Discriminating scalar ultralight dark matter from quasi-monochromatic gravitational waves in LISA

Abstract: A scalar ultralight dark matter (ULDM) candidate would induce oscillatory motion of freely falling test masses via its coupling to Standard Model fields. Such oscillations would create an observable Doppler shift of light exchanged between the test masses, and in particular would be visible in space-based gravitational waves (GW) detectors, such as LISA. While this kind of detection has been proposed multiple times in the recent years, we numerically investigate if it is possible to extract a scalar ULDM signal in a space-based GW detector, and in particular how to differentiate such a signal from a GW signal. Using one year of realistic orbits for the LISA spacecrafts and Bayesian methods, we find that LISA will indeed be able to discriminate between the two signals.