First Search for Dark Photon Dark Matter with a madmax Prototype

We report the first result from a dark photon dark matter search in the mass range from 78.62 to 83.95 μeV/c² with a dielectric haloscope prototype for madmax (Magnetized Disc and Mirror Axion eXperiment). Putative dark photons would convert to detectable photons within a stack consisting of three sapphire disks and a mirror. The emitted power of this system is received by an antenna and successively digitized using a low-noise receiver. No significant signal attributable to dark photons has been observed above the expected background. Assuming unpolarized dark photon dark matter with a local density of ρχ=0.3 GeV cm³ we exclude a dark photon to photon mixing parameter χ>2.7×10⁻¹² over the full mass range and χ>1.1×10⁻¹³ at a mass of 80.57 μeV/c² with a 95% confidence level. This is the first physics result from a madmax prototype and exceeds previous constraints on χ in this mass range by up to almost three orders of magnitude.