Searching for dilaton dark matter with atomic clocks
We propose an experiment to search for ultralight scalar dark matter (DM) with dilatonic interactions. Such couplings can arise for the dilaton as well as for moduli and axion-like particles in the presence of $CP$ violation. Ultralight dilaton DM acts as a background field that can cause tiny but coherent oscillations in Standard Model parameters such as the fine-structure constant and the proton-electron mass ratio. These minute variations can be detected through precise frequency comparisons of atomic clocks. Our experiment extends current searches for drifts in fundamental constants to the well-motivated high-frequency regime. Our proposed setups can probe scalars lighter than $1{0}^{\ensuremath{-}15}\text{ }\text{ }\mathrm{eV}$ with a discovery potential of dilatonic couplings as weak as $1{0}^{\ensuremath{-}11}$ times the strength of gravity, improving current equivalence principle bounds by up to 8 orders of magnitude. We point out potential $1{0}^{4}$ sensitivity enhancements with future optical and nuclear clocks, as well as possible signatures in gravitational-wave detectors. Finally, we discuss cosmological constraints and astrophysical hints of ultralight scalar DM, and show they are complimentary to and compatible with the parameter range accessible to our proposed laboratory experiments.