Feasibility, engineering aspects and physics reach of microwave cavity experiments searching for hidden photons and axions

TL;DR

This work analyzes the feasibility of microwave cavity light-shining-through-walls experiments to search for hidden sector photons and axion-like particles. It develops detection strategies based on narrowband methods (lock-in and FFT) and derives the scaling of the detectable power $\mathcal{P}_{\rm det}$ with model parameters, quality factors, and geometry through $\mathcal{P}_{\rm det} = \chi^4\, \frac{m_{\gamma'}^{8}}{\omega_0^{8}}\, |G|^2\,Q Q'\,\mathcal{P}_{\rm em}$ for HSPs and $\mathcal{P}_{\rm det} \sim (\frac{gB}{\omega_0})^4\, |\tilde{G}|^2\, Q Q'\,\mathcal{P}_{\rm em}$ for ALPs. It proposes a box-in-box shielding scheme with leakage monitoring to validate shielding in real-time and discusses experimental pathways—from room temperature to cryogenic and superconducting cavities, including high-field magnets for ALP searches. The paper provides concrete sensitivity projections (e.g., $P_{\rm det}$ down to $10^{-22}$–$10^{-26}$ W) and outlines design routes (dielectrics, hard superconductors) that could extend the reach beyond current laboratory and some astrophysical bounds, illustrating the practical potential of microwave cavity LSW in exploring new light weakly interacting particles.

Abstract

Using microwave cavities one can build a resonant ``light-shining-through-walls'' experiment to search for hidden sector photons and axion like particles, predicted in many extensions of the standard model. In this note we make a feasibility study of the sensitivities which can be reached using state of the art technology.

Figures (7)

• Figure 1: Schematic of a "light-shining-through a wall" experiment. An incoming photon $\gamma$ is converted into a new particle $X$ which interacts only very weakly with the opaque wall. It passes through the wall and is subsequently reconverted into an ordinary photon which can be detected. In the case of an axion the conversion is facilitated by a magnetic field that interacts with the incoming photon. In contrast, photon $\leftrightarrow$ hidden photon oscillations occur via a non-diagonal mass term and can therefore also occur in vacuum (analog to neutrino oscillations).
• Figure 2: The black line shows the Fourier transform of a signal which is concentrated around the reference frequency $\omega_0=\pm 5$ (arbitrary units). After multiplying it with the reference frequency the signal is now concentrated around $0,\pm 2\omega_{0}$ (red curves). Using the a measurement time $\Delta t$ we can then analyze the spectrum with a resolution $\delta \omega=2\pi/\Delta t$ (thin black lines denote the border of the bins). The black shaded bins shows the bin measured by a simple lock-in amplifier.
• Figure 3: In the upper panel a test setup for microwave detection with the methods described in the text (cf. also technical). A weak signal is generated in the signal generator and is then further attenuated down to $-190$ dBm$=10^{-22}$ W. This signal is then amplified by a total of 33.8 dB and combined (i.e. mixed) with the reference signal in the vector spectrum analyzer which also records the signal and performs the FFT. In the lower panel the observed signal averaged over 10 measurements (this smoothes out the fluctuations of the envelope of the narrowband filtered and peak detected noise) is shown (the resolution bandwidth is 3 mHz). A narrow signal line is observed technical which is clearly distinct from thermal background (-199 dBm) plus the noise from the amplifier (2dB).
• Figure 4: Setup of the experiment with "test signals" at slightly different frequencies for leakage monitoring. For details see main text.
• Figure 5: Expected structure for a "true" signal (arbitrary units). The black peak at the frequency $\Delta f_{1}$ is the signal. Our leakage monitoring leads to sidebands at frequencies $\Delta f_{1}\pm\Delta f_{2}$ (red) and $\Delta f_{1}\pm \Delta f_{3}$ (blue) which are hopefully much smaller.