Self-heating of Strongly Interacting Massive Particles

TL;DR

This paper investigates the unique temperature evolution of self-heating strongly interacting dark matter, its impact on small-scale structure, and proposes a particle physics model involving semi-annihilation in a QCD-like dark sector.

Contribution

It introduces the concept of self-heating in strongly interacting dark matter and explores its effects on cosmological perturbations and structure formation, along with a concrete particle physics realization.

Findings

Self-heating DM causes a cutoff in the matter power spectrum at subgalactic scales.

A viable particle physics model with semi-annihilating pion-like particles and dark radiation is proposed.

The model evades constraints from the effective number of neutrino species.

Abstract

It was recently pointed out that semi-annihilating dark matter (DM) may experience a novel temperature evolution dubbed as self-heating. Exothermic semi-annihilation converts the DM mass to the kinetic energy. This yields a unique DM temperature evolution, $T_{\chi} \propto 1 / a$, in contrast to $ T_{\chi} \propto 1 / a^{2}$ for free-streaming non-relativistic particles. Self-heating continues as long as self-scattering sufficiently redistributes the energy of DM particles. In this paper, we study the evolution of cosmological perturbations in self-heating DM. We find that sub-GeV self-heating DM leaves a cutoff on the subgalactic scale of the matter power spectrum when the self-scattering cross section is $\sigma_{\rm self} / m_{\chi} \sim {\cal O} (1) \,{\rm cm}^{2} /{\rm g}$. Then we present a particle physics realization of the self-heating DM scenario. The model is based on recently proposed strongly interacting massive particles with pion-like particles in a QCD-like sector. Pion-like particles semi-annihilate into an axion-like particle, which is thermalized with dark radiation. The dark radiation temperature is smaller than the standard model temperature, evading the constraint from the effective number of neutrino degrees of freedom. It is easily realized when the dark sector is populated from the standard model sector through a small coupling.