Probing Axion-like Particles via CMB Polarization

TL;DR

This paper investigates how axion-like particles (ALPs) affect CMB polarization, deriving relations between ALP properties and polarization rotation, and assessing observational sensitivities to ALP-photon coupling.

Contribution

It provides a detailed analysis of ALP-induced polarization rotation, including the effects of ALP field fluctuations, and forecasts sensitivities of future CMB experiments to ALP parameters.

Findings

Future CMB observations can detect ALP-photon couplings as low as 4×10^{-21} GeV^{-1}.

ALP field fluctuations can cause significant isotropic polarization rotation.

Detection of anisotropic rotation constrains the tensor-to-scalar ratio r > 5×10^{-9}.

Abstract

Axion-like particles (ALPs) rotate the linear polarization of photons through the ALP-photon coupling and convert the cosmic microwave background (CMB) $E$-mode to the $B$-mode. We derive the relation between the ALP dynamics and the rotation angle by assuming that the ALP $\phi$ has a quadratic potential, $V=m^2\phi^2/2$. We compute the current and future sensitivities of CMB observations to the ALP-photon coupling $g$, which can reach $g=4\times 10^{-21}\,\mathrm{GeV}^{-1}$ for $10^{-32}\,\mathrm{eV}\lesssim m\lesssim 10^{-28}\,\mathrm{eV}$ and extensively exceed the other searches for any mass $m\lesssim 10^{-25}\,\mathrm{eV}$. We find that the fluctuation of the ALP field at the observer, which has been neglected in previous studies, can induce significant isotropic rotation of the CMB polarization. The measurements of isotropic and anisotropic rotation allow us to put bounds on relevant quantities such as the ALP mass $m$ and the ALP density parameter $\Omega_\phi$. In particular, if LiteBIRD detects anisotropic rotation, we obtain the lower bound on the tensor-to-scalar ratio as $r > 5 \times 10^{-9}$.