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 (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.
