Speaker
Description
Axions or axion-like particles (ALPs) are hypothetical particles predicted by various BSM theories, which also make one of the dark matter candidates. If ALPs exist in nature, the CMB photons as they pass through galaxy clusters will convert to ALPs (of mass range $10^{-14}$ to $10^{-11}$ eV), resulting in a polarized spectral distortion in the CMB, and an astrophysics dependent non-Gaussian secondary anisotropy. The resonant conversions dominate over the non-resonant ones, and occur when the effective masses of the photon and ALP are equal. The probability of this conversion will depend on the mass of ALPs, photon-ALP coupling constant $g_{a\gamma}$, electron density and transverse magnetic field profiles of the clusters, as well as the photon frequency at the conversion location. If galaxy clusters are resolvable in various frequency bands, their astrophysical information can be obtained. We have developed a multi-band framework, SpectrAx, which uses radio synchrotron observations (say, with SKA), to obtain the transverse magnetic field profiles of clusters. Through X-ray observations (say, with eROSITA), their electron density and temperature profiles can be constrained. Using the spectral and spatial information of the CMB, the ALP signal from these clusters can be estimated. The clusters that are unresolved in various frequency bands, will create a diffused ALP background in the microwave sky. Such a signal will result in an increase in the CMB power spectrum at high multipoles, following the spectrum of the ALP signal. The two regimes will enable us to probe axions using the upcoming CMB experiments, such as the Simons Observatory and CMB-S4, which will be able to provide bounds ($g_{a\gamma}< 4\times{10}^{-12} \, \mathrm{GeV}^{-1}$) more than an order better than the current bounds from CAST ($g_{a\gamma}< 6.6\times{10}^{-11} \, \mathrm{GeV}^{-1}$).
