Prospects for direct detection of ultralight dark matter candidates in deci-Hz band with IndIGO-D

We investigate the sensitivity of IndIGO-D, a proposed space-based decihertz gravitational-wave interferometer, to different classes of ultralight dark matter. IndIGO-D will probe the $\sim0.01$--$10~\mathrm{Hz}$ frequency band between those accessible to current ground- and future space-based gravitational-wave interferometers, providing access to ultralight dark-matter masses beyond the reach of existing instruments. We consider two complementary signatures: interferometric displacements induced by coherently oscillating scalar (dilaton), vector (dark-photon, $U(1)_B$ and $U(1)_{B-L}$ gauge groups), and tensor fields with masses $m_{\rm DM}\sim10^{-17}$--$10^{-14}~\mathrm{eV}$; and changes in laser polarization induced by pseudoscalar axion dark matter at higher masses, around $m_a\sim10^{-12}~\mathrm{eV}$. For the dilatons, dark photons and tensors, we compute the expected sensitivities using two pipelines -- cross-correlation and BSD excess-power -- assuming L-shaped and triangular interferometer layouts and three representative noise power spectral densities (S1, S2, S3), each for two years of continuous observation. We find that the projected sensitivities agree to within a factor of order unity across the search pipelines and interferometer geometries. In particular, we show that IndIGO-D could open previously unconstrained coupling parameter space for vector and tensor dark matter across $m_{\rm DM}\sim10^{-16}$--$10^{-14}~\mathrm{eV}$, bridging the sensitivity of space- and ground-based experiments. For axion dark matter, we demonstrate that a complementary detection for laser light polarization shifts, limited primarily by photon shot noise, could probe the axion-photon coupling $g_{aγ}$ at masses around $m_a\sim10^{-12}~\mathrm{eV}$ at a level potentially better than that of other future experiments without degrading sensitivity to gravitational waves.