Cosmological constraints on decaying axion-like particles: a global analysis

TL;DR

This paper performs a comprehensive global analysis of decaying axion-like particles (ALPs) in the keV-MeV mass range, using cosmological data to constrain their properties and predict future observational improvements.

Contribution

It introduces a novel global fitting framework combining multiple cosmological and astrophysical constraints to analyze ALP parameter space comprehensively.

Findings

Lower bound on ALP mass around 300 keV

Constraints can be evaded if ALPs are stable

Future CMB spectral distortion observations could improve bounds by two orders of magnitude

Abstract

Axion-like particles (ALPs) decaying into photons are known to affect a wide range of astrophysical and cosmological observables. In this study we focus on ALPs with masses in the keV-MeV range and lifetimes between $10^4$ and $10^{13}$ seconds, corresponding to decays between the end of Big Bang Nucleosynthesis and the formation of the Cosmic Microwave Background (CMB). Using the CosmoBit module of the global fitting framework GAMBIT, we combine state-of-the-art calculations of the irreducible ALP freeze-in abundance, primordial element abundances (including photodisintegration through ALP decays), CMB spectral distortions and anisotropies, and constraints from supernovae and stellar cooling. This approach makes it possible for the first time to perform a global analysis of the ALP parameter space while varying the parameters of $\Lambda$CDM as well as several nuisance parameters. We find a lower bound on the ALP mass of around $m_a > 300\,\text{keV}$, which can only be evaded if ALPs are stable on cosmological timescales. Future observations of CMB spectral distortions with a PIXIE-like mission are expected to improve this bound by two orders of magnitude.