Photon proliferation from multi-body dark matter annihilation

TL;DR

This paper reveals that multi-body dark matter annihilation can significantly enhance photon production in the early Universe, especially in nonthermal scenarios, leading to stronger constraints on ultralight dark matter.

Contribution

It demonstrates that multi-body annihilation processes are important in nonthermal dark matter models and can produce a photon proliferation effect, impacting cosmological bounds.

Findings

Photon proliferation increases photon temperature after neutrino decoupling.

Stronger bounds on ultralight dark matter couplings than previous constraints.

Multi-body processes are crucial in early Universe dark matter phenomenology.

Abstract

Multi-body dark matter annihilation is commonly expected to be suppressed by higher-order couplings and phase-space factors, therefore being ignored thus far. We show that, however, this does not hold for a class of nonthermal dark matter scenarios, where the dark matter particle becomes nonrelativistic at temperatures much higher than its mass. We exemplify such a multi-body process via ultralight pseudoscalar dark matter annihilation to diphotons, which leads to a novel photon proliferation effect in the early Universe. As a phenomenological application, we consider the photon temperature shift after neutrino decoupling, showing that the photon proliferation effect can render bounds on the ultralight dark matter couplings stronger than the existing constraints by several orders of magnitude. Our research can be extended to other interactions and dark matter candidates, highlighting the importance of multi-body processes in the early Universe.