Deepest sensitivity to wavelike dark photon dark matter with superconducting radio frequency cavities

Abstract: Wavelike, bosonic dark matter candidates like axions and dark photons can be detected using microwave cavities known as haloscopes. Traditionally, haloscopes consist of tunable copper cavities operating in the TM${010}$ mode, but ohmic losses have limited their performance. In contrast, superconducting radio frequency (SRF) cavities can achieve quality factors of $\sim 10^{10}$, perhaps five orders of magnitude better than copper cavities, leading to more sensitive dark matter detectors. In this paper, we first derive that the scan rate of a haloscope experiment is proportional to the loaded quality factor $Q_L$, even if the cavity bandwidth is much narrower than the dark matter halo line shape. We then present a proof-of-concept search for dark photon dark matter using a nontunable ultrahigh quality SRF cavity. We exclude dark photon dark matter with kinetic mixing strengths of $\chi > 1.5\times 10^{-16}$ for a dark photon mass of $m{A^{\prime}} = 5.35\mu$eV, achieving the deepest exclusion to wavelike dark photons by almost an order of magnitude.