Axion-like Dark Matter Constraints from CMB Birefringence

Abstract

Axion-like particles are dark matter candidates motivated by the Peccei-Quinn mechanism and also occur in effective field theories where their masses and photon couplings are independent. We estimate the dispersion of circularly polarized photons in a background of oscillating axion-like particles (ALPs) with the standard $g_{a\gamma }a{F}_{\mu \nu }{\tilde{F}}^{\mu \nu }/4$ coupling to photons. This leads to birefringence or rotation of linear polarization by ALP dark matter. Cosmic microwave background (CMB) birefringence limits $\Delta \alpha \lesssim (1.0{)}^{\circ }$ enable us to constrain the axion-photon coupling $g_{a\gamma }\lesssim 10^{-17}-10^{-12}{\mathrm{GeV}}^{-1}$, for ultra-light ALP masses $m_a\sim 10^{-27}-10^{-24}$ eV. This improves upon previous axion-photon coupling limits by up to four orders of magnitude. Future CMB observations could tighten limits by another one to two orders.

Comments: 6+2 pages, 1 figure, 1 table; substantially revised version, overall conclusions essentially unchanged