Effects of cosmic-ray muons on µeV-to-meV scale axion dark matter searches

We estimate the synchrotron radiation emitted by cosmic-ray muons in a uniform magnetic field, focusing on the photon radiation from μeV to meV. Such events can potentially bring backgrounds to the axion dark matter searches. The GEANT4 software package is utilized to simulate the muon tracks in a cylindrical region of interest with an 8 T solenoid magnetic field applied. We further develop an analytical estimation of the angular-frequency-differential synchrotron radiation power spectra in this work as the cosmic-ray muons span a wide range of Lorentz factor γ and pitch angle α. We verify that the cosmic-ray muons are not the dominant noise background for the current axion dark matter experiments on the μeV scale because of the high quality factor Q and fine energy resolution in the readout. However, without sufficient energy resolution in the detector readout, future broadband axion dark matter experiments will be vulnerable to the
synchrotron radiation of these charged particles.