Light Mediators at the Neutrino-Dark Matter Frontier: Coherent Scattering, Precision Constraints, and Complementary Searches

Low-threshold neutrino and dark-matter experiments constrain light mediators through different momentum transfers, targets, and coupling combinations. We examine scalar and vector interactions in coherent neutrino--nucleus scattering, neutrino--electron scattering, and dark-matter direct detection. Three representative interaction models are used to compare recoil spectra and coupling degeneracies. For ⁷³Ge and ¹³¹Xe, the nuclear-charge response matrix has a condition number of approximately 115, so two different targets may still leave nearly parallel blind directions. An analytic profile-likelihood calculation shows how spectral overlap and uncertainties in neutrino interactions affect dark-matter discovery sensitivity in the neutrino fog. We discuss the implications of measurements from COHERENT, CONUS+, LZ, XENONnT, and PandaX-4T and specify which theoretical inputs, detector responses, and statistical correlations are needed to combine their constraints.