A power spectrum approach to the search for Axion-like Particles from resolved galaxy clusters using CMB as a backlight

Abstract

Axions or ALPs are hypothetical particles predicted by BSM theories, which make one of the dark matter candidates. These particles can convert into photons and vice-versa in the presence of magnetic field, with a probability decided by its coupling strength ${\mathrm{g}}_{\mathrm{a}\gamma }$. One of the ways to detect these particles is using the CMB as a backlight. As the CMB photons pass through a galaxy cluster, they can get converted into ALPs in the mass range $10^{-15}$ eV to $10^{-11}$ eV through resonant conversion in the presence of cluster magnetic fields. This leads to a polarized spectral distortion ($\alpha$-distortion) in the CMB as the photon polarization parallel to the magnetic field in the galaxy cluster is involved in the conversion. The fluctuations in the magnetic field and electron density in a galaxy cluster lead to spatially varying $\alpha$-distortion around the cluster, with a power spectrum that is different from the lensed CMB polarization power spectrum for the standard model of cosmology. By measuring the difference in the polarization power spectrum around a galaxy cluster from the all-sky signal, one can find new $\alpha$-distortion in the sky. For galaxy clusters resolvable in multiple EM bands, one can measure the coupling strength ${\mathrm{g}}_{\mathrm{a}\gamma }$ from the ALP power spectrum. Using multi-frequency techniques like ILC to clean the foregrounds, we show that the new power spectrum-based approach of the resolved galaxy clusters from upcoming CMB experiments such as Simons Observatory and CMB-S4 can detect (or put constraints) on the ALP-photon coupling strength of ${\mathrm{g}}_{\mathrm{a}\gamma }<5.2\times 10^{-12}\mathrm{Ge}{\mathrm{V}}^{-1}$ and ${\mathrm{g}}_{\mathrm{a}\gamma }<3.6\times 10^{-12}\mathrm{Ge}{\mathrm{V}}^{-1}$ at 95\% C.I. respectively for ALPs of masses $10^{-13}$ eV or for smaller ${\mathrm{g}}_{\mathrm{a}\gamma }$ for lighter ALP masses (Abridged).

Comments: 34 pages, 19 figures, 5 tables, Published in JCAP