New bounds for axions and axion-like particles with keV-GeV masses

We give updated constraints on hypothetical light bosons with a two-photon coupling such as axions or axion-like particles (ALPs). We focus on masses and lifetimes where decays happen near big bang nucleosynthesis (BBN), thus altering the baryon-to-photon ratio and number of relativistic degrees of freedom between the BBN epoch and the cosmic microwave background's (CMB) last scattering epoch, in particular such that N₄₅₅^CMB3 by the combination of CMB+D/H measurements if only ALPs and three thermalized neutrino species contribute to N₄₅₅. The bound relaxes if there are additional light degrees of freedom present which, in this scenario, have their contribution limited to N₄₅₅=1. 10. 3. We give forecasts showing that a number of experiments are expected to reach the sensitivity needed to further test this region, such as Stage-IV CMB and SUPER-KEKB, the latter a direct test insensitive to any extra degrees of freedom.