Probing Axion-Photon conversion via circular polarization imprints in the CMB $V$-mode observations

Abstract

In the presence of a background magnetic field, axions or axion-like particles (ALPs) can be resonantly converted to photons when their mass is nearly equal to the effective photon mass. In this paper, we propose a novel method to constrain the parameter space of ALPs by investigating the resulting imprints of axion-photon conversion in the cosmic microwave background (CMB) observations. We show that a helical magnetic field existing prior to the CMB epoch can generate an excess population of photons carrying net circular polarization due to the axion-photon conversion mechanism. Consequently, current measurements of the angular power spectrum of circular polarization ($V$-mode) in the CMB can be used to constrain the parameter space of ALP mass and its coupling to photons. In the optimistic scenario of a maximally helical magnetic field with strength $\sim {\rm nG}$, we find that CLASS observations at $40 \, {\rm GHz}$ can probe the previously unconstrained regions of axion-photon coupling corresponding to ALP masses in the range $10^{-10}-10^{-8} \, {\rm eV}$.

Figures (4)

• Figure 1: The conversion probability for three typical ALPs masses going through resonance between $1100 \lesssim z_{\rm res} \lesssim 10^4$ for CLASS observation frequency, $\omega_0 = 2\pi\times40 \, {\rm GHz}$.
• Figure 2: The resulting constraints plot on the ALPs parameter space $(g_{\phi\gamma},m_\phi)$ from various CMB $V$-mode measurements. For simplicity, we have fixed $\Delta \chi_B = 1$, and the solid and dashed curves represent $\mathcal{B}_{0}=1\, {\rm nG}$ and $\mathcal{B}_{0}= 0.1\, {\rm nG}$, respectively. The green shaded region shows the current constraints from magnetic white dwarf (MWD) polarization Benabou:2025jcv.
• Figure 3: The behaviour of various terms in Eq. \ref{['ap-con-prob-pm-z-fin-num']} for three typical masses, $m_{\phi}=1.44301\times 10^{-8} \, {\rm eV} , 7.07209\times10^{-9} \, {\rm eV} ,2.47505 \times 10^{-10}\, {\rm eV}$ going through the resonance at redshifts $z_c \simeq 8550,5314.41, 1094.18$ (upper, middle and lower panel), respectively.
• Figure 4: Left plot: shows the mass $m_{\phi}$ goes through resonance at redshifts $z_{res}$ in Eq.\ref{['res-mass-eq']}. Right plot: shows the comparison of the plasma frequency term $X_e (z)$ (black curve) and frequency-dependent term, $7.3\times 10^{-3} \left(\frac{\omega_0 (z)}{1.5\times 10^{15} \, {\rm Hz}}\right)^2 (1+z)^2(1-X_e (z) \,)$ in Eq.\ref{['effective-mass-cmp']} for three frequencies of our interest, we used $\omega (z) = \omega_0 (1+z)$ and $1\, {\rm eV} = 1.5\times 10^{15} \, {\rm Hz}$.