Discovering Axion-Like Particles Using Cosmic Microwave Background as the Backlight

Particle physicists and cosmologists have sought axion-like particles (ALPs) ever since they were postulated in quantum chromodynamics and string theory. We propose probing the electromagnetic interaction between ALPs and photons using the inter-conversion of cosmic microwave background (CMB) photons and ALPS in the magnetic field of galaxy clusters. Importantly, the unique, polarized non-gravitational signature of ALPs we propose can be measured from high resolution CMB imaging experiments even if ALPs are not the dark matter; our approach is therefore directly competitive with laboratory experiments. The search for this signal from the large sample of galaxy clusters from the upcoming CMB missions can provide orders of magnitude better constraints on the coupling between photons and ALPs than the current bounds available from any experiments. The ALP mass range and couplings that will be probed by the CMB experiments are unexplored to date and complementary with other cosmological searches from the imprints of ALPs on the cosmic density field.