Microwave cavity light shining through a wall optimization and experiment

TL;DR

This work analyzes the viability of using microwave cavity LSW experiments to search for hidden-sector photons via kinetic mixing, introducing the full geometric function $\mathcal{F}^2$ that encapsulates cavity mode, geometry, and separation effects on the transmission probability $P_{trans}$. Through numerical evaluation of axially stacked cylindrical cavities, it identifies mode-, aspect-ratio-, and separation-dependent behavior and outlines practical optimal configurations. The authors report a first experiment with two copper cavities at ~9.6 GHz, achieving a limit $\chi = 2.9\times 10^{-5}$ at $m_{\gamma'} \sim 3.79\times 10^{-5}$ eV, limited by microwave leakage rather than fundamental sensitivity, thereby validating the concept. They project that significant improvements are feasible, potentially reaching $\chi \sim 10^{-12}$ with higher-Q cavities, physical separation to reduce leakage, and cryogenic operation, establishing a concrete path toward substantially stronger constraints on hidden-sector photons.

Abstract

It has been proposed that microwave cavities can be used in a photon regeneration experiment to search for hidden sector photons. Using two isolated cavities, the presence of hidden sector photons could be inferred from a 'light shining through a wall' phenomenon. The sensitivity of the experiment has strong a dependence on the geometric construction and electromagnetic mode properties of the two cavities. In this paper we perform an in depth investigation to determine the optimal setup for such an experiment. We also describe the results of our first microwave cavity experiment to search for hidden sector photons. The experiment consisted of two cylindrical copper cavities stacked axially inside a single vacuum chamber. At a hidden sector photon mass of 37.78 micro eV we place an upper limit on the kinetic mixing parameter chi = 2.9 * 10^(-5). Whilst this result lies within already established limits our experiment validates the microwave cavity `light shining through a wall' concept. We also show that the experiment has great scope for improvement, potentially able to reduce the current upper limit on the mixing parameter chi by several orders of magnitude.

Figures (11)

• Figure 1: Diagram of cavity setup with radius $a$, length $L$ and separation distance $d$.
• Figure 2: Curves for the $\mathrm{TM}_{\,0\,1\,p}$ mode family labeled ($p$) where $p$ corresponds to the axial mode number. Each was calculated for a cavity aspect ratio of one and zero separation distance between the cavities.
• Figure 3: Lines of $\mathcal{F}^2$ maximums for various families of modes against relevant mode numbers. All for a cavity aspect ratio of one and zero separation distance between the cavities.
• Figure 4: Curves of $\mathcal{F}^2$ for the $\mathrm{TM}_{\,0\,1\,2}$ mode with aspect ratios (diameter divided by length) from (1)$1/3$ to (17)$5$, and zero separation distance between the cavities.
• Figure 5: Trends of the full geometric function maximum against aspect ratio for various modes. In all cases the separation distance between the cavities is zero.