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’s (CMB) last scattering epoch, in particular such that and . New constraints presented here come from Planck measurements of the CMB power spectrum combined with the latest inferences of primordial and D/H abundances. We find that a previously allowed region in parameter space near and , consistent with a QCD axion arising from a symmetry breaking near the electroweak scale, is now ruled out at by the combination of measurements if only ALPs and three thermalized neutrino species contribute to . The bound relaxes if there are additional light degrees of freedom present which, in this scenario, have their contribution limited to . 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.
1 More- Received 16 January 2015
DOI:https://doi.org/10.1103/PhysRevD.92.023010
© 2015 American Physical Society