New Target for Cosmic Axion Searches

Future cosmic microwave background experiments have the potential to probe the density of relativistic species at the subpercent level. This sensitivity allows light thermal relics to be detected up to arbitrarily high decoupling temperatures. Conversely, the absence of a detection would require extra light species never to have been in equilibrium with the Standard Model. In this Letter, we exploit this feature to demonstrate the sensitivity of future cosmological observations to the couplings of axions to photons, gluons, and charged fermions. In many cases, the constraints achievable from cosmology will surpass existing bounds from laboratory experiments and astrophysical observations by orders of magnitude.

Figure 1

Contribution of a single thermally decoupled Goldstone boson to the effective number of neutrinos, $\mathrm{\Delta }{N}_{\mathrm{eff}}$, as a function of the freeze-out temperature $T_F$. Shown are also the current $2\sigma$ sensitivity of the Planck satellite [1] and an (optimistic) estimate of the sensitivity of a future CMB-S4 mission [3]. The gray band indicates the QCD phase transition.

Figure 2

Comparison between current constraints on the axion-photon coupling and the sensitivity of a future CMB-S4 mission (figure adapted from Ref. [34]). Future laboratory constraints (IAXO and ADMX) are shown as shaded regions. The yellow band indicates a range of representative models for the QCD axion. The future CMB bound is a function of the reheating temperature $T_R$. We note that ADMX assumes that the axion is all of the dark matter, while all other constraints do not have this restriction.

Figure 3

Comparison between current constraints on the axion-gluon coupling and the sensitivity of a future CMB-S4 mission (figure adapted from Refs. [39, 41]). The dotted lines are the projected sensitivities of the NMR experiment CASPEr [42]. We note that CASPEr, the static EDM [39], and BBN constraints [41] assume that the axion is all of the dark matter, while SN 1987A [15] and the future CMB constraint do not have this restriction.