Dark radiation from particle decay: cosmological constraints and opportunities

TL;DR

This paper investigates how particle decay can produce dark radiation, deriving model-independent cosmological constraints and exploring potential observational signatures, with implications for dark matter and structure formation.

Contribution

It provides a comprehensive, model-independent framework for constraining dark radiation from particle decay using cosmological data, and explores novel scenarios linking dark radiation and dark matter.

Findings

Bounds on decay parameters depend on decay timing.

Decay scenarios can explain small-scale structure issues.

Possible to distinguish dark radiation from neutrinos after discovery.

Abstract

We study particle decay as the origin of dark radiation. After elaborating general properties and useful parametrisations we provide model-independent and easy-to-use constraints from nucleosynthesis, the cosmic microwave background and structure formation. Bounds on branching ratios and mass hierarchies depend in a unique way on the time of decay. We demonstrate their power to exclude well-motivated scenarios taking the example of the lightest ordinary sparticle decaying into the gravitino. We point out signatures and opportunities in cosmological observations and structure formation. For example, if there are two dark decay modes, dark radiation and the observed dark matter with adjustable free-streaming can originate from the same decaying particle, solving small-scale problems of structure formation. Hot dark matter mimicking a neutrino mass scale as deduced from cosmological observations can arise and possibly be distinguished after a discovery. Our results can be used as a guideline for model building.