Search for scalar ultralight dark matter using two optical cavities

Direct detection of dark matter (DM) poses one of the most urgent questions in fundamental physics. Scalar ultralight DM (ULDM) is a promising candidate whose coupling to the standard model (SM) of particle physics is expected to cause variations in the fundamental constants like the fine-structure constant and electron mass and thereby the Bohr radius. The latter causes an oscillation in the size of atoms and chemical bonds and consequently the size of macroscopic solids at the ULDM's Compton frequency. We experimentally demonstrate an optomechanical scheme to detect ULDM involving measurement of the ULDM-driven differential length change between two optical Fabry-Perot sapphire cavities. For Compton frequencies in the 5 kHz to 100 kHz range, we improve upon existing limits on the coupling of ULDM to the SM by up to two orders of magnitude. The new limits are presented for two cases: (i) the galactic DM halo model, and (ii) a relaxion star gravitationally bound to Earth.