Spectral distortion constraints on photon injection from low-mass decaying particles

Spectral distortions (SDs) of the cosmic microwave background (CMB) provide a powerful tool for studying particle physics. Here we compute the distortion signals from decaying particles that convert directly into photons at different epochs during cosmic history, focusing on injection energies Einj ≲ 20 keV. We deliver a comprehensive library of SD solutions, using CosmoTherm to compute the SD signals, including effects on the ionization history and opacities of the Universe, and blackbody-induced stimulated decay. Then, we use data from COBE/FIRAS and EDGES to constrain the properties of the decaying particles. We explore scenarios where these provide a dark matter (DM) candidate or constitute only a small fraction of DM. We complement the SD constraints with CMB anisotropy constraints, highlighting new effects from injections at very-low photon energies (hν ≲ 10-4 eV). Our model-independent constraints exhibit rich structures in the lifetime-energy domain, covering injection energies Einj ≲ 10-10 eV - 10 keV and lifetimes τX ≲ 105 - 1033 s. We discuss the constraints on axions and axion-like particles, revising existing SD constraints in the literature. Our limits are competitive with other constraints for axion masses mac2 ≳ 27 eV and we find that simple estimates based on the overall energetics are generally inaccurate. Future CMB spectrometers could significantly improve the obtained constraints, thus providing an important complementary probe of early-universe particle physics.