ALPHA – A Plasma Haloscope for the Post-Inflation Axion

Axions were hypothesized to explain the invariance of interactions under the transformations P and CP. Motivated by recent theoretical predictions for the mass of the post-inflation axion, ALPHA (Axion Longitudinal Plasma HAloscope) under construction, aims to probe axions above 40 μeV with a tunable metamaterial resonator inside the bore of a 200 mm diameter, 9 T solenoid magnet. As the plasma frequency depends on the unit cell rather than boundary conditions, as is the case for conventional microwave cavities, resonators of arbitrarily high frequency can be built without sacrificing volume. The optimum design of such a resonator for ALPHA is being explored by both the University of California, Berkeley, and Stockholm University. The resonator consists of thin copper rods with a Photonic Band Gap (PBG) enclosure to suppress Transverse Electric (TE) modes, which would otherwise overwhelm the TM₀₁₀ mode of interest, to which the axion couples. The initial phase of the experiment will cover 10-20 GHz. In this talk, I will provide an overview of the ALPHA experiment and discuss the theory and prototyping results of the metamaterial resonator in detail.