Detectability of ultralight scalar field dark matter with gravitational-wave detectors

An ultralight scalar field is one of the dark matter candidates. If it couples with Standard Model particles, it oscillates mirrors in gravitational-wave detectors and generates detectable signals. We focus on the case where the scalar field has nonminimal parity-even couplings with Standard Model particles and study the spectra of the signals, taking into account the motion of the detectors due to the Earth's rotation/the detectors' orbital motion around the Sun. Then, we formulate suitable data- analysis methods to detect it and estimate the future constraint with our methods. We find that gravitational-wave experiments can improve the existing constraints given by fifth-force experiments on one of the scalar field's coupling constants by factors of similar to 30, similar to 100, and similar to 350 for m(phi) = 2 x 10(-17) eV, 10(-14) eV, and 10(-12) eV, respectively, where m(phi) is the scalar field's mass. Our study demonstrates that experiments with gravitational-wave detectors are complementary to tests of the weak equivalence principle.