Reannihilation of self-interacting dark matter

TL;DR

This paper investigates a late-time dark matter reannihilation process driven by self-interactions and a light mediator, which could influence cosmological observations and help resolve small-scale structure issues.

Contribution

It introduces a model of dark matter reannihilation with Sommerfeld enhancement, analyzing its effects on cosmology and distinguishing between mediator types.

Findings

Reannihilation affects the Hubble expansion rate and CMB observations.

Late kinetic decoupling and strong self-interactions can address small-scale problems.

Reannihilation may reduce tensions in H0 and sigma8 measurements.

Abstract

We explore the phenomenology of having a second epoch of dark matter annihilation into dark radiation long after the standard thermal freeze-out. Such a hidden reannihilation process could affect visible sectors only gravitationally. As a concrete realization we consider self-interacting dark matter (SIDM) with a light force mediator coupled to dark radiation. We demonstrate how resonantly Sommerfeld enhanced cross sections emerge to induce the reannihilation epoch. The effect is a temporally local modification of the Hubble expansion rate and we show that the Cosmic Microwave Background (CMB) measurements -- as well as other observations -- have a high sensitivity to observe this phenomenon. Special attention is given to the model region where late kinetic decoupling and strong self-interactions can alleviate several small-scale problems in the cold dark matter paradigm at the same time. Interestingly, we find that reannihilation might here also simultaneously lower the tension between CMB and low-redshift astronomical observations of $H_{0}$ and $\sigma_{8}$. Moreover, we identify reannihilation as a clear signature to discriminate between the phenomenologically otherwise almost identical vector and scalar mediator realizations of SIDM.