Detecting Axion-like particles using Cosmic Variance Cancellation with CMB and Radio surveys

Abstract

Axions and axion-like particles (ALPs) arise naturally in many extensions of the Standard Model and are among the well-motivated candidates for dark matter. In the presence of magnetic fields of galaxy clusters, Cosmic Microwave Background (CMB) photons can convert to ALPs, with the efficiency of the process governed by the cluster electron density and magnetic field profiles, the photon--ALP coupling strength ($g_{aγ}$), as well as the frequency ($ν$) of the photon at the redshift of the cluster. The CMB blackbody spectrum suggests that this resonant conversion also takes place at radio wavelengths, following the spectral behaviour of the ALP distortion signal. This opens a new window to search for ALPs using cosmic variance cancellation (CVC), with multi-frequency tracers of the same phenomenon in CMB photon--ALP resonant conversion. The constraints on the ALP signal ratios from different combinations of microwave and radio bands of the Simons Observatory (SO) and the Square Kilometre Array (SKA) can be significantly improved using CVC compared to the case of using auto-only spectra from the two experiments. With the large number of galaxy clusters that will be observed by SO and SKA, we will be able to obtain much more information using CVC, especially for low-mass ALPs with stronger signals. Using the auto-only spectra from galaxy clusters up to redshift $z = 1$ for inference of the normalized ratio parameter, we obtain a standard deviation of $5.9 \times 10^{-2}$ for an ALP mass $m_a = 10^{-14} \, \mathrm{eV}$, which improves to $1 \times 10^{-2}$ using CVC. This method provides a universal probe of the ALP distortion signal using its spectral dependence and can also invalidate false detections of the ALP signal based on its frequency behaviour in different bands.

Figures (12)

• Figure 1: This figure shows how the ALP distortion signal will show up in both the microwave and radio frequencies. The CMB photon-ALP resonant conversion will result in a polarized spectral distortion of the CMB blackbody, accompanied with a loss in CMB intensity, and impacting both the radio and microwave frequencies. Using the cross CMB-radio observations to probe the ALP signal will provide additional information by drastically improving the signal ratio inferences.
• Figure 2: This figure depicts the change that is caused in the CMB polarization map due to the photon-ALP conversion in a cluster (shown as a dashed circle). If there are no ALPs in nature, the fluctuations in the cluster region will be frequency-independent and follow the smooth behaviour of the primordial CMB. But if ALPs exist, there will be additional fluctuations in the cluster region, which will increase with frequency.
• Figure 3: This figure shows how the variation of ALP distortion intensity fluctuations in a cluster region with frequency changes in magnitude, but the pattern of the ALP distortion fluctuations is similar in the cases of microwave (145 GHz) and radio (1.4 GHz) signals. The hot (cold) regions in the microwave band appear as hot (cold) regions in the radio band as well. When the cross-signal of the two bands is taken, i.e, $\Delta T_{\nu_1,\nu_2} = \sqrt{\Delta T_{\nu_1} \Delta T_{\nu_2}}$, we see that the signal fluctuations still show a similar pattern as in the microwave and radio maps, hot (cold) fluctuations show up as hot (cold) fluctuations in the cross signal.
• Figure 4: This figure depicts the power spectrum of the auto and cross spectra for the frequency bands 93 GHz from SO and 6.7 GHz from SKA. The modeled ALP power spectra are shown for $g_{a\gamma} = 3 \times 10^{-12} \, \rm{GeV}^{-1}$. The CVC uses the information that the information that the tracers at different frequencies show similar fluctuations, improving the cross-signal inferences.
• Figure 5: This figure shows the distribution of galaxy clusters across redshifts that will be observed by SO.