A directional force template for quadratically coupled ultralight dark matter

Abstract

Quadratic couplings between ultralight scalar dark matter and Standard Model fields can produce a distorted dark-matter field profile around the Earth. Gradients in the field induce a non-radial, composition-dependent force that can be suppressed at the Earth's surface while remaining accessible to space-based experiments. The MICROSCOPE satellite, which searched for violations of the equivalence principle, can constrain this force, but existing results assume a radial force, and they cannot be directly translated into an optimal bound in the anisotropic regime. We develop a signal template for this regime by organizing the force into radial and polar multipole coefficients and projecting the force onto the MICROSCOPE measurement axis. We use this template to recast the published MICROSCOPE constraint using the component of the signal that overlaps with the radial-force template. We estimate the sensitivity gain that would be provided by an analysis utilizing the additional non-overlapping signal. Such an analysis could improve sensitivity to the couplings of quadratically coupled scalar dark matter by more than an order of magnitude relative to the radial-force recast for dark matter masses $\gtrsim 10^{-9}$ eV.