Self-heating dark matter via semi-annihilation

TL;DR

This paper introduces a novel self-heating mechanism in semi-annihilating dark matter, where the DM temperature evolves differently, potentially addressing small-scale structure issues in cosmology.

Contribution

It presents the concept of self-heating in semi-annihilating dark matter and explores its implications for structure formation and halo profiles.

Findings

DM temperature redshifts like radiation during self-heating

Self-heating suppresses small-scale structure similar to warm DM

Large self-scattering cross section flattens inner halo density profiles

Abstract

The freeze-out of dark matter (DM) depends on the evolution of the DM temperature. The DM temperature does not have to follow the standard model one, when the elastic scattering is not sufficient to maintain the kinetic equilibrium. We study the temperature evolution of the semi-annihilating DM, where a pair of the DM particles annihilate into one DM particle and another particle coupled to the standard model sector. We find that the kinetic equilibrium is maintained solely via semi-annihilation until the last stage of the freeze-out. After the freeze-out, semi-annihilation converts the mass deficit to the kinetic energy of DM, which leads to non-trivial evolution of the DM temperature. We argue that the DM temperature redshifts like radiation as long as the DM self-interaction is efficient. We dub this novel temperature evolution as self-heating. Notably, the structure formation is suppressed at subgalactic scales like keV-scale warm DM but with GeV-scale self-heating DM if the self-heating lasts roughly until the matter-radiation equality. The long duration of the self-heating requires the large self-scattering cross section, which in turn flattens the DM density profile in inner halos. Consequently, self-heating DM can be a unified solution to apparent failures of cold DM to reproduce the observed subgalactic scale structure of the Universe.