New limits on scalar ultralight dark matter using Fabry-Pérot optical cavities

Scalar ultralight dark matter (ULDM) represents a well-motivated extension to the standard model of particle physics, coupling to parameters such as the electron mass and fine-structure constant, thereby inducing coherent oscillations in the size of macroscopic solids at the ULDM Compton frequency. This talk presents a novel optomechanical sensor employing two sapphire Fabry-Pérot optical cavities as high-quality-factor mechanical resonators to detect ULDM-induced differential length variations. After a four-day observing run, up to two orders of magnitude improved limits on ULDM couplings to SM for Compton frequencies (ULDM masses) spanning 5 kHz (20 peV) to 100 kHz (400 peV) are established, for both the standard galactic halo model and the model of an Earth-bound relaxion halo, nearing theoretical targets for Higgs-relaxion mixing in the latter. Projections for upgraded apparatus configurations suggest up to five orders of magnitude improvement in ULDM limits over a broader range of Compton frequencies (100 Hz to 1 MHz) or ULDM masses (400 feV to 4 neV) and surpassing the naturalness threshold.