Search for Dark Photons between 16.96--19.52 $\mu$eV with the HAYSTAC Experiment

We report dark photon results from HAYSTAC Phase II using data from previously reported axion searches. Additionally, we present an analysis of an unpublished dataset covering a region between 19.46–19.52 $\mu$eV. This region overlaps with a recently reported dark photon signal at 19.5 $\mu$eV with a kinetic coupling strength of $\mathrm{\mid }{\chi }_{\text{rand}}\mathrm{\mid }\simeq 6.5\times {10}^{-15}$ resulting from a reanalysis of previously published data from the TASEH collaboration. Given HAYSTAC’s sensitivity, if such a signal were present, it would have appeared as a large $17.1\sigma$ excess above the noise. However, no such signal was observed. We thus exclude couplings $\mathrm{\mid }{\chi }_{\text{rand}}\mathrm{\mid }\ge 4.90\times {10}^{-15}$ at the 90% confidence level over the newly reported region. In addition, using our previously reported axion data, we exclude couplings $\mathrm{\mid }{\chi }_{\text{rand}}\mathrm{\mid }\ge 2.90\times {10}^{-15}$ between 16.96–19.46 $\mu$eV at the 90% confidence level.