Effects of Cosmic Muons on $μ$eV-to-meV Scale Axion Dark Matter Searches

Abstract

We estimate the synchrotron radiation of cosmic muons in a uniform magnetic field in the $μ$eV-to-meV energy scale. 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 muons span a wide range of Lorentz factor $γ$ and pitch angle $α$. We verify that the cosmic muons are not the dominant noise background for the current axion dark matter experiments on the $μ$eV 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.

Figures (8)

• Figure 1: Examples of cosmic muon trajectories passing through a copper cavity with GEANT4. Blue and red tracks are $\mu^{+}$ and $\mu^{-}$, respectively. An 8 T vertical magnetic field (dark blue arrows) along $z$-axis is applied to cover the whole copper cylindrical cavity (pink). A larger cylindrical space (grey) is defined to terminate the events.
• Figure 2: Vector diagram for a muon in helical motion in a uniform magnetic field. Both $\alpha$ and $\theta$ are defined in the lab frame.
• Figure 3: Integrated angular-frequency-differential synchrotron radiation power spectra at different pitch angles $\alpha$ and Lorentz factors $\gamma$. The summations with $n$ (dashed lines) agree with substituted $\omega$ integrals (dotted lines) when the integration window from $E = \hbar\omega$ to $\hbar(\omega+\omega_0)$ spreads a large number of $n$.
• Figure 4: The angular-frequency-differential synchrotron radiation power spectra at different $\omega$s with the same integral window $\omega_{0}$ as a function of pitch angle $\alpha$ and Lorentz factor $\gamma$.
• Figure 5: Pitch angle $\alpha$ and Lorentz factor distributions $\gamma$ of cosmic muons at sea level