New Bounds for Axions and Axion-Like Particles with keV-GeV Masses

TL;DR

This paper updates constraints on axions and axion-like particles with keV-GeV masses, showing that certain parameter regions are now ruled out by CMB and primordial abundance measurements, and forecasts future experimental sensitivities.

Contribution

It provides new bounds on axion-like particles near 1 MeV mass, incorporating recent CMB and primordial element data, and discusses implications for axion models and future experiments.

Findings

A region near 1 MeV mass and 100 ms lifetime is ruled out at >3σ.

Constraints depend on the number of additional light degrees of freedom.

Future experiments like Stage-IV CMB and SUPER-KEKB can further test these bounds.

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.