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

TL;DR

This paper proposes a novel power spectrum method to detect axion-like particles (ALPs) by analyzing polarized spectral distortions in the CMB caused by ALP-photon conversion in galaxy clusters, using upcoming CMB experiments.

Contribution

It introduces a new power spectrum-based approach to constrain ALP-photon coupling using resolved galaxy clusters and multi-frequency CMB data.

Findings

Can detect ALP-photon coupling strength down to 5.2e-12 GeV^{-1} with Simons Observatory.

Can improve constraints to 3.6e-12 GeV^{-1} with CMB-S4.

Method distinguishes ALP-induced distortions from standard CMB polarization fluctuations.

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 $\mathrm{g_{a\gamma}}$. 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 $\mathrm{g_{a\gamma}}$ 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 $\mathrm{g_{a\gamma} < 5.2 \times 10^{-12} \, GeV^{-1}}$ and $\mathrm{g_{a\gamma} < 3.6 \times 10^{-12} \, GeV^{-1}}$ at 95\% C.I. respectively for ALPs of masses $10^{-13}$ eV or for smaller $\mathrm{g_{a\gamma}}$ for lighter ALP masses (Abridged).