Design and development of high-frequency resonant structures for future axion searches*

The axion is a bosonic dark matter candidate which can be detected by its conversion to photons in a magnetic field. In this talk I will give an overview of two projects that aim to test high frequency, tunable resonator designs for use in axion haloscopes. The first experiment is named Orpheus, and the second is yet unnamed and is located at Stanford. The current generation of tunable axion searches range from about 1-6 GHz (axion masses of about 1-25 $mu$eV). However, these experiments lose sensitivity when probing higher axion masses, largely due to the design of the resonators used. As the current searches continue, novel resonator designs will need to be developed to search at these higher frequencies. 

The Stanford experiment utilizes a unique geometry and novel tuning mechanism to search at higher frequencies while keeping cavity volume large, and thus not diminishing sensitivity. It has a working room temperature prototype with resonant frequencies around 7 GHz (corresponding to axion mass of about 30 $mu$eV). Orpheus uses dielectric plates and a Fabry-Perot resonator, and utilizes the 18th-order resonant mode to search for higher mass axions, with a resonant frequency around 16 GHz. Orpheus has excluded dark photon masses, and the resonator has operated and tuned successfully at liquid helium temperatures. Furthermore, the dipole magnet for Orpheus is nearing completion and is going through a round of testing. Completion of the magnet will allow Orpheus to exclude axions with masses around 60 $mu$eV. Design overviews will be given of both haloscopes, and projected axion sensitivities and future directions will be discussed.