Abstract
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 (CMB) last scattering epoch, in particular such that $N_{\rm eff}^{\rm CMB} < N_{\rm eff}^{\rm BBN}$ and $\eta^{\rm CMB} < \eta^{\rm BBN}$. New constraints presented here come from Planck measurements of the CMB power spectrum combined with the latest inferences of primordial $^4$He and D/H abundances. We find that a previously allowed region in parameter space near $m=1\,\rm MeV$ and $\tau=100\,\rm ms$, 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 $N_{\rm eff}$. The bound relaxes if there are additional light degrees of freedom present which, in this scenario, have their contribution limited to $\Delta N_{\rm eff}=1.1\pm0.3$. 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.
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