mathrm{DM} + \mathrm{L}$ imprint a non-thermal momentum distribution on the DM population and suppress small-scale structure. It develops the decay framework, derives the coupled evolution of energy densities and the non-thermal DM phase-space distribution, and shows that decays inject large velocities that yield a characteristic free-streaming scale $\lambda_c$ and a one-parameter power spectrum suppression described by $\lambda_c$ and the phase-space density $Q$. A linear perturbation analysis in the presence of decays leads to a damped $P(k)$ on small scales, captured by a simple fitting form for $P_{\rm DM}(k)$; phase-space constraints via the excess mass function $D(f)$ provide a robust bound on inner halo cores. The work also discusses reionization constraints, mixed dark matter scenarios, and observational prospects (e.g., lensing and CMB) to test decaying dark matter models. Overall, it argues that early decays can naturally alleviate small-scale structure issues without spoiling large-scale predictions, offering a testable link to supersymmetric scenarios with gravitino LSP.

Abstract

Two leading dark matter candidates from supersymmetry and other theories of physics beyond the standard model are WIMPs and weak scale gravitinos. If the lightest stable particle is a gravitino, then a WIMP will decay into it with a natural lifetime of order a month ~ M_{pl}^2/M_{weak}^3. We show that if the bulk of dark matter today came from decays of neutral particles with lifetimes of order a year or smaller, then it could lead to a reduction in the amount of small scale substructure, less concentrated halos and constant density cores in the smallest mass halos. Such beneficial effects may therefore be realized naturally, as discussed by Cembranos, Feng, Rajaraman, and Takayama, in the case of supersymmetry.