Speaker
Description
Abstract: Axions or axion-like particles are hypothetical particles predicted by various BSM theories, which also make one of the dark matter candidates. The CMB is the primordial radiation that surrounds us and it follows an ideal blackbody spectrum, hence deviation in its behaviour can be used to probe new physics. If ALPs exist in nature, the CMB photons as they pass through galaxy clusters will convert to ALPs, resulting in a polarized spectral distortion in the CMB. 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 using multi-band observations. Using radio synchrotron observations (say, with SKA), their transverse magnetic field profiles can be inferred. Through X-ray observations (say, with eROSITA), their electron density and temperature profiles can be constrained. These profile inferences will provide an estimate of the ALP signal from these clusters and bounds on the ALP coupling can be obtained using a pixel-based or power spectrum-based approach. The clusters that are unresolvable in multiple frequencies, will create a diffused ALP background in the sky that can be modelled using the distribution of clusters of different masses across various redshifts. This will result in an increase in the CMB power spectrum at high multipoles, following the spectrum of the ALP signal. Also, the presence of turbulence in profiles will lead to varying non-Gaussianity of the ALP distortion signal. The upcoming CMB experiments, such as the Simons Observatory, LiteBIRD and CMB-S4, will be able to provide bounds ($g_{a\gamma} < O[10^{-12}] \, \mathrm{GeV}^{-1}$) more than an order better than the current bounds from CAST ($g_{a\gamma} < 6.6 \times 10^{-11} \, \mathrm{GeV}^{-1}$).