Search for ultra-light axions with CMB polarization

When coupled to electromagnetism via a Chern-Simons interaction, axion-like particles (ALP) produce a rotation of the plane of linear polarization of photons known as cosmic birefringence. Recent measurements of cosmic birefringence obtained from the polarization of the cosmic microwave background (CMB) hint at the existence of an isotropic birefringence angle of $\beta\approx 0.3^\circ$, currently excluding $\beta=0$ with a statistical significance of $3.6\sigma$. Were such measurement to be confirmed as a cosmological signal, CMB information alone could constrain the ALP parameter space for masses $m_\phi\lesssim 10^{-27}$eV and axion-photon coupling constants $g_{\phi\gamma}\gtrsim 10^{-20}$GeV$^{-1}$.