The Maximal Gravitational Wave Signal from Asteroid-Mass Primordial Black Hole Mergers At Resonant Microwave Cavities

Abstract: Primordial black holes can be the entirety of the dark matter in a broad, approximately five-orders-of-magnitude-wide mass range, the ``asteroid mass range'', between $10^{-16}\ M_{\rm Sun}$ -- where constraints originate from evaporation -- and $10^{-11}\ M_{\rm Sun}$ -- from microlensing. A direct detection in this mass range is very challenging with any known observational or experimental methods. Here we update the calculation of the sight distance for narrow-band detectors such as resonant microwave cavities, and the resulting maximal event rate. We find that the largest detection rates are associated with binaries from non-monochromatic mass functions in early-formed three-body systems. Even in the most optimistic setup, these events are anticipated to be extremely rare.