Speaker
Description
Relativistic axions produced in the early universe can form a Cosmic axion Background (C$a$B), providing a target that is distinct from conventional cold axion dark matter. In this talk, I will present a framework for searching for the C$a$B using arrays of resonant cavities in a strong magnetic field as in the future ADMX project. Although the C$a$B is broadband and has a much shorter coherence length than cold dark matter, a high-$Q$ cavity filters the axion field into a narrow-band electromagnetic response whose spatial correlations can be exploited across multiple detectors. I derive the axion-induced electric-field two-point correlation function for multi-cavity arrays and show that sensitivity depends non-trivially on cavity aspect ratio and array layout. In particular, I find that ADMX's prospective arrays suffer a relativistic suppression, while a new vertically stacked fat-cavity array provides a more improved sensitivity. These results illustrate how detector geometry and inter-cavity correlations can open a new avenue for probing relativistic axion backgrounds.