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 E_\mathrm{inj}\lesssim 20\,\mathrm{keV} We deliver a comprehensive library of SD solutions that can be used to study a wide range of particle physics...