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

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 gaγ. 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 (α-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 α-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 α-distortion in the sky. For galaxy clusters resolvable in multiple EM bands, one can measure the coupling strength gaγ 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 gaγ < 5.24 × 10⁻¹² \, GeV⁻¹ and gaγ < 3.61 × 10⁻¹² \, GeV⁻¹ at 95% C.I. respectively for ALPs of masses 10⁻¹³ eV or for smaller gaγ for lighter ALP masses (Abridged).