Search for Dark Photon Dark Matter in the Mass Range $74\mbox{--}110\,\mu\mathrm{eV}/c^2$ with a Cryogenic Millimeter-Wave Receiver

We search for the dark photon dark matter (DPDM) using a cryogenic millimeter-wave receiver. DPDM has a kinetic coupling with electromagnetic fields with a coupling constant of $\chi$, and is converted into ordinary photons at the surface of a metal plate. We search for signal of this conversion in the frequency range $18\text{--}26.5\,\mathrm{GHz}$, which corresponds to the mass range $74\text{--}110\,mu\mathrm{eV}/c^2$. We observed no significant signal excess, allowing us to set an upper bound of $\chi < (0.3\text{--}2.0)\times 10^{-10}$ at 95\% confidence level. This is the most stringent constraint to date, and tighter than cosmological constraints. Improvements from previous studies are obtained by employing a cryogenic optical path and a fast spectrometer.