The Sensitivity of Dielectric Haloscopes to Dark Matter Axions and Dark Photons

Abstract

Axions and dark photons are dark matter candidates that have gathered increasing interest over the years. They are thought to be very lightweight with masses of less than $\SI{1}{\eV}$, where their wave-like nature needs to be taken into account. Haloscopes are experiments aimed at detecting axions or dark photons from the galactic dark matter halo in the laboratory by converting them into detectable ordinary photons. Dielectric haloscopes like the upcoming MAgnetized Disk and Mirror Axion eXperiment (MADMAX) are designed to have a detector volume that can scale independently of the wavelength of the converted photons, facilitating the search for higher axion or dark photon masses from $\SIrange{40}{400}{\micro\eV}$. They rely on the strategic placing of a stack of large dielectric disks, collectively known as the booster. Each disk emits converted photons sourced by either axions or dark photons. The spacing between disks is controlled such that the emissions of each disk can resonate and interfere constructively, leading to a large amplification of the expected signal power.

The most important characteristic of a haloscope is the expected sensitivity to a given axion or dark photon mass and interaction strength. This has proven to be particularly challenging for dielectric haloscopes as the large detector volume makes accurate numerical simulation infeasible. In this thesis, a novel approach to determining the sensitivity of dielectric haloscopes, which does not rely on numerical simulations, is developed and verified experimentally. By precisely measuring an electric test field, the expected signal power of the haloscope can be determined using reciprocity theorems. This \textit{reciprocity approach} applies not only to dielectric haloscopes but also to cavity and dish antenna haloscopes. The electric test field is measured on a dish antenna and dielectric haloscope with up to three disks using the bead-pull method. Measuring the electric fields allows to characterize systematic effects that have been out of reach for numerical simulations. Most notably, this includes standing waves between the receiver antenna and booster. The expected signal power is determined from electric field measurements and with minimal model input, enabling a full calibration of a dielectric haloscope for the first time.

Solving this major challenge made it possible to perform a competitive dark photon search using a three-disk dielectric haloscope and a custom-built receiver chain. No dark photon signal was found, excluding dark photon dark matter with a mixing angle $\chi > \num{5.7e-12}$ over a mass range of $\SIrange{78.62}{83.95}{\micro\eV}$. With a peak sensitivity of $\chi\sim\num{2e-13}$, existing bounds are improved by almost three orders of magnitude.