Asteroids for ultralight dark-photon dark-matter detection

Gravitational-wave (GW) detectors that monitor fluctuations in the separation between inertial test masses (TMs) are sensitive to new forces acting on those TMs. Ultralight dark-photon dark matter (DPDM) coupled to $U(1{)}_B$ or $U(1{)}_{B-L}$ charges supplies one such force that oscillates with a frequency set by the DPDM mass. GW detectors operating in different frequency bands are thus sensitive to different DPDM mass ranges. A recent GW detection proposal based on monitoring the separation of certain asteroids in the inner Solar System would have sensitivity to $\mu$Hz frequencies [arXiv:2112.11431]. In this paper, we show how that proposal would also enable access to new parameter space for DPDM coupled to $B$ [respectively, $B-L$] charges in the mass range $5[9]\times {10}^{-21}\text{eV}\lesssim m_{\text{DM}}\lesssim 2\times {10}^{-19}\text{eV}$, with peak sensitivities about a factor of 500 [50] beyond current best limits on ${\epsilon }_B$ [${\epsilon }_{B-L}$] at $m_{\text{DM}}\sim 2\times {10}^{-19}\text{eV}$. Sensitivity could be extended up to $m_{\text{DM}}\sim 2\times {10}^{-18}\text{eV}$ only if noise issues associated with asteroid rotational motion could be overcome.