# A Fourth Exception in the Calculation of Relic Abundances

## TL;DR

This paper introduces a new mechanism called coscattering, where dark matter abundance is determined by inelastic scattering decoupling, leading to lighter dark matter and potential observable effects on the cosmic microwave background.

## Contribution

It proposes the coscattering mechanism as a novel way to set dark matter relic abundance, differing from traditional annihilation-based models.

## Key findings

- Coscattering can produce dark matter lighter than the weak scale.
- Dark matter with suppressed annihilation rates avoids indirect detection constraints.
- Late decays of upscattered states may distort the CMB blackbody spectrum.

## Abstract

We propose that the dark matter abundance is set by the decoupling of inelastic scattering instead of annihilations. This coscattering mechanism is generically realized if dark matter scatters against states of comparable mass from the thermal bath. Coscattering points to dark matter that is exponentially lighter than the weak scale and has a suppressed annihilation rate, avoiding stringent constraints from indirect detection. Dark matter upscatters into states whose late decays can lead to observable distortions to the blackbody spectrum of the cosmic microwave background.