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 N-eff(CMB) < N-eff(BBN) and eta(CMB) < eta(BBN). New constraints presented here come from Planck measurements of the CMB power spectrum combined with the latest inferences of primordial He-4 and D/H abundances. We find that a previously allowed region in parameter space near m = 1 MeV and tau = 100 ms, consistent with a QCD axion arising from a symmetry breaking near the electroweak scale, is now ruled out at >3 sigma by the combination of CMB + D/H measurements if only ALPs and three thermalized neutrino species contribute to N-eff. The bound relaxes if there are additional light degrees of freedom present which, in this scenario, have their contribution limited to Delta N-eff = 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.