A new probe of Axion-Like Particles: CMB polarization distortions due to
  cluster magnetic fields
  We propose using the upcoming Cosmic Microwave Background (CMB) ground based
experiments to detect the signal of ALPs (Axion like particles) interacting
with magnetic fields in galaxy clusters. The conversion between CMB photons and
ALPs in the presence of the cluster magnetic field can cause a polarized
spectral distortion in the CMB around a galaxy cluster. The strength of the
signal depends upon the redshift of the galaxy cluster and will exhibit a
distinctive spatial profile around it depending upon the structure of electron
density and magnetic field. This distortion produces a different shape from the
other known spectral distortions like $y$-type and $\mu$-type and hence are
separable from the multi-frequency CMB observation. The spectrum is close to
kinematic Sunyaev-Zeldovich (kSZ) signal but can be separated from it using the
polarization information. For the future ground-based CMB experiments such as
Simons Observatory and CMB-S4, we estimate the measurability of this signal in
the presence of foreground contamination, instrument noise and CMB
anisotropies. This new avenue can probe the photon-ALP coupling over the ALP
mass range from $10^{-13}$ eV to $10^{-12}$ eV with two orders of magnitude
better accuracy from CMB-S4 than the current existing bounds.
