http://arxiv.org/abs/1811.07873
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 the rotation of linear polarization that can constrain the axion-photon coupling $g_{a\gamma}$. The cosmic microwave background (CMB) polarization is a sensitive probe of cosmic birefringence arising from an effective refractive index that would be induced by ALP dark matter. The birefringence limit from CMB observations $\Delta \alpha \lesssim (0.5)^\circ$ enables us to constrain the axion-photon coupling $g_{a\gamma} \lesssim 10^{-18}-10^{-13}\,{\rm GeV}^{-1}$, for ultra-light ALP masses $m_a \sim 10^{-27} – 10^{-22}$ eV. This represents an improvement of one to a few orders of magnitude upon the tightest prior limits derived from x-ray observations of active galaxies in clusters, although the techniques have different assumptions. Future CMB polarization experiments and their birefringence forecasts have the potential to further refine ALP dark matter parameters.
G. Sigl and P. Trivedi
Tue, 20 Nov 18
41/73
Comments: 7 pages, 1 figure
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