Search for Dark-Matter-Induced Oscillations of Fundamental Constants Using Molecular Spectroscopy

A possible implication of an ultralight dark matter field interacting with the standard model degrees of freedom is oscillations of fundamental constants. Here, we establish direct experimental bounds on the coupling of an oscillating ultralight dark matter field to the up, down, and strange quarks and to the gluons, for oscillation frequencies between 10 and 10⁸ Hz. We employ spectroscopic experiments that take advantage of the dependence of molecular transition frequencies on the nuclear masses. Our results apply to previously unexplored frequency bands and improve on existing bounds at frequencies >5 MHz. We also improve on the bounds for coupling to the electromagnetic field and the electron field, in particular spectral windows. We identify a sector of ultralight dark matter and standard model coupling space where the bounds from equivalence principle tests may be challenged by next-generation experiments of the present kind.