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 NeffCMB<NeffBBN and ηCMB<ηBBN. New constraints presented here come from Planck measurements of the CMB power spectrum combined with the latest inferences of primordial He4 and D/H abundances. We find that a previously allowed region in parameter space near m=1MeV and τ=100ms, consistent with a QCD axion arising from a symmetry breaking near the electroweak scale, is now ruled out at >3σ by the combination of CMB+D/H measurements if only ALPs and three thermalized neutrino species contribute to Neff. The bound relaxes if there are additional light degrees of freedom present which, in this scenario, have their contribution limited to ΔNeff=1.1±0.3. 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.