New Limit on Dark Photon Kinetic Mixing in the 0.2-1.2 μeV Mass Range From the Dark E-Field Radio Experiment

We report new limits on the kinetic mixing strength of the dark photon spanning the mass range 0.21 -- 1.24 μeV corresponding to a frequency span of 50 -- 300 MHz. The Dark E-Field Radio experiment is a wide-band search for dark photon dark matter. In this paper we detail changes in calibration and upgrades since our proof-of-concept pilot run. Our detector employs a wide bandwidth E-field antenna moved to multiple positions in a shielded room, a low noise amplifier, wideband ADC, followed by a 2²⁴-point FFT. An optimal filter searches for signals with Q ≈10⁶. In nine days of integration, this system is capable of detecting dark photon signals corresponding to ε several orders of magnitude lower than previous limits. We find a 95% exclusion limit on ε over this mass range between 6× 10⁻¹⁵ and 6× 10⁻¹³, tracking the complex resonant mode structure in the shielded room.